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2020

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Szabados J, Werner C, Herr S, Breunig I, Buse K
Electro-optic eigenfrequency tuning of potassium tantalate-niobate microresonators
2020 APL Photonics, Band: 5, Seite: 016106
KurzfassungEigenfrequency tuning in microresonators is useful for a range of applications including frequency-agile optical filters and tunable optical frequency converters. In most of these applications, eigenfrequency tuning is achieved by thermal or mechanical means, while a few non-centrosymmetric crystals such as lithium niobate allow for such tuning using the linear electro-optic effect. Potassium tantalate-niobate (KTa1−xNbxO3 with 0 ≤ x ≤ 1, KTN) is a particularly attractive material for electro-optic tuning purposes. It has both non-centrosymmetric and centrosymmetric phases offering outstandingly large linear as well as quadratic electro-optic coefficients near the phase transition temperature. We demonstrate whispering-gallery resonators made of KTN with quality factors of Q > 107 and electro-optic eigenfrequency tuning of more than 100 GHz at λ = 1040 nm for moderate field strengths of E = 250 V/mm. The tuning behavior near the phase transition temperature is analyzed by introducing a simple theoretical model. These results pave the way for applications such as electro-optically tunable microresonator-based Kerr frequency combs.
Szabados J, Puzyrev D, Minet Y, Reis L, Buse K, Villois A, Skryabin D, Breunig I
Frequency Comb Generation via Cascaded Second-Order Nonlinearities in Microresonators
2020 Phys Rev Lett, Band: 124, Seite: 203902
KurzfassungOptical frequency combs are revolutionizing modern time and frequency metrology. In the past years, their range of applications has increased substantially, driven by their miniaturization through microresonator-based solutions. The combs in such devices are typically generated using the third-order χ(3) nonlinearity of the resonator material. An alternative approach is making use of second-order χ(2) nonlinearities. While the idea of generating combs this way has been around for almost two decades, so far only few demonstrations are known, based either on bulky bow-tie cavities or on relatively low-Q waveguide resonators. Here, we present the first such comb that is based on a millimeter-sized microresonator made of lithium niobate, that allows for cascaded second-order nonlinearities. This proof-of-concept device comes already with pump powers as low as 2 mW, generating repetition-rate-locked combs around 1064 and 532 nm. From the nonlinear dynamics point of view, the observed combs correspond to Turing roll patterns.
Szabados J, Sturman B, Breunig I
Frequency comb generation threshold via second-harmonic excitation in X(2)optical microresonators
2020 APL Photonics, Band: 5, Nummer: 11, Seite: 116102
KurzfassungWe investigate the threshold of X(2) frequency comb generation in lithium niobate whispering gallery microresonators theoretically and experimentally. When generating a frequency comb via second-harmonic excitation, also commonly known as second-harmonic generation, the threshold for the onset of cascaded second-order processes leading to a comb is found to be ∼85 µW. The second-harmonic generation efficiency up to this value is in excellent agreement with a previously known theoretical framework. This framework is extended here, showing that the onset of cascaded X(2) processes and the maximum of the second-harmonic generation efficiency coincide. The model introduced here allows us to determine the frequency comb generation threshold analytically. Furthermore, we observe that the frequency distance between the comb lines is a function of the pump power. It changes from four free spectral ranges at the oscillation threshold to one free spectral range at 590 µW.
Podivilov E, Sturman B, Breunig I
Frequency comb solutions for driven X(2) optical microresonators
2020 J Opt Soc Am B, Band: 37, Nummer: 11, Seiten: 3316 - 3324
KurzfassungFrequency combs in X(2) optical microresonators caused by cascaded second-order nonlinear processes currently attract great research interest. In contrast to X(3) resonators, two light amplitudes relevant to the first and second harmonics (FH and SH), two dispersion coefficients, and a considerable difference of the FH and SH group velocities (the walk-off) must be taken into account to investigate localized coherent structures (solitons) propagating with a common velocity. Finding such comb solutions taking into account external pumping is a crucial step toward the X(2) combs. We report on two new families of driven soliton-comb solutions for X(2) microresonators. They are strongly localized, corresponding to spectrally broad combs, and possess well-defined FH and SH frequency detunings, propagation velocities, and, generally, nonzero spatial backgrounds.
Podivilov E, Smirnov S, Breunig I, Sturman B
Nonlinear solutions for χ(2) frequency combs in optical microresonators
2020 Phys Rev A, Band: 101, Seite: 023815
KurzfassungExperimental and theoretical studies of nonlinear frequency combs in χ(3) optical microresonators attracted tremendous research interest during the last decade and resulted in prototypes of soliton-based steadily working devices. Realization of similar combs owing to χ(2) optical nonlinearity promises new breakthroughs and is a big scientific challenge. We analyze the main obstacles for realization of the χ(2) frequency combs in high-Q microresonators and propose two families of steady-state nonlinear solutions, including soliton and periodic solutions, for such combs. Despite periodicity of light fields inside microresonators, the nonlinear solutions can be topologically different and relevant to periodic and antiperiodic boundary conditions. The antiperiodic states are expected to be the most favorable for the comb generation. The found particular solutions exist owing to a large difference in the group velocities between the first and second harmonics, typical of χ(2) microresonators, and to the presence of the pump. They have no zero-pump counterparts relevant to conservative solitons. The stability issue for the found comb solutions remains open and requires further numerical analysis.
Minet Y, Reis L, Szabados J, Werner C, Zappe H, Buse K, Breunig I
Pockels-effect-based adiabatic frequency conversion in ultrahigh-Q microresonators
2020 Opt Express, Band: 28, Nummer: 3, Seiten: 2939 - 2947
KurzfassungAdiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of 100 % down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. Frequency shifts of more than 5 GHz are achieved by applying just 20 V to a 70-µm-thick resonator. Furthermore, we demonstrate that with the same setup positive and negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.
Smirnov S, Sturman B, Podivilov E, Breunig I
Walk-off controlled self-starting frequency combs in χ(2) optical microresonators
2020 Opt Express, Band: 28, Seite: 18006
KurzfassungInvestigations of frequency combs in χ(3) optical microresonators are burgeoning nowadays. Changeover to χ(2) resonators promises further advances and brings new challenges. Here, the comb generation entails not only coupled first and second harmonics (FHs and SHs) and two dispersion coefficients but also a substantial difference in the group velocities – the temporal walk-off. We predict walk-off controlled highly stable comb generation, which is drastically different from that known in the χ(3) case. This includes the general notion of antiperiodic states; formation of localized coherent antiperiodic steady states (solitons), where the FH and SH envelopes move with a common velocity without shape changes; characterization of a new vast family of antiperiodic solitons; and the dependence of comb spectra on the pump power and the group velocity difference.

2019

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Villouis A, Kondratiev N, Breunig I, Puzyrev D N, Skryabin D V
Frequency combs in a microring optical parametric oscillator
2019 Opt Lett, Band: 44, Seite: 4443
KurzfassungWe report the soliton frequency comb generation in microring optical parametric oscillators operating in the downconversion regime and with the simultaneous presence of the χ(2) and Kerr nonlinearities. The combs are studied considering a typical geometry of a bulk LiNbO3 toroidal resonator with the normal group velocity dispersion spanning an interval between the pump and the downconverted signal. We have identified critical power signaling a transition between the relatively low pump power predominantly χ(2) combs and the high pump power ones shaped by the competition between the χ(2) and Kerr nonlinearities.
Herr S, Buse K, Breunig I
Tunable single-frequency lasing in a microresonator
2019 Opt Express, Band: 27, Nummer: 11, Seiten: 15351 - 15358
KurzfassungWhispering-gallery-mode resonators made of laser-active materials can serve as efficient microphotonic coherent light sources. However, the majority of experimental realizations relies on expensive pump light sources like narrow-linewidth or pulsed laser systems, which is inappropriate for most applications. In order to overcome this, we present a whispering-gallery laser system without the need for an expensive pump light source and at the same time with unprecedented laser performance: A laser-active resonator made of Nd:YVO 4 is non-resonantly excited, employing a low-cost laser diode without any external frequency stabilization, emitting up to 100 mW optical power around 810 nm wavelength. Continuous-wave single-frequency lasing at 1064 nm wavelength is achieved with directed laser light emission in the mW-regime. The temporal power and frequency stability are within ±1.5 % and ±30 MHz, respectively. Modehop-free frequency fine-tuning is achieved exceeding 11 GHz tuning range by changing the temperature of the cavity. Faster tuning can be expected when applying geometric or electro-optic instead of thermal tuning.
Sturman B, Podivilov E, Werner C, Breunig I
Vectorial perturbation theory for axisymmetric whispering gallery resonators
2019 Phys Rev A, Band: 99, Seite: 013810
KurzfassungWe propose a vectorial perturbation theory for axially symmetric, generally nonspherical whispering gallery resonators made of isotropic and anisotropic optical materials. It is based on analysis of the leading terms in the coupled equations for independent light-field components, as derived from Maxwell's equations, and true boundary conditions. Strong localization of the whispering gallery modes (WGMs) near the resonator rim, controlled by the azimuth modal number m, is the main prerequisite for our analysis. The theory gives high-precision expressions for the WGM frequencies and modal functions, including the evanescent effects. One of important applications of the theory is analysis of anticrossings of the WGM resonances in anisotropic resonators detected in experiments. Simple relations for the frequency avoidance gaps during the anticrossings are derived and compared with experimental data obtained in lithium-niobate-based WGM resonators. We show also that the vectorial effects substantially restrict the field of applicability of the scalar WGM models.

2018

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Jia Y, Hanka K, Zawilski K, Schunemann P, Buse K, Breunig I
Continuous-wave whispering-gallery optical parametric oscillator based on CdSiP₂
2018 Opt Express, Band: 26, Nummer: 8, Seiten: 10833 - 10841
KurzfassungContinuous-wave (cw) optical parametric oscillators (OPOs) are ideally suited for applications, for example high-resolution spectroscopy, that need coherent sources combining narrow-linewidth emission with good wavelength tunability. Here, we demonstrate for the first time cw OPOs based on a millimeter-sized whispering gallery resonator (WGR) made of cadmium silicon phosphide (CdSiP₂). By employing a compact laser diode at 1.57-μm wavelength for pumping, a cw OPO with wavelength tunability from 2.3 μm to 5.1 μm is realized based on such a resonator. The oscillation thresholds are in the milliwatt range. The maximum total power conversion efficiency reaches more than 15%. The intrinsic quality factor at 1.57 μm is determined to be 3.5 × 10⁶. This work suggests that CdSiP₂ is a very promising alternative for constructing mid-infrared parametric devices.
Werner C, Sturman B, Podivilov E, Kini Manjeshwar S, Buse K, Breunig I
Control of mode anticrossings in whispering gallery microresonators
2018 Opt Express, Band: 26, Nummer: 2, Seiten: 762 - 771
KurzfassungOptical microresonators attract strong interest because of exciting effects and applications ranging from sensing of single atoms and molecules to quantum and nonlinear optics. For all this, control and tuning of the discrete resonances are vital. In resonators made of anisotropic materials that are beneficial for nonlinear-optical applications, anticrossings of ordinarily (o) and extraordinarily (e) polarized modes occur regularly. This effect is badly understood and harmful for mode control and tuning. We show that the anticrossings are inherent in the o- and e-modes because of the vectorial properties of Maxwell’s equations. Within a novel pertubative approach employing a strong localization of the modes near the resonator rim, we have quantified the anticrossings. The values of avoidance gaps strongly exceeding the linewidths and selection rules for the interacting modes are predicted. The inferred values of the avoidance gaps are confirmed experimentally in resonators made of lithium niobate. Furthermore, based on theory, we have eliminated the anticrossings completely by spatially-controlled introduction of defects. This paves the way for unperturbed tuning of anisotropic microresonators.
Herr S, Brasch V, Szabados J, Obrzud E, Jia Y, Lecomte S, Buse K, Breunig I, Herr T
Frequency comb up- and down-conversion in synchronously driven χ(2) optical microresonators
2018 Opt Lett, Band: 43, Seite: 5745
KurzfassungOptical frequency combs are key to optical precision measurements. While most frequency combs operate in the near-infrared (NIR) regime, many applications require combs at mid-infrared (MIR), visible (VIS), or even ultra-violet (UV) wavelengths. Frequency combs can be transferred to other wavelengths via nonlinear optical processes; however, this becomes exceedingly challenging for high-repetition-rate frequency combs. Here it is demonstrated that a synchronously driven high- microresonator with a second-order optical nonlinearity can efficiently convert high-repetition-rate NIR frequency combs to VIS, UV, and MIR wavelengths, providing new opportunities for microresonator and electro-optic combs in applications including molecular sensing, astronomy, and quantum optics
Jia Y, Winkler M, Cheng C, Chen F, Kirste L, Cimalla V, Zukauskaite A, Szabados J, Breunig I, Buse K
Pulsed laser deposition of ferroelectric potassium tantalate-niobate optical waveguiding thin films
2018 Opt Mater Express, Band: 8, Nummer: 3, Seiten: 541 - 548
KurzfassungWe present the epitaxial growth of ferroelectric potassium tantalate-niobate (KTN) thin films by pulsed laser deposition. As a result of the optimization of the deposition and the surface finishing processes, a c-axis oriented KTa₀.₅Nb₀.₅O₃ thin film with homogeneous polarization phase grown on KTaO₃ and an efficient KTa₀.₅Nb₀.₅O₃ waveguiding thin film grown on MgO are demonstrated. The highly improved crystalline and optical quality of KTN layers grown in this work reveal the great potential of such films for integrated nonlinear optics.
Wolf R, Jia Y, Bonaus S, Werner C, Herr S, Breunig I, Buse K, Zappe H
Quasi-phase-matched nonlinear optical frequency conversion in on-chip whispering galleries
2018 Optica, Band: 5, Seite: 872
KurzfassungChip-integrated whispering-gallery resonators (WGRs) enable compact and wavelength-agile nonlinear optical frequency synthesizers. So far, the most flexible phase-matching technique, i.e., quasi-phase matching, has not been applied in this configuration. The reason is the lack of suitable thin films with alternating crystal structure on a low-refractive-index substrate. Here, we demonstrate an innovative method of realizing thin film substrates suitable for quasi-phase matching by field-assisted domain engineering of lithium niobate, and subsequent direct bonding and polishing. We are able to fabricate high- on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The -factors of the resonators are up to one million, which allows us to demonstrate quasi-phase-matched second-harmonic generation in on-chip WGRs for the first time, to the best of our knowledge. The normalized conversion efficiency is 9×10−4 mW−1. This method can also be transferred to other material systems.
Herr S, Werner C, Buse K, Breunig I
Quasi-phase-matched self-pumped optical parametric oscillation in a micro-resonator
2018 Opt Express, Band: 26, Nummer: 8, Seiten: 10813 - 10819
KurzfassungLasing and self-pumped optical parametric oscillation (self-OPO) are achieved in a high-Q whispering-gallery-mode micro-resonator, made of neodymium-doped lithium niobate. A laser process providing 5 mW output power at 1.08 μm wavelength is sufficient to pump a self-OPO process within the same high-Q cavity. At 6 mW lasing output power, the sum of signal and idler output powers exceeds 1.2 mW. The wavelength of the generated light ranges from 1.5 to 3.8 μm. Phase matching is provided by a radial quasi-phase-matching structure, which is generated by a current-controlled calligraphic poling technique. To the best of our knowledge, this is the first demonstration of a quasi-phase-matched self-pumped nonlinear optical process in a micro-resonator, as well as the first self-OPO in a micro-resonator. The concept bears the potential for a highly integrated and wavelength-tunable coherent light source at low cost.
Wolf R, Breunig I, Zappe H, Buse K
Scattering-loss reduction of ridge waveguides by sidewall polishing
2018 Opt Express, Band: 26, Seite: 19815
KurzfassungRidge waveguides provide a large refractive index contrast and thus strong mode confinement, making them highly attractive for building compact photonic integrated circuits. However, ridge waveguides suffer from scattering losses. We demonstrate scattering-loss reduction of ridge waveguides made of lithium-niobate-on-insulator (LNOI) substrates by more than one order of magnitude. This is achieved by gently polishing of the ridge’s sidewalls and simultaneous protection of the top surfaces by a metal layer. Whispering-gallery-resonator loss measurements reveal ultra-low losses down to 0.04 dB/cm of the processed waveguides. Our approach pushes ridge waveguides further towards their fundamental absorption-loss limit, enabling highly efficient integrated devices.

2017

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Wolf R, Breunig I, Zappe H, Buse K
Cascaded second-order optical nonlinearities in on-chip micro rings
2017 Opt Express, Band: 25, Seite: 29927
KurzfassungWe demonstrate cascaded Stimulated Raman Scattering (SRS), Second-Harmonic Generation (SHG), and Sum-Frequency Generation (SFG) in integrated on-chip whispering-gallery resonators (WGRs). These lithium niobate-based WGRs are fabricated using highly-parallel semiconductor manufacturing techniques coupled with specialized polishing as a post-processing step and thus represent a novel means for batch fabrication of this family of non-linear devices. We achieved record high Q-factors for on-chip lithium niobate WGRs reaching up to 3 × 10⁶. Furthermore, we present a flexible but stable coupling scheme, which gives us the opportunity to optimize the coupling regarding the non-linear optical processes we observe.
Meisenheimer S, Fürst J, Buse K, Breunig I
Continuous-wave optical parametric oscillation tunable up to an 8 μm wavelength
2017 Optica, Band: 4, Seite: 189
KurzfassungHigh-resolution spectroscopy of molecular gases requires sources of mid-infrared laser light combining narrow linewidths and wavelength tunability. Continuous-wave optical parametric oscillators (cw OPOs) fulfill these demands; however, their mid-infrared tuning range has been limited to wavelengths below 5.5 μm so far. Here, we demonstrate the first cw OPO emitting mid-infrared light at wavelengths up to 8 μm. This device is based on a 3.5-mm-diameter whispering gallery resonator made of silver gallium selenide (AgGaSe₂) pumped by a compact distributed feedback laser diode emitting at the 1.57 μm wavelength. The oscillation thresholds are in the mW region, while the output powers range from 10 to 800 μW. By changing the radial mode number of the pump wave, wavelengths of up to 8 μm are achieved. Temperature variation enables 100-nm-wide wavelength tuning. The measured tuning branches are in good accordance with the simulations. Furthermore, the latter show that whispering-gallery OPOs based on AgGaSe₂ with diameters around 2 mm can generate idler waves exceeding the 10 μm wavelength.
Schiller A, Beckmann T, Fratz M, Belzer D, Bertz A, Carl D, Buse K
Digital holography on moving objects: interference contrast as a function of velocity and aperture width
2017 Appl Optics, Band: 56, Nummer: 16, Seiten: 4622 - 4628
KurzfassungDigital holographic measurements on planar moving objects are investigated. We discuss the dependence of the interference contrast on velocity and aperture width for both diffusely and specularly reflecting objects. Using spatial phase shifting, the experimental results for motion in parallel and perpendicular to the optical axis are in good agreement with the theoretical considerations. Measurements with object velocities of up to 100 mm/s are conducted using only less than 1 mW of continuous-wave laser light. These considerations are used to determine the optimal angle between the direction of motion and the illuminating beam, resulting in the lowest decrease in contrast with increasing velocity.
Werner C, Yoshiki W, Herr S, Breunig I, Buse K
Geometric tuning: spectroscopy using whispering gallery resonator frequency-synthesizers
2017 Optica, Band: 4, Seite: 1205
KurzfassungContinuous fine-tuning of nonlinear processes in whisperinggallery resonators is still in its infancy. We demonstrate an innovative, fast, and compact tuning-method by enlarging a ring-like whispering-gallery resonator made out of lithium niobate with the help of a piezo actuator disk embedded inside the resonator. The radial expansion of the actuator forces the resonator to change its size and shape. This process is much faster than thermal tuning and has the additional benefit of a smaller dispersion. Using just this method to tune the resonator, a second-harmonic process pumped at around 1 μm wavelength can be tuned mode-hop-free over a range of 28 GHz, and an optical parametric oscillator pumped at the same wavelength achieved 4.5 GHz continuous tuning. In addition to presenting and demonstrating this novel concept, the influence of the elasto-optic effect and the refractive-index dispersion are discussed, and an analytic model to predict the wavelength dependent tuning efficiency is presented.
Herr S, Buse K, Breunig I
LED-pumped whispering-gallery laser
2017 Photonics Res, Band: 5, Nummer: 6, Seiten: B34 - B38
KurzfassungA low-cost light-emitting diode (LED) is sufficient to pump a quasi-continuous-wave bidirectional high-Q whispering-gallery resonator laser made of Nd:YVO₄. This is remarkable because of the very limited spatial and spectral coherence of an LED. The LED, delivering up to 3.5 W, centered around 810 nm, is turned on in intervals of 100 μs duration, and for these periods a laser output exceeding 0.8 mW has been verified. Furthermore, 0.1-s-long laser pulses are demonstrated. To the best of our knowledge, this is the first demonstration of an LED-pumped high-Q whispering-gallery laser. The concept can be extended easily to other laser active materials. A prospect is also to pump several of such lasers with a single LED.
Werner C, Herr S, Buse K, Sturman B, Soergel E, Razzaghi C, Breunig I
Large and accessible conductivity of charged domain walls in lithium niobate
2017 Sci Rep-uk, Band: 7, Seite: 9862
KurzfassungFerroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals – a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.
Herr S, Folwill Y, Buse K, Breunig I
Self-frequency doubling in a laser-active whispering-gallery resonator
2017 Opt Lett, Band: 42, Seite: 2627
KurzfassungLasing and self-frequency doubling are achieved in a millimeter-sized laser-active whispering-gallery resonator made of neodymium-doped lithium niobate. A low-cost 808-nm laser diode without external frequency stabilization is sufficient to pump the neodymium ions. Laser oscillation around 1.08 μm drives a frequency-doubling process within the same cavity providing green light. The electrical-optical efficiency of the system reaches up to 2×10⁻⁴. To the best of our knowledge, this is the first demonstration of combining lasing and χ⁽²⁾ frequency conversion in a single high-Q whispering-gallery resonator. This approach is general and can be applied to other materials and other nonlinear optical processes.
Wolf S, Trendle T, Kiessling J, Herbst J, Buse K, Kühnemann F
Self-gated mid-infrared short pulse upconversion detection for gas sensing
2017 Opt Express, Band: 25, Nummer: 20, Seiten: 24459 - 24468
KurzfassungPulsed nonlinear-optical upconversion is used for mid-infrared signal detection. A setup for both mid-infrared generation and upconversion based on a single pump laser enables sensitive light detection and is utilized for gas spectroscopy. With the demonstrated pulsed setup, quantum efficiencies above 80 % for the upconversion of a Gaussian beam signal and 25 % for the upconversion of backscattered radiation are achieved and in agreement with theoretical predictions. Combined with efficient background suppression due to spectral and temporal gating, this results in highly sensitive detection of the infrared signals. As a demonstration of application, the presented system is used for methane sensing in an open path geometry, highlighting the potential for stand-off leak detection with a concentration resolution better than 1.5 ppm·m.
Wolf S, Kiessling J, Kunz M, Popko G, Buse K, Kühnemann F
Upconversion-enabled array spectrometer for the mid-infrared, featuring kilohertz spectra acquisition rates
2017 Opt Express, Band: 25, Nummer: 13, Seiten: 14504 - 14515
KurzfassungMid-infrared spectroscopy is an essential analytical method in science and industry. Unlike in the near-infrared range, grating spectrometers for the mid-infrared are rarely employed, mostly due to the limited availability and performance of suitable line array detectors. In this work, continuous-wave nonlinear-optical upconversion is used to enable mid-infrared spectroscopy. A broad spectral window between 3.7 and 4.7 µm is upconverted to 825 - 867 nm for detection on a silicon-camera-based near-infrared grating spectrometer with a high sensitivity down to sub-picowatt of input power. A theoretical model is presented that accurately describes the upconversion process and the total system behavior. Spectroscopic flame emission measurements demonstrate the applicability towards the analysis of highly dynamic processes.

2016

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Leidinger M, Werner C, Yoshiki W, Buse K, Breunig I
Impact of the photorefractive and pyroelectric-electro-optic effect in lithium niobate on whispering-gallery modes
2016 Opt Lett, Band: 41, Seite: 5474
KurzfassungWhispering-gallery resonators made of undoped and MgO-doped congruently grown lithium niobate are used to study electro-optic refractive index changes. Hereby, we focus on the volume photovoltaic and the pyroelectric effect, both providing an electric field driving the electro-optic effect. Our findings indicate that the light-induced photorefractive effect, combining the photovoltaic and electro-optic effect, is present only in the non-MgO-doped lithium niobate for exposure with light having wavelengths of up to 850 nm. This leads to strong resonance frequency shifts of the whispering-gallery modes. No photorefractive effect was observed in the MgO-doped material. One has to be aware that surface charges induced by the pyroelectric effect result in a similar phenomenon and are present in both materials.
Leidinger M, Buse K, Breunig I
Influence of dry-oxygen-annealing on the residual absorption of lithium niobate crystals in the spectral range from 500 to 2900 nanometers
2016 Opt Mater Express, Band: 6, Seite: 264
KurzfassungRecent highly sensitive absorption measurements of lithium niobate (LiNbO₃) show that within the spectral range between 500 and 2900 nm the transparency is limited by impurities such as iron or hydrogen. In order to reduce the residual absorption, 5-mol.-%-MgO-doped and undoped congruent as well as undoped stoichiometric LiNbO₃ crystals are annealed in dry oxygen atmosphere. The extinction coefficient of the treated crystals is measured using whispering-gallery-resonator-based absorption spectroscopy. The conducted measurements show that the treatment of stoichiometric crystals leads to scattering centers. For the congruent material residual metallic ions like iron and copper dominate the absorption in the spectral region from 400-2000 nm, and oxidization only shifts the center of the absorption in our case from that of iron to that of copper, thus inhibiting reaching the theoretical loss limit. Nevertheless, starting from 2000 nm, where absorption caused by hydrogen dominates, annealing leads to a significant drop in absorption, narrowing here the gap to the theoretical loss limit.
Hentschel M, Metzger B, Knabe B, Buse K, Giessen H
Linear and nonlinear optical properties of hybrid metallic-dielectric plasmonic nanoantennas
2016 Beilstein J Nanotech, Band: 7, Seiten: 111 - 120
KurzfassungWe study the linear and nonlinear optical properties of hybrid metallic–dielectric plasmonic gap nanoantennas. Using a two-step-aligned electron beam lithography process, we demonstrate the ability to selectively and reproducibly fill the gap region of nanoantennas with dielectric nanoparticles made of lithium niobate (LiNbO₃) with high efficiency. The linear optical properties of the antennas are modified due to the large refractive index of the material. This leads to a change in the coupling strength as well as an increase of the effective refractive index of the surrounding. The combination of these two effects causes a red- or blue-shift of the plasmonic modes, respectively. We find that the nonlinear optical properties of the combined system are only modified in the range of one order of magnitude. The observed changes in our experiments in the nonlinear emission can be traced to the changed dielectric environment and thus the modified linear optical properties. The intrinsic nonlinearity of the dielectric used is in fact small when compared to the nonlinearity of the metallic part of the hybrid antennas. Thus, the nonlinear signals generated by the antenna itself are dominant in our experiments. We demonstrate that the well-known nonlinear response of bulk dielectric materials cannot always straightforwardly be used to boost the nonlinear response of nanoscale antenna systems. Our results significantly deepen the understanding of these interesting hybrid systems and offer important guidelines for the design of nanoscale, nonlinear light sources.
Meisenheimer S, Fürst J, Schiller A, Holderied F, Buse K, Breunig I
Pseudo-type-II tuning behavior and mode identification in whispering gallery optical parametric oscillators
2016 Opt Express, Band: 24, Seite: 15137
KurzfassungWavelength tuning of conventional mirror-based optical parametric oscillators (OPOs) exhibits parabolically-shaped tuning curves (type-0 and type-I phase matching) or tuning branches that cross each other with a finite slope (type-II phase matching). We predict and experimentally prove that whispering gallery OPOs based on type-0 phase matching show both tuning behaviors, depending on whether the mode numbers of the generated waves coincide or differ. We investigate the wavelength tuning of optical parametric oscillation in a millimeter-sized radially-poled lithium niobate disk pumped at 1 μm wavelength generating signal and idler waves between 1.7 and 2.6 μm wavelength. Our experimental findings excellently coincide with the theoretical predictions. The investigated whispering gallery optical parametric oscillator combines the employment of the highest nonlinear-optical coefficient of the material with a controlled type-II-like wavelength tuning and with the possibility of self-phase locking.
Leidinger M, Sturman B, Buse K, Breunig I
Strong forward-backward asymmetry of stimulated Raman scattering in lithium-niobate-based whispering gallery resonators
2016 Opt Lett, Band: 41, Seite: 2823
KurzfassungWe show experimentally and prove theoretically that the pump-power thresholds of stimulated Raman scattering (SRS) in lithium-niobate-based whispering gallery resona- tors (WGRs) are strongly different for the signal waves propagating in the backward and forward directions with respect to the pump wave. This feature is due to a strong polaritonic effect. It leads to a cascade of alternating forward-backward Raman lines with increasing pump power. The measured polarization and spectral properties of SRS are in good agreement with theory. Similar properties have to be inherent in other WGRs made of polar crystals.
Fürst J, Sturman B, Buse K, Breunig I
Whispering gallery resonators with broken axial symmetry: Theory and experiment
2016 Optics Express, Band: 24, Seite: 20143
KurzfassungAxial symmetry is the cornerstone for theory and applications of high-Q optical whispering gallery resonators (WGRs). Nevertheless, research on birefringent crystalline material persistently pushes towards breaking this symmetry. We show theoretically and experimentally that the effect of broken axial symmetry, caused by optical anisotropy, is modest for the resonant frequencies and Q-factors of the WGR modes. Thus, the most important equatorial whispering gallery modes can be quantitatively described and experimentally identified. At the same time, the effect of broken axial symmetry on the light field distribution of the whispering gallery modes is typically very strong. This qualitatively modifies the phase-matching for the χ(2) nonlinear processes and enables broad-band second harmonic generation and optical parametric oscillation. The effect of weak geometric ellipticity in nominally symmetric WGRs is also considered. Altogether our findings pave the way for an extensive use of numerous birefringent (uniaxial and biaxial) crystals with broad transparency window and large χ(2) coefficients in nonlinear optics with WGRs

2015

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Sturman B, Breunig I
Acoustic whispering gallery modes within the theory of elasticity
2015 J Appl Phys, Band: 118, Seite: 013102
KurzfassungInvestigations of nonlinear phenomena in optical whispering gallery mode (WGM) microresonators are booming because of rich physics and applications. Stimulated Brillouin scattering is one of the strongest processes in these devices. Here, the optical WGMs interact with acoustic counterparts. The acoustic WGMs are well known for resonators based on liquids and gases, where the sound waves are longitudinal. The situation with solid-state resonators is different because of the presence of the longitudinal (l) and transverse (t) sound waves with substantially different velocities v_{l,t}. Moreover, the l- and t-parts of the acoustic displacement are coupled at the resonator surface breaking the separation of modes into longitudinal and transverse. Investigation of the acoustic WGMs is of high priority. Here, analytically and numerically we investigate the resonant frequencies and the eigenfunctions (displacement vector distributions) for acoustic WGMs in microresonators made of isotropic solid-state materials. Cylindrical and spherical resonators are considered. Each mode has the azimuth, radial, and orbital (for sphere) numbers m, q, and l; its properties are controlled also by the ratio v_l/v_t. All modes are either transverse (t) or hybrid transverse-longitudinal (tl). Pure l-modes, providing the strongest interaction with optical modes in fibers and bulk crystals, are absent. The tl-modes include distorted Rayleigh waves, the modes with q ~ 1 and dominating t-part, and pseudo-longitudinal modes with q >> 1, closely spaced frequencies, and weakly localized t -part. They have no analogies to the optical WGMs and are of high relevance for Brillouin lasing in optical microresonators. The actual values of l and m are 10^2...10^5, and the lasing thresholds lie in the µW range. Our findings include exact dispersion equations for acoustic WGMs, which can be solved numerically for l, m < 10^4, asymptotic tools for l, m > 10^3, and particular illustrations.
Sturman B, Breunig I
Brillouin lasing in whispering gallery micro-resonators
2015 New J Phys, Band: 17, Seite: 125006
KurzfassungThresholds of stimulated Brillouin scattering (SBS) in solid-statewhispering gallery mode (WGM) microresonators are analyzed. It is shown that the SBS interaction is substantially different here from that known in the bulk case and in the case of water droplet resonators.The reason is the absence of pure longitudinal acoustic WGMs owing to strong coupling of the longitudinal (l) and transverse (t) acoustic displacements at the surface of the resonator. As a result, a considerable increase of the SBS thresholds takes place, and the lowest thresholds correspond to the hybrid tl-modes with very large radial indices. Nevertheless, the thresholds lie in the μW range of the pump power.Dependence of the SBS power thresholds on themodal numbers and the possibility of self-tuning to the SBS resonance are analyzed.
Meisenheimer S, Fürst J, Werner C, Beckmann T, Buse K, Breunig I
Broadband infrared spectroscopy using optical parametric oscillation in a radially-poled whispering gallery resonator
2015 Opt Express, Band: 23, Seite: 24042
KurzfassungWe demonstrate optical parametric oscillation in a millimeter-sized whispering gallery resonator suitable for broadband infrared spectroscopy. This nonlinear-optical process is quasi-phase-matched using a radial domain pattern with 30 µm period length, inscribed by calligraphic poling. The output wavelengths are selected in a controlled way over hundreds of nanometers. We achieve this by increasing the temperature of the resonator in steps such that the azimuthal mode number of the pump wave rises by one. As a proof-of-principle experiment, we measure a characteristic resonance of polystyrene in the spectral range of 2.25 – 2.45 µm.
Leidinger M, Fieberg S, Waasem N, Kühnemann F, Buse K, Breunig I
Comparative study on three highly sensitive absorption measurement techniques characterizing lithium niobate over its entire transparent spectral range
2015 Opt Express, Band: 23, Seite: 21690
KurzfassungWe employ three highly sensitive spectrometers: a photoacoustic spectrometer, a photothermal common-path interferometer and a whispering-gallery-resonator-based absorption spectrometer, for a comparative study of measuring the absorption coefficient of nominally transparent undoped, congruently grown lithium niobate for ordinarily and extraordinarily polarized light in the wavelength range from 390 to 3800 nm. The absorption coefficient ranges from below 10^(−4)/cm up to 2/cm. Furthermore, we measure the absorption at the Urbach tail as well as the multiphonon edge of the material by a standard grating spectrometer and a Fourier-transform infrared spectrometer, providing for the first time an absorption spectrum of the whole transparency window of lithium niobate. The absorption coefficients obtained by the three highly sensitive and independent methods show good agreement.
Werner C, Buse K, Breunig I
Continuous-wave whispering-gallery optical parametric oscillator for high-resolution spectroscopy
2015 Opt Lett, Band: 40, Seite: 772
KurzfassungWe achieve a continuous operation of a whispering gallery optical parametric oscillator by stabilizing the resonator temperature T on the mK level and simultaneously locking the pump frequency to a cavity resonance using the Pound-Drever–Hall technique. The millimeter-sized device converts several mW of a pump wave at 1040 nm wavelength to signal and idler waves around 2000 nm wavelength with more than 50 % efficiency. Over 1 h, power and frequency of the signal wave vary by < 1 % and by < 25 MHz, respectively. The latter can be tuned over 480 MHz without a mode hop by changing T over 120 mK. In order to prove the suitability for high-resolution spectroscopy, we scan the signal frequency across the resonance of a Fabry–Perot interferometer resolving nicely its 10 MHz linewidth.
Fürst J, Buse K, Breunig I, Becker P, Liebertz J, Bohaty L
Second-harmonic generation of light at 245 nm in a lithium tetraborate whispering gallery resonator
2015 Opt Lett, Band: 40, Seite: 1932
KurzfassungA millimeter-sized, monolithic whispering gallery resonator made of a lithium tetraborate, Li2B4O7, crystal was employed for doubly resonant second-harmonic generation with a continuous-wave laser source at 490 nm. An intrinsic quality factor of 2×10^8 was observed at the pump wavelength. A conversion efficiency of 2.2 % was attained with 5.9 mW of mode-matched pump power. In the lithium tetraborate resonator, it is feasible to achieve phase-matching of second-harmonic generation for pump wavelengths between 486 and 506 nm.

2014

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Metzger B, Hentschel M, Schumacher T, Lippitz M, Ye X, Murray C B, Knabe B, Buse K, Giessen H
Doubling the efficiency of third harmonic generation by positioning ITO nanocrystals into the hot-spot of plasmonic gap-antennas
2014 Nano Lett, Band: 14, Seite: 2867
KurzfassungWe incorporate dielectric indium tin oxide nanocrystals into the hot-spot of gold nanogap-antennas and perform third harmonic spectroscopy on these hybrid nanostructure arrays. The combined system shows a 2-fold increase of the radiated third harmonic intensity when compared to bare gold antennas. In order to identify the origin of the enhanced nonlinear response we perform finite element simulations of the nanostructures, which are in excellent agreement with our measurements. We find that the third harmonic signal enhancement is mainly related to changes in the linear optical properties of the plasmonic antenna resonances when the ITO nanocrystals are incorporated. Furthermore, the dominant source of the third harmonic is found to be located in the gold volume of the plasmonic antennas.
Fieberg S, Sturman B, Kühnemann F, Buse K
Strong polarization effects in photothermal common-path interferometry
2014 Opt Lett, Band: 39, Seite: 3880
KurzfassungWe show experimentally and theoretically that heating of highly transparent materials by continuous-wave pump beams induces strong optical anisotropy for probe beams in primarily optically isotropic configurations. This is due to a uniaxial thermally induced strain and the elasto-optic effect. The cases of glasses and lithium niobate crystals are considered. The discovered effect is important for the development of techniques aimed at the analysis of absorptive properties of transparent optical materials.

2013

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Sowade R, Kießling J, Breunig I
Cw laser light tunable from blue to red: OPOs pave the way
2013 Photonik international, Band: 1, Seite: 63
KurzfassungOptical parametric oscillators (OPOs) are widely tunable and thus turn out as very flexible light sources. Until recently, OPOs for the visible frequency range were commercially available as pulsed systems only. In this article, two concepts are presented, which pave the way into the visible spectrum for continuous-wave OPOs.
Sowade R, Kießling J, Breunig I
Durchstimmbares cw-Laserlicht von Blau bis Rot : OPOs machen es jetzt möglich
2013 Photonik, Band: 2, Seite: 42
KurzfassungOptisch-parametrische Oszillatoren (OPOs) sind weit abstimm- bare und dadurch variabel einsetzbare Lichtquellen. Diese sind für den sichtbaren Spektralbereich bislang allerdings nur im gepulsten Betrieb kommerziell erhältlich. In diesem Artikel stellen wir zwei Konzepte vor, mit deren Hilfe kontinuierlich emittierende OPO-Systeme das sichtbare Spektrum erobern.
Waasem N, Markosyan A, Fejer M M, Buse K
Green-induced blue absorption in MgO-doped lithium niobate crystals
2013 Opt Lett, Band: 38, Seite: 2953
KurzfassungWe have measured the intrinsic and 532 nm-induced optical absorption of 5 mol. % MgO-doped lithium niobate crystals at 488 nm wavelength. The measurements have been conducted employing a photothermal common-path interferometer. The absorption at 488 nm increases on simultaneous illumination with 532 nm light. This induced absorption rises linearly with the stimulating 532 nm radiation power and saturates for intensities larger than 50 kW/cm2. A model developed recently, considering the excitation of electrons from levels near the valence band into iron centers, is applied to explain the observations.
Kießling J, Breunig I, Schunemann P G, Buse K, Vodopyanov K L
High power and spectral purity continuous-wave photonic THz source tunable from 1 to 4.5 THz for nonlinear molecular spectroscopy
2013 New J Phys, Band: 15, Seite: 105014
KurzfassungWe report a diffraction-limited photonic terahertz (THz) source with linewidth < 10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near \lambda = 2 µm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2μm was attained with GaAs inside cavity.
Sowade R, Kießling J, Breunig I
One light source - all colors
2013 Optik & Photonik, Band: 8, Seite: 46
KurzfassungA single source emitting single-fre- quency light with tunable color and without the need of changing toxic chemicals, as it is the case for dye la- sers, is desired by many scientists. The Department of Microsystems Engi- neering IMTEK at the University of Freiburg together with the Fraunhofer Institute for Physical Measurement Techniques IPM now extend the tun- ing range of continuous-wave optical parametric oscillators into the visible spectrum by integrated, intra-cavity frequency doubling. Combination of these two nonlinear-optical effects enables the operator to deliberately choose any wavelength between 430 and 690 nm.
Waasem N, Fieberg S, Hauser J, Gomes G, Haertle D, Kühnemann F, Buse K
Photoacoustic absorption spectrometer for highly transparent dielectrics with parts-per-million sensitivity
2013 Rev Sci Instrum, Band: 84, Seite: 023109
KurzfassungA sensitive photoacoustic absorption spectrometer for highly transparent solids has been built and tested. As the light source an optical parametrical oscillator pumped by a nanosecond pulse laser with 10 Hz repetition rate is employed, covering the complete wavelength range from 407 to 2600 nm. A second-harmonic-generation unit extends the range of accessible wavelengths down to 212 nm. A lead-zirconate-titanate piezo transducer, directly coupled to the sample, detects the photoacoustically generated sound waves. Absorption spectra of lithium triborate, lithium niobate, and alpha barium borate crystals with absorption coefficients down to 10−5 cm−1 are presented.
Breunig I, Sturman B, Bückle A, Werner C, Buse K
Structure of pump resonances during optical parametric oscillation in whispering gallery resonators
2013 Opt Lett, Band: 38, Seite: 3316
KurzfassungIn optical parametric oscillators, the line shape of the pump resonance becomes strongly distorted above the oscillation threshold. We model this behavior and find good agreement with the literature data and our original experimental data. A fit of the model to the data provides valuable information about the loss mechanisms in the parametric process. In particular, the modal properties of the parametric waves can be gained, which is important for both classical and quantum aspects of optical parametric oscillation.
Kießling J, Buse K, Breunig I
Temperature-dependent Sellmeier equation for the extraordinary refractive index of 5 mol. % MgO-doped LiNbO3 in the terahertz range
2013 J Opt Soc Am B, Band: 30, Seite: 950
KurzfassungWe present a Sellmeier equation for the temperature- and wavelength-dependent extraordinary refractive index of 5 mol. % MgO-doped congruent lithium niobate, which is valid for frequencies from 1 to 3 THz in a temperature range from 30°C to 200°C. The determination of the Sellmeier coefficients is based on the tuning behavior of a quasi-phase-matched, continuous-wave optical parametric oscillator with its idler wave in the terahertz regime.
Breunig I, Sturman B, Sedlmeir F, Schwefel H G L, Buse K
Whispering gallery modes at the rim of an axisymmetric optical resonator: Analytical versus numerical description and comparison with experiment
2013 Opt Express, Band: 21, Seite: 30683
KurzfassungOptical whispering gallery modes (WGMs) of mm-sized axisymmetric resonators are well localized at the equator. Employing this distinctive feature, we obtain simple analytical relations for the frequencies and eigenfunctions of WGMs which include the major radius of the resonator and the curvature radius of the rim. Being compared with results of finite-element simulations, these relations show a high accuracy and practicability. High-precision free-spectral-range measurements with a millimeter-sized disc resonator made of MgF2 allow us to identify the WGMs and confirm the applicability of our analytical description.

2012

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Werner C, Beckmann T, Buse K, Breunig I
Blue-pumped whispering gallery optical parametric oscillator
2012 Opt Lett, Band: 37, Seite: 4224
KurzfassungWe demonstrate a whispering gallery optical parametric oscillator pumped at 488 nm wavelength. This millimetersized device has a pump threshold of 160 μW. The signal field is tunable between 707 and 865 nm wavelength and the idler field between 1120 and 1575 nm through temperature variation. Although the conversion efficiency is fundamentally limited to several percent because of absorption loss for the pump wave, the results provide evidence that such oscillators will be able to cover finally the entire visible range.
Luedtke F, Buse K, Sturman B
Hidden Reservoir of Photoactive Electrons in LiNbO3 Crystals
2012 Phys Rev Lett, Band: 109, Seite: 026603
KurzfassungWe show that a continuous-wave (cw) pump beam at a wavelength of 532 nm produces substantial light-induced (LI) absorption in the visible range in initially transparent undoped LiNbO3 crystals. The LI absorption coefficient stays linear in the pump intensity Ip up to Ipmax⁡=48 kW/cm2. Together with other features including long-term stretched-exponential relaxation of the LI absorption, it indicates that the present concept of LI electron processes in this important optical material must be revised: the amount of photoactive electrons increases already within the cw intensity range. A quantitative model is proposed that explains the experimental data and employs two-step excitations from filled localized states near the valence band via intermediate deep centers into the conduction band. The introduced localized states serve as a hidden reservoir of electrons.
Beckmann T, Buse K, Breunig I
Optimizing pump threshold and conversion efficiency of whispering gallery optical parametric oscillators by controlled coupling
2012 Opt Lett, Band: 37, Seite: 5250
KurzfassungThreshold and efficiency of optical parametric oscillation in whispering gallery resonators have been predicted to depend strongly on the coupling strength. Our experiments, varying the coupling strength continuously over 3 orders of magnitude, confirm these predictions. The pump threshold changes by a factor of 20, allowing the adaptation of the system to a wide range of input powers. The minimum threshold of 0.20mWoccurs for significant pump undercoupling. The efficiency increases monotonically with coupling, spanning 4 orders of magnitude with a maximum efficiency of 30% in contact yielding output powers exceeding 1 mW.
Sturman B, Beckmann T, Breunig I
Quasi-resonant and quasi-phase-matched nonlinear second-order phenomena in whispering-gallery resonators
2012 J Opt Soc Am B, Band: 29, Seite: 3087
KurzfassungRecently achieved radial poling of whispering-gallery resonators (WGRs) strongly extends the capabilities of tailoring the second-order nonlinear phenomena, such as second-harmonic generation and optical parametric oscillation, and transferring them to the range of low-power continuous-wave light sources. Owing to discreteness of the frequency spectrum, the resonance and phase-matching conditions for interacting waves cannot be fulfilled simultaneously in WGRs in the general case. Using Yariv’s generic approach to the description of WGR phenomena, we analyze two closely related issues: the possibilities to achieve the resonant and phase-matching conditions using the temperature tuning and the impact of detunings and phase mismatches on the nonlinear transformation efficiencies. It is shown that the radial poling provides important necessary conditions for the subsequent fine tuning to the nonlinear resonances. The requirements to the temperature tuning, as exemplified by the case of lithium niobate, are substantially dependent on the nonlinear process in question, the actual wavelength range, and the pump intensity.
Knabe B, Buse K, Assenmacher W, Mader W
Spontaneous polarization in ultrasmall lithium niobate nanocrystals revealed by second harmonic generation
2012 Phys Rev B, Band: 86, Seite: 195428
KurzfassungNanocrystals of lithium niobate (LiNbO3) with diameters of between 5 and 50 nm were produced by a combination of sol-gel synthesis and wet-chemical etching. The phase purity and nearly spherical shape of the crystals were verified by transmission electron microscopy. Power and polarization dependences of the second harmonic signal of single nanocrystals reveal that the nanocrystals have the noncentrosymmetric point group 3m and that the second-order nonlinear optical coefficients known from the bulk material are fully present in all nanocrystals studied. The volume of the crystals is still the main source of frequency conversion and they are spontaneously polarized.

2011

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Kießling J, Sowade R, Camara Mayorga I, Buse K, Breunig I
Coherent detection of terahertz radiation employing a continuous wave optical parametric source
2011 Rev Sci Instrum, Band: 82, Seite: 026108
KurzfassungThe combination of an all-optical terahertz source with a photoconductive antenna to achieve coherent detection is presented. This approach aims to overcome the frequency limits introduced by optoelectronic terahertz sources commonly used. Here the Gaussian-shaped and linearly polarized terahertz waves are generated by a continuous wave optical parametric oscillator with a power of 3 μW at 1.4 THz. The infrared signal light of the optical parametric oscillator can be used to coherently detect the generated terahertz wave with a photoconductive antenna. As a proof-of-principle experiment we determine the thickness profile of a plastic lens using phase shifting interferometry.
Steigerwald H, Ying Y J, Eason R E, Buse K, Mailis S, Soergel E, Buse K
Direct writing of ferroelectric domains on the x- and y-faces of lithium niobate using a continuous wave ultraviolet laser
2011 Appl Phys Lett, Band: 98, Seite: 062902
KurzfassungFerroelectric domain reversal has been achieved by scanning a tightly focused, strongly absorbed ultraviolet laser beam across the x- and y-faces of lithium niobate crystals. The domains were investigated by piezoresponse force microscopy. The emergence and width of any domain was found to depend on the scanning direction of the irradiating laser beam with respect to the polar z-axis. Full width and half width domains or no domain formation at all could be achieved for scanning along specific directions. We interpret the results by a direct correlation between the local temperature gradient and the resulting polarization direction.
Sturman B, Breunig I
Generic description of second-order nonlinear phenomena in whispering-gallery resonators
2011 J Opt Soc Am B, Band: 28, Seite: 2465
KurzfassungExtending Yariv’s generic approach to the description of optical microresonators, we describe the secondharmonic generation and the optical parametric oscillation in whispering-gallery resonators (WGRs). The output characteristics of these nonlinear processes are expressed in terms of conventional cavity/coupling parameters and nonlinear material coefficients. The found relations are relevant to the description and optimization of experiments with nonlinearly active WGRs at ultralow input light powers.
Beckmann T, Linnenbank H, Steigerwald H, Sturman B, Haertle D, Buse K, Breunig I
Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators
2011 Phys Rev Lett, Band: 106, Seite: 143903
KurzfassungWhispering-gallery resonators (WGR’s), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04 µm pump wave, the signal and idler waves are tuned from 1.78 to 2.5 µm - including the point of degeneracy - by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.
Kießling J, Fuchs F, Buse K, Breunig I
Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation
2011 Opt Lett, Band: 36, Seite: 4374
KurzfassungWe demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.
Schwesyg J R, Falk M, Phillips C R, Jundt D H, Buse K, Fejer M M
Pyroelectrically induced photorefractive damage in magnesium-doped lithium niobate crystals
2011 J Opt Soc Am B, Band: 28, Seite: 1973
KurzfassungBeam distortion due to photorefraction limits the usability of lithium niobate (LiNbO3) crystals for frequency conversion applications. To prevent beam distortion in LiNbO3, 5 mol% magnesium-doped LiNbO3 (MgO:LN) is usually used. However, we show that strong beam distortion of green laser light can occur within seconds in MgO:LN, starting at light intensity levels in the 100 mW/cm2 regime, if the crystal is heated by several degrees Celsius during or before illumination. Beam distortion does not occur in undoped congruent LiNbO3 (CLN) under the same conditions. We show that the pyroelectric effect together with an elevated photoconductivity compared to that of CLN causes this beam distortion and that this effect also influences frequency conversion experiments in the infrared even if no external heating is applied.
Knabe B, Schütze D, Jungk T, Svete M, Assenmacher W, Mader W, Buse K
Synthesis and characterization of Fe-doped LiNbO3 nanocrystals from a triple-alkoxide method
2011 Phys Status Solidi A, Band: 208, Seite: 857
KurzfassungNanocrystals of Fe/LiNbO3 were synthesized from Li(OC2H5), Nb(OC2H5)5, and Fe(OC2H5)3 using a triple alkoxide sol–gel method with Fe-dopings of 0.5 and 1mol%, respectively. X-ray diffraction proves the samples to be phase-pure and crystalline. The size of the crystals as determined by Scherrer equation and transmission electron microscopy is below 50 nm. Iron is distributed homogeneously within the crystals as shown with energy-dispersive X-ray spectroscopy. Ferroelectricity is verified by piezoresponse force microscopy. Vacuum annealing enables to control the Fe2+/Fe3+-concentration ratio from 0.1 to 0.9 with a reduction onset temperature of 200 °C.

2010

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Kalkum F, Peter M, Barbastathis G, Buse K
External-resonance-enhanced transmission of light through sub-wavelength holes
2010 Appl Phys B-lasers O, Band: 100, Seite: 169
KurzfassungUsing plasmonic resonances of metal films, enhanced transmission of light through sub-wavelength holes has been demonstrated. Here we show that external resonances can be employed as well: the transmission of 1.5-μm wavelength light through 600-nm holes is enhanced by a factor of 20 using a Fabry–Pérot arrangement. The maximal enhancement factor is determined by the limited reflectivity of metal surfaces. It seems promising to combine both effects—plasmonic resonances plus tailored photoniccrystal structures on top of the metal film—in order to realize efficient sub-wavelength light sources as they are required for, e.g., advanced spectroscopy and lithography.
Steigerwald H, von Cube F, Luedtke F, Dierolf V, Buse K
Influence of heat and UV light on the coercive field of lithium niobate crystals
2010 Appl Phys B-lasers O, Band: 101, Seite: 535
KurzfassungHeating and UV illumination reduce the coercive field of lithium niobate crystals. It is found that both effects add up, leading to the conclusion that the underlying mechanisms are different. We present arguments that heating leads to an increased ionic conductivity that changes defect structures which pin domain walls, while UV illumination enables electrical screening of the pinning defects by photoexcited electrons.
Schwesyg J R, Kajiyama M C C, Falk M, Jundt D H, Buse K, Fejer M M
Light absorption in undoped congruent and magnesium-doped lithium niobate crystals in the visible wavelength range
2010 Appl Phys B-lasers O, Band: 100, Seite: 109
KurzfassungLight absorption measurements of nominally undoped congruent lithium niobate crystals (CLN) as well as 5 mol%magnesium-oxide-doped lithium niobate (MgO:LN) crystals were performed in the light wavelength range of 350 to 800 nm. Absorption spectra reveal that—besides iron (Fe) impurities—chromium (Cr) impurities of less than 0.5 wt. ppm concentration contribute significantly to the total optical absorption in the CLN crystals with a maximum of 0.035 cm−1 around 500 nm. The axial distribution of Cr within a CLN boule is examined, revealing that the bottom part of the boule contains less Cr and therefore light absorption is reduced as well. In the case of the MgO:LN crystals, Cr impurities also contribute significantly to the total optical absorption, which is on the order of 0.025 cm−1 for ordinarily polarized light and 0.015 cm−1 for extraordinarily polarized light around 500 nm.
Sowade R, Breunig I, Tulea C, Buse K
Nonlinear coefficient and temperature dependence of the refractive index of lithium niobate crystals in the terahertz regime
2010 Appl Phys B-lasers O, Band: 99, Seite: 63
KurzfassungThe effective nonlinear coefficient deff of lithium niobate is determined to be 94 pm/V for a process that converts infrared light to 1.35 THz radiation. This value is inferred from the performance of a continuous-wave, singlyresonant optical parametric oscillator, in which the cavityenhanced signal wave of a primary parametric process acts as a pump wave for a cascaded process, generating terahertz waves. To quantify the nonlinear coefficient, the coupled wave equations including absorption are evaluated. Furthermore, from our measurements we also determine the temperature dependence of the refractive index to be dnTHz/dT = 0.0013/K around 1.4 THz.
Schütze D, Knabe B, Ackermann M, Buse K
Orientation of colloidally suspended LiNbO3 nanocrystals in externally applied electric fields
2010 Appl Phys Lett, Band: 97, Seite: 242908
KurzfassungElectric fields are applied to lithium niobate nanoparticles of 41 nm radius colloidally suspended in heptane. Interferometric measurements of field-induced refractive index changes show that the particle orientation is dominated by induced dipole moments. There are indications that a permanent dipole moment of the nanoparticles of 10−26 Cm along their crystallographic c-axis is present as well. An electric field of 2 kV/mm is sufficient to align 80% of the particles, such that the angle between their c-axis and the applied electric field direction is less than 70°, making applications of such a liquid with electrically controlled optical properties feasible.
Steigerwald H, Lilienblum M, von Cube F, Ying Y J, Eason R W, Mailis S, Sturman B, Soergel E, Buse K
Origin of UV-induced poling inhibition in lithium niobate crystals
2010 Phys Rev B, Band: 82, Seite: 214105
KurzfassungShort-term exposure of the +z face of LiNbO3 crystals to focused UV laser light leads to persistent inhibition of ferroelectric domain reversal at the irradiated area, a phenomenon referred to as “poling inhibition.” Different types of crystals (stoichiometric, congruent, or Mg-doped ones) are exposed, creating the so-called “latent state” and domain growth during subsequent electric-field poling is visualized. The latent state is robust against thermal annealing up to 250 °C and uniform illumination. With the tip of a scanning force microscope the coercive field is mapped, showing not only the expected resistance against domain reversal in the UVirradiated region but also easier poling adjacent to the UV-irradiated section. These results and theoretical estimates point to the following mechanism of poling inhibition: the UV light-induced heating results in a local reduction of the lithium concentration, via thermodiffusion. The required charge compensation is provided by UV-excited free electrons/holes. After cooling, the lithium ions become immobile, and the reduced lithium concentration causes a strong local increase in the coercive field in the exposed area, while the increased Li concentration next to this area reduces the coercive field.
Schwesyg J R, Phillips C R, Ioakeimidi K, Kajiyama M C C, Falk M, Jundt D H, Buse K, Fejer M M
Suppression of mid-infrared light absorption in undoped congruent lithium niobate crytals
2010 Opt Lett, Band: 35, Seite: 1070
KurzfassungAbsorption measurements on 5 mol. % MgO-doped and undoped congruent lithium niobate (LiNbO3) crystals revealed absorption bands in the wavelength range of 2500–2800 nm, caused by incorporated hydrogen ions. High-temperature annealing was applied to the congruent LiNbO3 (CLN) crystals, which decreased the absorption significantly. Then the annealed CLN crystals were periodically poled. As an application of the low-loss annealed CLN crystal, the operation of a 1550 nm pumped singly resonant CW optical parametric oscillator, resonant around 2600 nm, using a periodically poled crystal was demonstrated.

2009

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Kießling J, Sowade R, Breunig I, Buse K, Dierolf V
Cascaded optical parametric oscillations generating tunable terahertz waves in periodically poled lithium niobate crystals
2009 Opt Express, Band: 17, Seite: 87
KurzfassungWe present a continuous-wave (cw) singly-resonant optical parametric oscillator (SROPO) based on MgO-doped periodically poled lithium niobate (PPLN) delivering single-frequency idler output from 2.33 to 5.32 μm. In this system, we observe additional spectral components that have been attributed to stimulated Raman lines in other studies. However, we are able to assign them unambiguously to cascaded optical parametric processes. The tunable forward and backward idler waves generated by these additional phase-matched oscillations have frequencies that are tunable around 3.5 and 1.5 THz, respectively.
Kösters M, Becher C, Haertle D, Sturman B, Buse K
Charge transport properties of undoped congruent lithium niobate crystals
2009 Appl Phys B-lasers O, Band: 97, Seite: 811
KurzfassungTime-resolved light-induced birefringence measurements based on a phase compensation technique are used to determine the charge transport properties of congruent, nominally-undoped lithium niobate crystals (LiNbO3). Some of the crystals are conventionally oxidized. The steady-state space-charge field Epv, the bulk photovoltaic coefficient β, and the photoconductivity σph are determined in the intensity range (30–30 000)W/cm2. The photovoltaic coefficient β increases by one order of magnitude over this intensity range, the space-charge field Epv even by two orders of magnitude. We discuss the results in the context of the known one- and two-center charge transport models for LiNbO3. The experimental findings presented here are of relevance for the long-standing problem of optical damage in such crystals, which inhibits their use in high-intensity applications like nonlinear optics.
Sowade R, Breunig I, Camara Mayorga I, Kießling J, Tulea C, Dierolf V, Buse K
Continuous-wave optical parametric terahertz source
2009 Opt Express, Band: 17, Seite: 22303
KurzfassungHere, we present a continuous-wave optical parametric terahertz light source that does not require cooling. It coherently emits a diffraction-limited terahertz beam that is tunable from 1.3 to 1.7 THz with power levels exceeding 1 µW. Simultaneous phase matching of two nonlinear processes within one periodically-poled lithium niobate crystal, situated in an optical resonator, is employed: The signal wave of a primary parametric process is enhanced in this resonator. Therefore, its power is sufficient for starting a second process, generating a backwards traveling terahertz wave. Such a scheme of cascaded processes increases the output power of a terahertz system by more than one order of magnitude compared with non-resonant difference frequency generation due to high intracavity powers. The existence of linearly polarized terahertz radiation at 1.35 THz is confirmed by analyzing the terahertz light with metal grid polarizers and a Fabry-Perot interferometer.
Luedtke F, Villarroel J, García-Cabañes A, Buse K, Carrascosa M
Correlation beween photorefractive index changes and optical damage thresholds in z-cut proton-exchanged-LiNbO3 waveguides
2009 Opt Express, Band: 17, Seite: 658
KurzfassungAn interferometric Mach-Zehnder technique very recently developed has been applied to measure photorefractive index changes in different types of z-cut proton-exchanged planar waveguides in LiNbO3. These measurements are complemented by determining the intensity threshold for the onset of optical damage with a standard single-beam setup. In the intensity region just below the threshold-intensity obtained in the single-beam experiment the refractive index change is found to saturate at values around 1×10−4 . Furthermore, we measure the dark conductivities of proton-exchanged waveguides by monitoring the decay of the light-induced index changes. Via the time constant of the decay we obtain dark conductivities of the order of about 5×10−16 Ω−1 cm−1 , that are negligible compared with the photoconductivity within the light intensity range used. The results of the measurements compare well with the predictions of a recent work, that uses a two-center model to explain the optical damage.
Ganguly P, Sones C L, Ying Y J, Steigerwald H, Buse K, Soergel E, Eason R W, Mailis S
Determination of refractive indices from the mode profiles of UV-written channel waveguides in LiNbO3 crystals for optimization of writing conditions
2009 J Lightwave Technol, Band: 27, Seite: 3490
KurzfassungWe report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in LiNbO3 for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of 0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.
Kalkum F, Peithmann K, Buse K
Dynamics of holographic recording with focused beams in iron-doped lithium niobate crystals
2009 Opt Express, Band: 17, Seite: 1321
KurzfassungHolograms are recorded with focused beams in an iron-doped lithium niobate crystal. The diffraction efficiency shows a maximum after several seconds of recording, unlike in the case of writing with two homogeneous plane waves in the same crystal. This behavior can be attributed to a compensation field caused by incomplete illumination of the crystal. The field finally stops the bulk photovoltaic effect, which is the main driving force of the process. Based on this assumptions, we derive an analytical expression for the evolution of the diffraction efficiency which correctly fits the experimental data.
Sliwinska E, Mansurova S, Hartwig U, Buse K, Meerholz K
Effect of co-sensitization in new hybrid photorefractive materials based on PVK polymer matrix and inorganic LiNbO3 nano-crystals
2009 Appl Phys B-lasers O, Band: 95, Seite: 519
KurzfassungA new class of photo-refractive (PR) composite materials based on poly(N-vinylcarbazole) (PVK) (co-) sensitized with nano-crystalline lithium niobate (LiNbO3) is presented. The steady-state performance, as well as the kinetics of the photo-refractive grating formation, was investigated by degenerate four-wave mixing (DFWM) and twobeam- coupling (TBC) experiments. We found an optimum content of LiNbO3 nano-particles of only 10−3 wt.%. Even at concentrations as low as 10−7 wt.% a notable effect of the nano-crystals was detected. This yields materials with improved PR performance compared to that of the standard PR polymer material sensitized by TNF only. The role of LiNbO3 nano-particles is discussed in detail: The particles support generation and transport of the free-charge carriers. Furthermore, they increase the electron trap density.
Mansurova S, Meerholz K, Sliwinska E, Hartwig U, Buse K
Enhancement of the charge carrier transport by doping PVK-based photoconductive polymers with LiNbO3 nanocrystals
2009 Phys Rev B, Band: 79, Seite: 174208
KurzfassungThe photoconductive properties of a hybrid material based on poly(N-vinylcarbazole) (PVK) doped with nanoparticles of lithium niobate (LiNbO3) have been studied. The experiments were carried out using modulated photocurrent and non-steady-state photo-emf techniques at 633 nm wavelength. The photoconductivity of the composite increases remarkably with the nanoparticle content. This growth is related to larger drift lengths of the photoholes at higher nanoparticle concentrations. The reason for this enhancement is an increase in the effective photohole lifetime due to trapping of electrons on the nanoparticles and creation of recombination barriers at the polymer/nanoparticle interface.
Schwesyg J R, Beckmann T, Zimmermann A S, Buse K, Haertle D
Fabrication and characterization of whispering-gallery-mode resonators made of polymers
2009 Opt Express, Band: 17, Seite: 2573
KurzfassungHigh-quality whispering-gallery-mode resonators made of polymethylmethacrylate (PMMA) are fabricated by simple mechanical turning and polishing according to a technique used by Ilchenko et al. to produce crystalline whispering-gallery-mode resonators with high quality factors (Q-factors). The high-Q PMMA resonators are investigated in two wavelength regimes: in the near infrared between the wavelengths 1470 and 1580 nm and at the wavelength 635 nm. The Q-factor in the infrared regime is limited by material absorption to 3×105. At 635 nm the Q-factor is limited by surface scattering only and reaches 4×107, which is a new record for polymers.
Kroeger F, Breunig I, Buse K
Frequency stabilization and output power undulations of diode lasers with feedback by volume holographic gratings
2009 Appl Phys B-lasers O, Band: 95, Seite: 603
KurzfassungThe mode-hop behavior and the power characteristics of a laser diode with wavelength-selective optical feedback are experimentally investigated. The feedback is provided by external volume holographic gratings, also called ‘Bragg mirrors’, at normal incidence. We demonstrate that a Bragg mirror forces a laser diode to operate only within a narrow wavelength range, and that the emission wavelength of the laser diode is stabilized against variations of the injection current. Moreover, we present periodic undulations of the power characteristics of the laser, depending on the driving current. They can be qualitatively explained with a simple model which takes into account that the threshold gain in the laser system strongly depends on the wavelength.
Kalkum F, Broch S, Brands T, Buse K
Holographic phase conjugation through a sub-wavelength hole
2009 Appl Phys B-lasers O, Band: 95, Seite: 637
KurzfassungHolographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3 × 10−5 compared to 3 × 10−2 in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the abovementioned issues.
Sowade R, Breunig I, Kießling J, Buse K
Influence of the pump threshold on the single-frequency output power of singly-resonant optical parametric oscillators
2009 Appl Phys B-lasers O, Band: 96, Seite: 25
KurzfassungWe demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, 1969). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 μm and 7 W at 1.5 μm could be generated simultaneously.
Maxein D, Buse K
Interaction of femtosecond laser pulses with lithium niobate crystals: transmission changes and refractive index modulations
2009 J Holography Speckle, Band: 5, Seite: 275
KurzfassungRecent insights about the interaction of femtosecond laser pulses with lithium niobate crystals are reviewed. These ultrashort pulses cause transmission changes because of two-photon absorption, light-induced population of photoexcitable states, and because of holographic scattering. Index changes are induced by the Kerr nonlinearity and the photorefractive effect.
Kösters M, Sturmann B, Haertle D, Buse K
Kinetics of photorefractive recording for circular light beams
2009 Opt Lett, Band: 34, Seite: 1036
KurzfassungWe show, theoretically and experimentally, that the buildup of the space-charge field in photorefractive crystals is far from monoexponential for circular light beams. This is a general property of the two-dimensional (2D) case, in contrast to the one-dimensional case. The results form a basis for determination of the photoelectric parameters of photorefractive crystals within a wide intensity range, which is important, e.g., for solving of the optical-damage problem in LiNbO3 and LiTaO3 crystals.
Bueckers J, Maxein D, Haertle D, Buse K
Light-induced scattering of femtosecond laser pulses in iron-doped lithium niobate crystals
2009 J Opt Soc Am B, Band: 26, Seite: 1018
KurzfassungSelf-amplification of weak scattered coherent light waves in photorefractive crystals leads to losses, known as light-induced scattering or holographic scattering. We find with 532 nm light that it is reduced in LiNbO3:Fe for femtosecond laser pulses as compared to cw laser light. Light-induced scattering of pulses is completely absent in samples with sufficiently small Fe2+ content, in contrast to the scattering of cw light. Additional differences include a slower buildup time, a weaker Bragg selectivity, and a narrower angular distribution of the scattered light for pulsed illumination. The differences can be attributed mainly to the smaller temporal coherence of pulses.
Luedtke F, Villarroel J, García-Cabañes A, Carrascosa M, Steigerwald H, Buse K
Mach-Zehnder method for optical damage characterization of planar waveguides
2009 Ferroelectrics, Band: 390, Seite: 41
KurzfassungA Mach-Zehnder interferometric technique has been implemented for measuring photorefractive index changes (“optical damage”) in planar waveguides in electrooptic materials, that can be useful for nonlinear-optical applications. First used with channel waveguides, it is here applied to proton-exchanged lithium niobate planar guides. The data is consistent with that obtained from similar measurements. Index changes of about 10−4 that build up within some seconds are detected.
Kösters M, Sturman B, Werheit P, Haertle D, Buse K
Optical cleaning of lithium niobate crystals
2009 Nat Photonics, Band: 3, Seite: 510
KurzfassungLithium niobate (LiNbO3), also called the 'silicon of photonics', is indispensable in advanced photonics and nonlinear optics. For many applications, however, the material is too polluted by transition metals, which are unavoidable at the parts per million level. These impurities serve as sources and traps for photoelectrons, causing optical damage and hampering the usability of LiNbO3. Efforts have therefore been made to get rid of the photoexcitable electrons. Here we introduce a method termed 'optical cleaning'. We show theoretically and experimentally that, if the material is heated to moderate temperatures, allowing ions to migrate and to maintain charge neutrality, an appropriately moving light beam pushes photoexcitable electrons out of the illuminated region like a brush, and provides exponential cleaning. This promises purification levels that are beyond the reach of current technologies.
Sturman B, Kösters M, Haertle D, Becher C, Buse K
Optical cleaning owing to the bulk photovoltaic effect
2009 Phys Rev B, Band: 80, Seite: 245319
KurzfassungIt is shown within the conventional photovoltaic charge-transport model that photoexcitable electrons, localized at deep impurity levels, can be effectively removed by light from the exposed area at sufficiently high temperatures. This allows to modify strongly the absorption and photoelectric properties of the material and, in particular, to suppress “optical damage” in LiNbO3 and LiTaO3 crystals. This optical cleaning method is applicable to numerous pyro- and piezo-electric optical materials. It employs the photovoltaic drift of electrons and ionic charge compensation at elevated temperatures. The physics of the optical cleaning is very rich; it has strong links to nonlinear dynamics and offers important handles for improvement of the cleaning performance. The use of properly moving light beams leads, e.g., to a strong enhancement of the cleaning rate and allows to reduce the electron concentration by several orders of magnitude. The theoretical predictions are supported by the data of our cleaning experiments with LiNbO3 crystals. In particular, the intensity threshold of optical damage is increased by three orders of magnitude.
Maxein D, Bueckers J, Haertle D, Buse K
Photorefraction in LiNbO3:Fe crystals with femtosecond pulses at 532 nm
2009 Appl Phys B-lasers O, Band: 95, Seite: 399
KurzfassungPhotorefractive index gratings are written into iron-doped lithium niobate crystals using femtosecond laser pulses and cw light, both having a wavelength of 532 nm. The saturation value of the refractive index changes in asgrown samples is found to decrease with increasing peak pulse intensity. Furthermore, in oxidized crystals, writing with femtosecond pulses is much faster than with cw light and retains about the same writing speed than in as-grown crystals.We propose a charge transport model that addresses the special case of recording with high intensity femtosecond pulses.
Japs J, Falk M, Woike Th, Buse K, Sturman B
Relaxation dynamics of space-charge gratings excited by nanosecond light pulses in highly iron-doped LiNbO3 crystals
2009 Appl Phys B-lasers O, Band: 95, Seite: 413
KurzfassungTransmission gratings are recorded with nanosecond light pulses in highly Fe-doped (0.5 to 3 wt%) as-grown and oxidized lithium niobate crystals. In as-grown samples, the diffraction efficiency decays exponentially after recording, and an exponential increase of the decay rate with decreasing mean distance between the Fe centers takes place. Such a behavior is typical for tunneling of electrons between Fe centers. A different and highly peculiar dark evolution of the gratings, which includes a strong transient increase of the diffraction efficiency during about 100 s after illumination, is observed in strongly oxidized crystals. This behavior is explained within a simple relaxation model which accounts for the pyroelectric effect. The long-time decay of the diffraction efficiency again follows an exponential law. In strongly oxidized crystals, the decay rate shows, however, a surprising decrease with decreasing mean distance between the Fe centers. This peculiarity is shown to be consistent with the present notion of hopping charge transport in highly doped strongly compensated semiconductors.
Vitova T, Hormes J, Falk M, Buse K
Site-selective investigation of site-symmetry and site-occupation of iron in Fe-doped lithium niobate crystals
2009 J Appl Phys, Band: 105, Seite: 013524
KurzfassungLithium niobate (LN) crystals, in particular after strong oxidation, are of great relevance for applications in photorefraction and nonlinear optics. Crystals doped with 2 or 4 wt % of iron are studied by extended x-ray absorption fine structure spectroscopy. The Fe atoms are observed to be sixfold coordinated by oxygen atoms and incorporated into the Li site. No evidence for formation of iron oxide clusters is found. In situ x-ray absorption spectroscopy measurements of the thermoelectric oxidation of a LN:Fe 2 wt % crystal and the analyses of the partially site-selective x-ray absorption near edge structure spectra of a LN:Fe 4 wt % crystal provide site-selective structural information. We found that the Fe2+ and Fe3+ sites have the same site symmetries and positions in the LN matrix, i.e., the Fe2+ and Fe3+ atoms are both octahedrally coordinated by six oxygen atoms and both occupy the Li site.
Steigerwald H, Luedtke F, Buse K
Ultraviolet-light assisted periodic poling of near-stoichiometric, magnesium-doped lithium niobate crystals
2009 Appl Phys Lett, Band: 94, Seite: 032906
KurzfassungFerroelectric domain reversal by application of electric-fields to near-stoichiometric lithium niobate crystals containing 1.3 mol % magnesium is investigated. The +z-face of the crystal is coated with a patterned photoresist layer that yields both a structured ultraviolet light absorber and a structured electric insulator. Periodic ferroelectric domain inversion in a z-cut crystal is then achieved by combination of illumination with light of the wavelength of 334 nm that reduces the coercive field strength inside the crystal by up to 50% and application of an external electrical field. Domain patterns with a period length of 10 µm are fabricated in a 500 µm thick crystal.

2008

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Breunig I, Kießling J, Sowade R, Knabe B, Buse K
Generation of tunable continuous-wave terahertz radiation by photomixing the signal waves of a dual-crystal optical parametric oscillator
2008 New J Phys, Band: 10, Seite: 073003
KurzfassungWe present a continuous-wave dual-crystal optical parametric oscillator that generates signal waves of about 1.4μm wavelength with a tunable difference frequency \Delta\nu between 0.64 and 5.3 THz. A model of the parametric gain in such a system indicates that the lowest reachable value for \Delta\nu depends on the refractive index dispersion of the nonlinear material used. As a proof of principle, we demonstrate the generation of terahertz radiation by photomixing the signal waves. Frequencies between 0.64 and 0.85 THz are created and detected coherently with ion-irradiated InGaAs interdigitated photomixers.
Breunig I, Kießling J, Knabe B, Sowade R, Buse K
Hybridly-pumped continuous-wave optical parametric oscillator
2008 Opt Express, Band: 16, Seite: 5662
KurzfassungWe present the first to our knowledge continuous-wave singlyresonant optical parametric oscillator (SROPO) generating tunable signal and idler waves with less than 100 mW single-frequency pump power. This low threshold is achieved by an additional intracavity gain medium that is pumped incoherently. The idler power with respect to the single-frequency pump power shows a bistable behavior which depends strongly on the pumping of the additional amplifier. Furthermore, we demonstrate that such a setup allows a SROPO to be completely diode pumped.
Brands K, Falk M, Haertle D, Woike Th, Buse K
Impedance spectroscopy of iron-doped lithium niobate crystals
2008 Appl Phys B-lasers O, Band: 91, Seite: 279
KurzfassungThe dc conductivity of as-grown and oxidized irondoped lithium niobate crystals is measured by impedance spectroscopy in the temperature range 30–700 °C. Electron and ion conduction at lower and higher temperatures are clearly distinguished, providing relevant information for understanding and optimizing the recently developed thermo-electric oxidization that makes crystals robust against optical damage.
Maxein D, Kratz S, Reckenthaeler P, Bueckers J, Haertle D, Woike Th, Buse K
Polarons in magnesium-doped lithium niobate crystals induced by femtosecond light pulses
2008 Appl Phys B-lasers O, Band: 92, Seite: 543
KurzfassungStrong light-induced absorption has been observed in lithium niobate crystals doped with magnesium after application of femtosecond illumination. In this material there are no Nb-on-Li-site defects and hence no antisite polarons occur, but small free polarons close to the conduction band can be generated. The light-induced absorption observed is attributed to these polarons. For LiNbO3:Mg, their decay times are about two orders of magnitude smaller than those of the Nb-on-Li-site polarons in undoped material. The results are relevant for a better understanding of the suppression of the so-called optical damage in these crystals and for their use in femtosecond applications.
Schwesyg J R, Eggert H A, Adleman J R, Buse K
Spatially resolved holographic measurements of the thermal diffusivity in liquids
2008 Appl Phys B-lasers O, Band: 92, Seite: 79
KurzfassungPhase gratings are recorded holographically with nanosecond light pulses by the interference of two plane waves in colored liquid samples using the thermooptic effect. The samples consist of toluene and distilled water which are immiscible and have different thermal conductivities. The exponential decay of the gratings caused by the thermal conductivity is monitored by diffraction of continuous wave light, and the thermal diffusivity is determined. Scanning of the read-out beam provides spatial resolution.
Gronenborn S, Sturman B, Falk M, Haertle D, Buse K
Ultraslow shock waves of electron density in LiNbO3 crystals
2008 Phys Rev Lett, Band: 101, Seite: 116601
KurzfassungWe show, experimentally and theoretically, that the application of modest voltages, U0 = (0.1–1) kV, to LiNbO3:Fe crystals at sufficiently high temperatures, T = (550–700) °C, leads to the formation of ultraslow shock waves (moving discontinuities) of the electron density owing to the removal of electrons from Fe2+ centers. Behind the sharp wave front, almost all iron centers are in the Fe3+ state, the sample is optically transparent, and its transport properties are strongly modified. The front velocity decreases during the propagation; it is controlled by the electron mobility.

2007

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Kalkum F, Eggert H A, Jungk T, Buse K
A stochastical model for periodic domain structuring in ferroelectric crystals
2007 J Appl Phys, Band: 102, Seite: 014104
KurzfassungA stochastic description is applied in order to understand how ferroelectric structures can be formed.The predictions are compared with experimental data of the so-called electrical fixing: domains are patterned in photorefractive lithium niobate crystals by the combination of light-induced space-charge fields with externally applied electrical fields. In terms of our stochastic model, the probability for domain nucleation is modulated according to the sum of external and internal fields. The model describes the shape of the domain pattern as well as the effective degree of modulation.
Falk M, Woike Th, Buse K
Charge compensation mechanism for thermo-electric oxidization of lithium niobate crystals
2007 J Appl Phys, Band: 102, Seite: 063529
KurzfassungThe charge compensation mechanism for thermo-electric oxidization of iron-doped lithium niobatecrystals is investigated. Iron ions are oxidized, i.e., Fe2+ ions are transformed to Fe3+. It is shown bybirefringence and absorption measurements that lithium and hydrogen ions leave the crystal duringthe thermo-electric oxidization process for charge compensation.
Falk M, Japs J, Woike Th, Buse K
Charge transport in highly iron-doped oxidized lithium niobate single crystals
2007 Appl Phys B-lasers O, Band: 87, Seite: 119
KurzfassungThe photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt.%. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe2+ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beamcoupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport.
Caballero-Calero O, Kösters M, Woike Th, Buse K, García-Cabañes A, Carrascosa M
Electric field periodical poling of lithium niobate crystals after soft-proton-exchanged waveguide fabrication
2007 Appl Phys B-lasers O, Band: 88, Seite: 75
KurzfassungWe report on the periodic polarization of α-phase proton- exchanged LiNbO3 planar waveguides down to 5 μm period lengths using structured electrodes and electrical fields. Unlike a majority of previous work, room-temperature electric field poling has been carried out after waveguide fabrication. Chemical etching of the large guiding surface has revealed homogeneous and high-quality domains with duty cycles close to 0.5. Moreover, it was verified by diffraction techniques and etching that the domain structure is preserved more than 2 μm along the thickness of the waveguide. The excellent optical performances of single-domain α-phase waveguides were not affected by the periodic poling.
Schwesyg J R, Eggert H A, Buse K, Sliwinska E, Khalil S, Maiser M, Meerholz K
Fabrication and optical characterization of stable suspensions of iron- or copper-doped lithium niobate nanocrystals in heptane
2007 Appl Phys B-lasers O, Band: 89, Seite: 15
KurzfassungStable suspensions of iron- or copper-doped lithium niobate nanocrystals in heptane are prepared by high-energy ball milling of iron- and copper-doped bulk crystals for 25 h. The distribution of particle sizes is determined by means of dynamic light scattering with more than 90% of the crystals in therange of 10–25 nm. The optical absorption of the suspensionssuggests a changed environment of the dopants.
Sturman B, Beyer O, Maxein D, Buse K
Femtosecond recording and time-resolved readout of spatial gratings in lithium niobate crystals
2007 J Opt Soc Am B, Band: 24, Seite: 419
KurzfassungExperimental and theoretical results on recording of spatial gratings with interfering femtosecond pulses at 388 nm and time-resolved Bragg-matched readout at 776 nm are presented for LiNbO3 crystals. These include dependences of the diffraction intensity on the time delay between probe and pump pulses, on the pump intensity and angle, and on the crystal composition. The grating buildup involves instantaneous and quasipermanent changes of the refractive index and the absorption coefficient. These changes are due to Kerr and two-photon absorption effects and the modulation of photoexcited carriers, respectively. A good agreement between theory and experiment is achieved. Our analysis provides an understanding of nonlinear optical phenomena in LiNbO3 crystals on the femtosecond time scale. The theory is applicable to a large range of optical materials with modestly wide, ~3 to 5 eV, bandgap.
Breunig I, Sowade R, Buse K
Limitations of the tunability of dual-crystal optical parametric Oscillators
2007 Opt Lett, Band: 32, Seite: 1450
KurzfassungWe demonstrate what is believed to be the first continuous-wave dual-crystal optical parametric oscillator (D-OPO) for generation of waves with a frequency difference of some terahertz. It is based on two magnesium-doped periodically poled lithium niobate crystals pumped with near-infrared light at 1030 nm.By changing the temperature difference of the crystals we achieve a difference-frequency tuning. This ranges from an initially unexpected lower threshold of 1.3 THz up to higher frequencies. The linewidth ofthe considered signal waves is smaller than 5 GHz. Smaller difference frequencies were not achievable. Our model, describing the dual-crystal parametric gain, explains the observed lower tuning limit by showing that the assumption of independent oscillation conditions in a D-OPO is not necessarily valid.
Falk M, Woike Th, Buse K
Reduction of optical damage in lithium niobate crystals by thermo-electric oxidization
2007 Appl Phys Lett, Band: 90, Seite: 251912
KurzfassungThe impact of conventional and of electric-field enhanced thermal oxidization on photorefractive index changes in nominally undoped lithium niobate crystals is investigated. Light-induced birefringence changes are measured with a Senarmont compensator for various light intensities and for crystals after different oxidization treatments. It is shown that the conventional annealing in oxygen atmosphere suppresses the unwanted index changes by a factor of 5. However, thermo-electric oxidization leads to a suppression by one order of magnitude. This effect is attributed to a permanent lowering of the amount of photoexcitable charge carriers achieved by the thermo-electric oxidization treatment. Such oxidized crystals can be very useful for, e.g., second harmonic generation or optical parametric oscillation.
Breunig I, Falk M, Knabe B, Sowade R, Buse K, Rabiei P, Jundt D H
Second harmonic generation of 2.6 W green light with thermoelectrically oxidized undoped congruent lithium niobate crystals below 100 °C
2007 Appl Phys Lett, Band: 91, Seite: 221110
KurzfassungWe demonstrate that thermoelectric oxidization suppresses unwanted light-induced refractive index changes in undoped congruent periodically poled lithium niobate crystals used for single-pass continuous-wave frequency doubling of 1030 nm light. The maximum second harmonic output power shows a strong dependence on the phase matching temperature. For 90 °C, a total of 2.6 W green light is generated with an efficiency of 22% corresponding to a normalized conversion efficiency of 1.8%/W.
Reckenthaeler P, Maxein D, Woike Th, Buse K, Sturman B
Separation of optical Kerr and free-carrier nonlinear responses with femtosecond light pulses in lithium niobate crystals
2007 Phys Rev B, Band: 76, Seite: 195117
KurzfassungTime-resolved four-wave pump-probe experiments with femtosecond light pulses, including readout of the fundamental K grating and also of the 2K grating, allow to separate the free-carrier nonlinearity caused by two-photon excitation from the instantaneous optical Kerr nonlinearity. The method is applicable to wide-gapoptical materials. For lithium niobate crystals, we show, using 200 fs pulses, that the buildup of the free-carriergrating adiabatically follows the absorbed energy. This finding and the results of modeling lead us to theconclusion that the relaxation (thermalization) time of the photoexcited band carriers is shorter than 100 fs.
Eggert H A, Kuhnert F Y, Buse K, Adleman J R, Psaltis D
Trapping of dielectric particles with light-induced space-charge fields
2007 Appl Phys Lett, Band: 90, Seite: 241909
KurzfassungLight-induced space-charge fields in lithium niobate crystals are used to trap and manipulate dielectric particles on the surface of such crystals. Without any external voltage source, strong field gradients are present in the proximity of the crystal surface. These are used to trap particles with diameters in the range between 100 nm and some tens of micrometers.

2006

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Heinemeyer U, Wengler M C, Buse K
Annihilation of the OH-absorption due to domain inversion in MgO-doped lithium niobate crystals
2006 Appl Phys Lett, Band: 89, Seite: 112910
KurzfassungThe influence of domain inversion on the infrared absorption of lithium niobate crystals with varying MgO doping is investigated. The authors observe a significant difference between crystals doped below and those doped above the so-called optical damage threshold. The shift of the OH absorption due to domain inversion for weakly doped or undoped samples, known from the literature, can be confirmed. However, for highly doped samples the peak vanishes completely after forward poling. It reemerges either by reverse poling or by annealing at 250 °C. The mechanism is yet not fully understood but has to be correlated with the defect structure of the crystal.
Eggert H A, Kalkum F, Buse K, Sturman B
Bragg selectivity of space-charge gratings in multidomain lithium niobate crystals
2006 Opt Lett, Band: 31, Seite: 1256
KurzfassungIt is shown experimentally and theoretically that a partial domain reversal in ferroelectric lithium niobate crystals containing a previously imprinted photorefractive space-charge grating results in a highly unusual change of the angular Bragg selectivity. This carries important in situ information about domain structures, including the quality of periodically poled samples.
Apitz D, Bertram R P, Benter N, Sommer-Larsen P, Johansen P M, Buse K
Conductivity of oriented bis-azo polymer films
2006 Chemphyschem, Band: 7, Seite: 468
KurzfassungThe conductivity properties of electro-optic, photoaddressable, dense bis-azo chromophore polymer films are investigated by using samples corona poled at various temperatures. A dielectric spectrometer is applied to measure the frequency dependence of the conductivity at different temperatures before and after heating the material to above the glass transition temperature. The results show that the orientation of the chromophores changes the charge-carrier mobility. Ionic conductivity dominates in a more disordered configuration of the material, while the competing process of hole hopping takes over as a transition to a liquid-crystalline phase occurs when the material is heated to much higher than the glass transition temperature. Such microcrystallization strongly enhances the conductivity.
Hartwig U, Peithmann K, Woike Th, Buse K
Determination of the absorption cross section of dopants in lithium niobate crystals
2006 J Phys-condens Mat, Band: 18, Seite: 447
KurzfassungA method is presented to acquire the absorption cross sections of dopants in photorefractive lithium niobate crystals utilizing doubly doped samples. The absorption cross section of one dopant must be well known. By illumination with ultraviolet light, electrons are transferred from one centre to the other. From the changes of the absorption spectra, the absorption cross section of the centre under investigation is deduced. For a wavelength of λ = 577 nm the absorption cross section of Mn3+ is determined by this method to be σo Mn3+ ,577 nm = (9.2 ± 1.3) × 10−19 cm2 for ordinarily polarized light. The described method can be adapted to other dopants and host materials.
Peithmann K, Zamani Meymian M R, Haaks M, Maier K, Andreas B, Buse K, Modrow H
Fabrication of embedded waveguides in lithium-niobate crystals by radiation damage
2006 Appl Phys B-lasers O, Band: 82, Seite: 419
KurzfassungIrradiation of lithium-niobate crystals (LiNbO3) with fast, high-energy 3He ions changes the refractive index in the interaction region where the ions speed through the material. Thus an inhomogeneous flux density profile can be used for a tailored modification of the optical properties of LiNbO3 crystals, without employing ion implantation. A new method to fabricate embedded, polarization sensitive channel waveguides in LiNbO3 utilizing accelerated 3He ions with an energy of 40 MeV is demonstrated.
Hartwig U, Kösters M, Woike Th, Buse K, Shumelyuk S, Odulov S
Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals
2006 Opt Lett, Band: 31, Seite: 583
KurzfassungHolographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K±G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.
Beyer O, Maxein D, Woike Th, Buse K
Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale
2006 Appl Phys B-lasers O, Band: 83, Seite: 527
KurzfassungLight pulses of 240 fs duration and 388-nm wavelength generate in lithium niobate crystals transient absorption. The absorption changes are observed with delayed ultra-short pulses (wavelength 776 nm) and with continuous-wave light (wavelength 785 nm) over 10 decades from 400 fs up to 10 s after the pump pulse. The decay is well described by one stretched-exponential function over the entire temporal range. We attribute the absorption changes to small bound polarons because of the temporal, spectral, and polarization fingerprints. The polarons are formed in less than 400 fs after the pump illumination, ruling out that phonons with wavenumbers below 100 cm−1 are the main cause for polaron generation and indicating that the breathing mode of the oxygen octahedra is of relevance. An upper limit of the generated polaron concentration is estimated as ≈ 4.4×1018 cm−3 with an intensity of ≈ 220GW/cm2.
Hartwig U, Kösters M, Woike Th, Buse K
High-temperature-recorded index gratings in periodically-poled lithium niobate
2006 Opt Lett, Band: 31, Seite: 3267
KurzfassungHolographic index gratings based on a zero-electric-field photorefractive effect are recorded at high temperatures in copper-doped periodically poled lithium niobate crystals. The interplay between the domain structure and the index grating is studied: the fundamental grating with spatial frequency K is strongly suppressed. Pronounced sideband gratings with Ks=K+sG appear, where G is the domain grating vector and s is an integer number. After development, an additional grating based on the electro-optic effect shows up. In contrast with the previously mentioned gratings, this grating allows anisotropic diffraction.
Adleman J R, Eggert H A, Buse K, Psaltis D
Holographic grating formation in a colloidal suspension of silver nanoparticles
2006 Opt Lett, Band: 31, Seite: 447
KurzfassungHolographic gratings are recorded in colloidal suspensions of silver nanoparticles by utilizing interfering nanosecond pulses. The diffraction efficiency is measured with continuous-wave light. An instantaneous response together with a transient grating are observed: the nanoparticles absorb the pump light and heat up. Heat is transferred to the solvent, and a delayed thermal grating appears. The final decay time constant of this grating depends quadratically on the period length and has a typical value of 1 µs for grating spacings of several micrometers.
Shumelyuk A, Hartwig U, Buse K, Odulov S, Woike Th
Linearity of index grating recording with spatially oscillating photovoltaic currents
2006 J Opt Soc Am B, Band: 23, Seite: 857
KurzfassungThe response of photorefractive crystals with bulk photovoltaic charge transport is usually highly nonlinear, and for illumination with a sinusoidal light pattern the recorded space-charge gratings possess, apart from the principal spatial frequency K, several higher spatial harmonics, 2K, 3K, etc. We show experimentally that purely sinusoidal index gratings can be recorded in LiNbO3 :Fe when the charge redistribution is governed by spatially oscillating photovoltaic currents. This property is especially beneficial for holographic data storage.
Beyer O, von Korff Schmising C, Luennemann M, Buse K, Sturman B
Long-living currents induced by nanosecond light pulses in LiNbO3 crystals
2006 Opt Express, Band: 14, Seite: 1533
KurzfassungWe have obtained first solid evidence of strong charge separation that is caused by relaxing localized electrons in a polar medium: Space-charge gratings induced in highly-doped LiNbO3:Fe crystals by interfering nanosecond light pulses at 532 nm show a highly peculiar longterm behavior (buildup or/and decay) in the dark. It depends strongly on the applied electric field E0 (ranging from −140 to +640 kV/cm) and occurs on a time scale of (1 – 100) s which is much larger than the relaxation time of photo-electrons and smaller than the dark dielectric relaxation time. All peculiarities observed are fully described by a charge-transport model that incorporates the energy relaxation of electrons within a band of localized Fe2+ states and a long-living, field-gradient-independent ”polar current“ directed along the polar axis.
Nau D, Bertram R P, Buse K, Zentgraf T, Kuhl J, Tigodeev S G, Gippius N A, Giessen H
Optical switching in metallic photonic crystal slabs with photoaddressable polymers
2006 Appl Phys B-lasers O, Band: 82, Seite: 543
KurzfassungWe report on a metal-polymer compound material with optical properties that can be reversibly switched all-optically. The key element is a metallic photonic crystal slab with an additional layer of photoaddressable material that provides a large variable birefringence and sharp resonances. Pump-probe experiments show a shift of the photonic crystal resonances that depends on the pump polarization and on the exposure. Comparison of the results with calculations from a scattering-matrix theory allows one to determine the refractive index changes for different polarization geometries and to model our compound material quantitatively.
Beyer O, Breunig I, Kalkum F, Buse K
Photorefractive effect in iron-doped lithium niobate crystals induced by femtosecond pulses of 1.5 µm wavelength
2006 Appl Phys Lett, Band: 88, Seite: 051120
KurzfassungIllumination of iron-doped lithium crystals (LiNbO3:Fe) with femtosecond pulses of 1.5 µm wavelength results in large refractive index changes \Delta n in the order of 10−3. The sign of the refractive index changes depends on the polarization of the recording light. The results can be very useful for fabrication of tailored holographic components for telecommunication.
Kösters M, Hartwig U, Woike Th, Buse K, Sturman B
Quantitative characterization of periodically poled lithium niobate by electrically induced Bragg diffraction
2006 Appl Phys Lett, Band: 88, Seite: 182910
KurzfassungBy applying strong electric fields, up to 3 kV/mm, to periodically poled lithium niobate (PPLN) and measuring the characteristics of Bragg diffraction from the periodic domain structure, we have determined the basic parameters of PPLN, such as the period length, the duty-cycle ratio, the internal field, and the in-wall refractive index changes. We thus propose a nondestructive optical method for the characterization of periodically poled ferroelectrics.
Olimov Kh, Falk M, Buse K, Woike Th, Hormes J, Modrow H
X-ray absorption near edge spectroscopy investigations of Valency and lattice occupation site of Fe in highly iron-doped lithium niobate crystals
2006 J Phys-condens Mat, Band: 18, Seite: 5135
KurzfassungThe oxidation state of Fe in highly doped lithium niobate crystals (2 wt% Fe2O3) in the as-grown, reduced, and oxidized states is determined by the combination of differentiation and integration methods applied to their Fe Kedge XANES (x-ray absorption near edge spectroscopy) spectra. The obtained valences are confirmed further by absorption measurements in the IR/vis (infrared and visible) spectral range. It is shown that reduction and thermoelectric oxidization of as-grown Fe:LiNbO3, respectively, leads to a noticeable lowering of the number of Fe3+ and to complete oxidization of all Fe2+ to Fe3+, respectively. It is found that Fe in highly iron-doped lithium niobate samples (0.5, 2, and 4 wt% Fe2O3) is incorporated onto the Li site comparing the experimental XANES spectra to the FEFF8 calculations. The difference of the XANES spectrum of the 4 wt% Fe2O3 doped sample as compared to those of the 0.5 and 2 wt% doped samples is explained by a co-phase formation in the former one. The results obtained can help to tailor such crystals for non-linear optical applications as well as for photorefraction.

2005

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Hsieh H T, Psaltis D, Beyer O, Maxein D, Buse K, Sturman B
Enhanced temporal resolution in femtosecond dynamic-grating experiments
2005 J Appl Phys, Band: 97, Seite: 113107
KurzfassungRecording of gratings by interference of two pump pulses and diffraction of a third probe pulse is useful for investigating ultrafast material phenomena. We demonstrate, in theory and experiment, that the temporal resolution in such configurations does not degrade appreciably even for large angular separation between the pump pulses. Transient Kerr gratings are generated inside calcium fluoride sCaF2d crystals by two interfering femtosecond spumpd pulses at 388 nm and read out by a Bragg-matched probe pulse at 776 nm. The solution to the relevant coupled-mode equations is well corroborated by the experimental results, yielding a value of the Kerr coefficient of ~4.4x10−7 cm2 /GW for CaF2.
Hsieh H T, Psaltis D, Beyer O, Maxein D, von Korff Schmising C, Buse K, Sturman B
Femtosecond holography in lithium niobate crystals
2005 Opt Lett, Band: 30, Seite: 2233
KurzfassungSpatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump–probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was ~1.0 x 10−5 cm2/GW.
Beyer O, Maxein D, Buse K, Sturman B, Hsieh H T, Psaltis D
Femtosecond time-resolved absorption processes in lithium niobate crystals
2005 Opt Lett, Band: 30, Seite: 1366
KurzfassungFemtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient bp ranges from ,3.5 cm/GW for lp=388 nm to ,0.1 cm/GW for 514 nm. In collinear pump–probe experiments the probe transmission at the double pump wavelength 2lp=776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (br ~0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump–probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of ~0.1 ps.
Kösters M, Hsieh H T, Psaltis D, Buse K
Holography in commercially available photo-etchable glasses
2005 Appl Optics, Band: 44, Seite: 3399
KurzfassungVolume holographic gratings are recorded and retrieved in two commercially available glasses: Schott Foturan and Hoya PEG3. These materials are photoetchable, which describes their major application, but they also allow storage of volume holograms without any chemical etching. The samples are illuminated with ultraviolet light at a wavelength of 325 nm and thermally processed to achieve a maximum diffraction efficiency of ~9% for a 1-mm-thick sample. The two glasses show similar behavior; the diffraction efficiencies in Foturan tend to be slightly larger, whereas PEG3 tends to have weaker light scattering.
Apitz D, Bertram R P, Benter N, Hieringer W, Andreasen J W, Nielsen M M, Johansen P M, Buse K
Investigation of chromophore-chromophore interaction by electro-optic measurements, linear dichroism, x-ray scattering and density-functional calculations
2005 Phys Rev E, Band: 72, Seite: 036610
KurzfassungFree-beam interferometry and angle-resolved absorption spectra are used to investigate the linear electrooptic coefficients and the linear dichroism in photoaddressable bis-azo copolymer thin films. From the first and second order parameters deduced, the chromophore orientation distribution is calculated and displayed for several poling temperatures and chromophore concentrations. The influence of dipole-dipole interaction on the overall polymer dynamics is discussed. The first order parameter, and therefore the Pockels effect, peaks for a poling temperature of around 10 °C above the glass transition. The decrease of the Pockels effect above this temperature region is triggered by a head-to-tail chromophore orientation, i.e., a transition to a microcrystalline phase, increasing the second order parameter. Comparison of the experimentally observed absorption spectra and those obtained by density-functional calculations support the picture of differently aligned bis-azo dye molecules in a trans,trans configuration. Complementary wide-angle x-ray scattering is recorded to confirm the various kinds of ordering in samples poled at different temperatures.
Beyer O, Maxein D, Buse K, Sturman B, Hsieh H T, Psaltis D
Investigation of nonlinear absorption processes with femtosecond light pulses in lithium niobate crystals
2005 Phys Rev E, Band: 71, Seite: 056603
KurzfassungThe propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNbO3) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is bp ~3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Dt between probe and pump pulses is characterized by a narrow dip sat Dt.0d and a long son the picosecond time scaled lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is br.0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sr~8x10−18 cm2. The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of ~0.24 ps.
Müller M, Langrock C, Soergel E, Buse K, Fejer M M
Investigation of periodically-poled lithium niobate crystals by light diffraction
2005 J Appl Phys, Band: 97, Seite: 044102
KurzfassungPeriodically poled lithium niobate crystals are investigated by illumination along the crystallographic c axis while applying an external electric field. The resulting diffraction patterns are in good agreement with the results of numerical simulations and directly yield first information such as periodicity, duty cycle, and homogeneity of the domain patterns.
Benter N, Bertram R P, Soergel E, Buse K, Apitz D, Bang Jacobsen L, Johansen P M
Large-area Fabry-Pérot modulator based on electro-optic polymers
2005 Appl Optics, Band: 44, Seite: 6235
KurzfassungWe present a large-area electro-optic Fabry–Perot modulator utilizing a photoaddressable bis-azo polymer placed between two dielectric mirrors with an open aperture of 2 cm. A modulation efficiency of 1% at an effective modulation voltage of 20 V for a wavelength of 1.55 µm is demonstrated. By comparing distance tuning of the cavity with wavelength tuning, an effective electro-optic coefficient of ~7 pm/V is measured.
Sones C L, Wengler M C, Valdivia C E, Mailis S, Eason R W, Buse K
Light-induced order-of-magnitude decrease in the electric field for domain nucleation in MgO-doped lithium niobate crystals
2005 Appl Phys Lett, Band: 86, Seite: 212901
KurzfassungWe report an order-of-magnitude reduction in the electric field required for domain nucleation in 1 mol % MgO-doped near-stoichiometric and 5 mol % MgO-doped congruently grown lithium niobate crystals induced by illumination from a focused continuous wave laser beam at wavelengths of 514, 488, and 457 nm. A smaller decrease of 31% is also observed for undoped congruently grown crystals. The effect is independent of the visible wavelengths explored. Light-controlled domain patterning is also demonstrated.
Andreas B, Breunig I, Buse K
Modeling of x-ray-induced refractive-index changes in poly (methyl methacrylate)
2005 Chemphyschem, Band: 6, Seite: 1544
KurzfassungThe refractive index of bulk poly(methyl methacrylate) samples is modified by X-ray exposure. The intensity of the radiation is distributed over an energy range of 1 to 20 keV with a maximum at 3 keV. For an absorbed dose of about 1 kJ cm-3, an increase of the refractive index of up to 3x10-4 is observed. However, for much lower doses of about 10 J cm-3 a decrease of 4x10-4 appears. These refractive index changes, as well as the simultaneously arising radiation-induced thickness changes, are detected interferometrically. Based on these observations, on dose calculations, and on literature data, a model is proposed which describes quantitatively the observed refractive index changes. The refractive index increase is caused by a radiation-induced chemical alteration of the polymer composition, while the refractive index decrease is correlated with a decrease of mass density.
Eggert H A, Kalkum F, Hecking B, Buse K
Optimization of electrical fixing in near-stoichiometric iron-doped lithium niobate crytals
2005 J Opt Soc Am B, Band: 22, Seite: 2553
KurzfassungHolographic gratings are recorded in photorefractive near-stoichiometric iron-doped lithium niobate crystals in the presence of an external electric field close to the coercive field of the material. The holograms are erased by homogeneous illumination. Subsequent application of an external field well above the coercive field reveals the holographic grating partly and transiently. The diffraction efficiency of the revealed grating increases for longer recording times and for larger grating period lengths. All results can be understood by a process known as electrical fixing: During recording the space-charge pattern is transferred to a pattern of ferroelectric domains.
Hartwig U, Peithmann K, Sturman B, Buse K
Strong permanent reversible diffraction gratings in copper-doped lithium niobate crystals caused by a zero-electric-field photorefractive effect
2005 Appl Phys B-lasers O, Band: 80, Seite: 227
KurzfassungPermanent reversible diffraction gratings with refractive-indexmodulations of up to ~10−4 and with a grating vector perpendicular to the polar axis are realized in LiNbO3:Cu crystals by making use of high temperature recording and charge compensation. The index changes do not result from light-induced space-charge fields and the linear electro-optic effect. They are linked to fixed high-contrast narrow-band absorption gratings of Cu+ /Cu2+ ions via the Kramers–Kronig relations.
Falk M, Buse K
Thermo-electric method for nearly complete oxidization of highly iron-doped lithium niobate crystals
2005 Appl Phys B-lasers O, Band: 81, Seite: 853
KurzfassungA new method is presented, allowing the nearly complete oxidization of lithium niobate crystals (LiNbO3), doped with large amounts of iron oxide (0.05–3 wt.% Fe2O3) utilizing annealing at 700 °C in the presence of externally applied electric fields. The treatment results in a concentration ratio of Fe2+ and Fe3+ ions of less than 2×10−3. Strong oxidization of iron in LiNbO3 reduces the photorefractive effect and is therefore of particular interest for nonlinear-optical applications.
Wengler M C, Heinemeyer U, Soergel E, Buse K
UV-assisted domain inversion in magnesium-doped lithium niobate crystals
2005 J Appl Phys, Band: 98, Seite: 064104
KurzfassungThe influence of ultraviolet (UV) light (wavelengths \lambda=334 and 305 nm) on the ferroelectric domain inversion of lithium niobate crystals doped with different amounts of magnesium ranging from 0 to 7.5 mol % is investigated. Illumination at \lambda=334 nm leads to a coercive field reduction of up to 50%, but only in samples doped with a magnesium concentration above the so-called optical damage threshold. For \lambda=305 nm the effective coercive field is reduced significantly in all samples. Different behavior of the coercive field reduction at both wavelengths indicates the presence of two mechanisms. To explain the effect occurring for 305 nm illumination a model is presented in which an UV-induced photoconductivity alters the electric-field distribution through the crystal thickness. Utilizing an UV-interference pattern and a suitable homogeneous external electrical field, periodically poled lithium niobate with a period length of 55 µm is produced.

2004

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Eggert H A, Hecking B, Buse K
Electrical fixing in near-stoichiometric lithium niobate crystals
2004 Opt Lett, Band: 29, Seite: 2476
KurzfassungPhotorefractive gratings are recorded in lithium niobate crystals in the presence of externally applied electric fields close to the coercive field. After optical erasure a voltage pulse above the poling threshold is applied, and by these means a photorefractive grating reappears that can be erased optically. The results point to electrical fixing, i.e., ferroelectric domain reversal induced by a space-charge field.
Wengler M C, Fassbender B, Soergel E, Buse K
Impact of ultraviolet light on coercive field, poling dynamics and poling quality of various lithium niobate crystals from different sources
2004 J Appl Phys, Band: 96, Seite: 2816
KurzfassungFerroelectric domain reversal by electric field poling of lithium niobate crystals (LiNbO3) with varying stoichiometry and magnesium (MgO) doping level obtained from various commercial suppliers is investigated. Magnesium doping lowers the domain-wall velocity, increases the uniformity of the growth of the domains, and reduces the impact of crystal symmetry on the shape of the domains. Illumination with ultraviolet (UV) laser light (305 nm) reduces the coercive field by up to 34% in MgO-doped crystals, but is accompanied by a degradation of poling quality. UV light of longer wavelengths (334 nm) has no influence on the coercive field except for the MgO:LiNbO3 material of one supplier, where the field is reduced by 27%. In this case the poling quality is excellent. UV-induced reduction of stress-induced birefringence is observed in some samples. The results are of crucial relevance for light-induced domain engineering of LiNbO3 crystals.
Bertram R P, Benter N, Apitz D, Soergel E, Buse K, Hagen R, Kostromine S G
Increased thermal stability of a poled electro-optic polymer using high-molar-mass fractions
2004 Phys Rev E, Band: 70, Seite: 041802
KurzfassungPolymer fractionation yields materials with higher average molar masses and thus substantially increased glass transition temperatures. We apply this technique to a photoaddressable polymer that exhibits high electrooptic coefficients, in order to enhance the thermal stability of the poled material. The degree of orientation of the dipolar chromophores is investigated with both pyroelectric and electro-optic measurements. The pyroelectric measurements at elevated temperatures show no significant contribution of b relaxation as compared to a relaxation.
Müller M, Soergel E, Wengler M C, Buse K
Light deflection from ferroelectric domain boundaries
2004 Appl Phys B-lasers O, Band: 78, Seite: 367
KurzfassungA new deflection phenomenon of light has been discovered in ferroelectric crystals that provides information about domain structures, their nucleation, and their dynamics: Illumination of a multi-domain crystal with a single light beam yields light patterns, if external electrical fields are applied. Deflection angles of up to ±8° are observed. We attribute this effect to light deflection from the boundaries of the domains. Besides providing a new method to study domains of any ferroelectric material, the method can be of practical importance for electrooptic beam scanning.
Müller M, Soergel E, Buse K
Light deflection from ferroelectric domain structures in congruently melting lithium tantalate crystals
2004 Appl Optics, Band: 43, Seite: 6344
KurzfassungCongruently grown lithium tantalate crystals containing ferroelectric domain structures have been illuminated with ultraviolet light along the crystallographic c axis with simultaneous application of an external electric field. This gives rise to a star-shaped light diffraction pattern in the transmitted beam. The characteristics of this phenomenon are investigated and compared with those observed in lithium niobate crystals.
Andreas B, Peitmann K, Maier K, Buse K
Modification of the refractive index of lithium niobate crystals by transmission of high-energy 4He2+ and D+ particles
2004 Appl Phys Lett, Band: 84, Seite: 3813
KurzfassungWe report reductions of the refractive index of congruently melting lithium niobate crystals of up to 6x10-3 by exposure of z-cut samples with high-energy 4He21 and D1 particles which are transmitted through the crystals.
Shumelyuk A, Shcherbin K, Odoulov S, Sturman B, Podivilov E, Buse K
Slowing-down of light in photorefractive crystals with beam intensity coupling reduced to zero
2004 Phys Rev Lett, Band: 93, Seite: 243604
KurzfassungA considerable deceleration of light pulses in crystals with nearly compensated space-charge field proves that the strong dispersion of dynamic photorefractive gratings in the vicinity of Bragg resonance is of primary importance for light slowing down.
Wengler M C, Schreder B, Soergel E, Zimmer J, Buse K
Spectral photosensitivity and holographic performance of europium-doped fluorophosphate glass
2004 Appl Phys B-lasers O, Band: 79, Seite: 597
KurzfassungSpectral investigation of europium-doped flourophosphate glass shows photosensitivity in the range of 244 nm to 351 nm. Holographic investigations of the dependence of the achievable refractive-index changes indicate a strong impact of the doping concentration. The refractive-index changes increase with europium concentrations up to 1.0×1020 Eu/cm3; for higher doping levels the effect becomes smaller. Based on these results, refractive-index changes in excess of 10−3 for optimized material and holographic recording are predicted. About 23% of the initially recorded holographic grating decay in a double-exponential way with time constants of 7 h and 240 h, but the other 77%of the primary refractive-index changes are persistent. The spatial resolution of this material is better than 1 μm.

2003

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Mützel M, Rasbach U, Meschede D, Burstedde C, Braun J, Kunoth A, Peithmann K, Buse K
Atomic nanofabrication with complex light fields
2003 Appl Phys B-lasers O, Band: 77, Seite: 1
KurzfassungThe method of neutral atom lithography allows one to transfer to a substrate a 2D intensity modulation of an atomic beam imposed by an inhomogeneous light field. The complexity of the pattern depends on the properties of the light field constructed from the superposition of multiple laser beams. For the design of suitable light fields we present a mathematical model with a corresponding numerical simulation of the socalled inverse problem. Furthermore, details of an experiment carried out with a holographically reconstructed light field are discussed.
Luennemann M, Buse K, Sturman B
Coupling effects for counter-propagating light beams in lithium niobate crystals studied by grating translation technique for extremely high external electric felds
2003 J Appl Phys, Band: 94, Seite: 6274
KurzfassungThe grating translation technique (GTT), which employs a momentary shift of light fringes during holographic recording, is a powerful tool for characterization of nonlinear materials. Earlier, it was used in the transmission geometry. We formulate the main relations of GTT for the reflection case. These allow us to determine in situ the real and imaginary parts of the coupling strength and to detect the formation of refractive-index and absorption gratings. We apply GTT to determine the photorefractive characteristics of two-wave coupling in z-cut iron-doped lithium niobate crystals for an extremely small grating spacing (\Lambda=105 nm) and extremely large externally applied electric fields (up to 650 kV/cm). Stabilization of the light fringes, crucially important in the reflection geometry, has been achieved with the use of an electronic feedback loop.We prove that the feedback introduces a frequency detuning between the light beams and modifies strongly the nonlinear response.
Bäumer Ch, Berben D, Buse K, Hesse H, Imbrock J
Determination of the composition of lithium tantalate crystals by zero-birefringence measurements
2003 Appl Phys Lett, Band: 82, Seite: 2248
KurzfassungWe present an approach to determine the exact composition of lithium tantalate (LiTaO3) crystals by measuring the temperature where the birefringence disappears. Various samples with compositions ranging from the congruently melting to the stoichiometric one have been prepared by vapor transport equilibration treatments and are investigated. The sensitivity is as high as 1.5 °C/0.01 mol% Li2O. The relative accuracy is better than 0.01 mol %, while the absolute accuracy is limited by the calibration.
Luennemann M, Hartwig U, Panotopoulos G, Buse K
Electrooptic properties of lithium niobate crystals for extremely high external electric fields
2003 Appl Phys B-lasers O, Band: 76, Seite: 403
KurzfassungThe electrooptic effect in lithium niobate crystals (LiNbO3) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10−3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s13| ≤ 2.3×10−21 m2/V2 is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated.
Gorkunov M, Sturman B, Luennemann M, Buse K
Feedback-controlled two-wave coupling in reflection geometry: application to lithium niobate crystals subjected to extremely high external electric fields
2003 Appl Phys B-lasers O, Band: 77, Seite: 43
KurzfassungAn active stabilization of photorefractive two-wave coupling by means of an electronic feedback loop has been used extensively during recent years in transmission geometry. It leads to 100% diffraction efficiency η and also to periodic states instead of familiar steady states. We investigate the feedback operation in the case of reflection geometry, especially for iron-doped lithium niobate (LiNbO3:Fe) crystals. This includesformulation of the feedback equations, numerical analysis of the operation regimes for LiNbO3 crystals, and comparison between theory and experiment, which is performed in the range of applied electric fields from0 to 650 kV cm−1. Themain findings are as follows: (i) the feedback does not lead to periodic states, (ii) itmodifies the photorefractive response by introducing a frequency shift and maximizes η, and (iii) there is a close relation between the enhancement of η and the resonant excitation of space-charge waves predicted earlier in ferroelectric materials.
Beyer O, Nee I, Havermeyer F, Buse K
Holographic recording of Bragg gratings for wavelength division multiplexing in doped and partially polymerized polymethylmethacrylate
2003 Appl Optics, Band: 42, Seite: 30
KurzfassungBragg gratings are recorded in doped and partially polymerized poly(methyl methacrylate) with green light (wavelength, 532 nm) in transmission geometry, and the gratings are read in reflection geometry with infrared light (wavelength, ~1550 nm). Diffraction efficiencies of more than 99% with a wavelength bandwidth of ~1 nm are obtained for single gratings with a typical length of 15 mm. Superposition of four gratings in a volume sample has been demonstrated as well. The material is promising for use in the fabrication of add–drop filters, attenuators, switches, and multiplexers–demultiplexers for optical networks that use wavelength division multiplexing.
Luennemann M, Hartwig U, Buse K
Improvements of sensitivity and refractive-index changes in photorefractive iron-doped lithium-niobate crystals by application of extremely large external electrical fields
2003 J Opt Soc Am B, Band: 20, Seite: 1643
KurzfassungApplying external electric fields of as much as 65 kV/mm reveals significant enhancement of refractive-index changes Dns and of photorefractive sensitivity S. Samples with different iron concentrations and oxidation– reduction states were investigated. Refractive-index changes of up to Dns = 11.5 x 10-4 and sensitivities as high as S = 40 cm/J were measured (ordinary light polarization; light wavelength, l = 488 nm). The refractive-index changes are space-charge limited, but the photorefractive sensitivity S exhibits for large externally applied electric fields a linear increase up to the highest field values used. No nonlinear enhancement of the charge transport was observed. However, the excellent high-field material performance and the record values for Dns and S obtained improve the applicability of LiNbO3 crystals.
Müller M, Soergel E, Buse K
Influence of UV illumination on the poling characteristics of lithium niobate crystals
2003 Appl Phys Lett, Band: 83, Seite: 1824
KurzfassungThe poling characteristics of lithium niobate crystals under ultraviolet laser illumination are investigated. Light with a wavelength around 334 nm permanently decreases the coercive field of the crystal and is used to demonstrate optical control of domain patterning.
Soergel E, Pankrath R, Buse K
Investigation of photorefractive SBN crystals with atomic force microscopy
2003 Ferroelectrics, Band: 296, Seite: 19
KurzfassungElectrostatic force detection with an atomic force microscope (AFM) is ideally suited for the study of the surface properties of dielectric materials. The AFM allows simultaneous detection of many physical quantities like free and trapped charge carrier density, ferroelectric domains and, of course, the topography of the sample. Usually, photorefractive materials are optically investigated: volume refractive index gratings are observed by light diffraction. In contrast, the AFM offers the possibility of directly observing the physical steps for the development of a photorefractive grating. Due to the high sensitivity and the unprecedented lateral resolution, the AFMprovides a wealth of newpossibilities for the investigation of photorefractive materials. We present results obtained with Sr0.61Ba0.39Nb2O6 (SBN) crystals. The samples had as-grown as well as polished surfaces. Spatial distribution and temporal dynamics of photorefractive charge patterns are successfully resolved. Furthermore, additional charge patterns are observed that may originate from local crystal imperfections and from ferroelectric domain structures.
Nee I, Beyer O, Müller M, Buse K
Multi-channel wavelength-division-multiplexing with thermally-fixed Bragg gratings in photorefractive lithium-niobate crystals
2003 J Opt Soc Am B, Band: 20, Seite: 1593
KurzfassungThe transmission capacity of fiber communication networks is enhanced by usage of dense wavelength division multiplexing (WDM). This technique requires wavelength filters for multiplexing of the channels. We report on the realization of a multiplexer device based on superimposed volume-phase gratings in a single lithium niobate crystal. The gratings are recorded through the photorefractive effect by interference of two green laser beams. Thermal fixing is employed to increase the lifetime of the gratings. Each grating diffracts light of a certain WDM channel (wavelengths of ;1500 nm). Simultaneous multiplexing of many channels is achieved by suitable arrangement of the gratings in the crystal. We present the basic concept of this technology as well as recent advances: (1) refined experimental methods about tailored recording of manychannel multiplexers, (2) characterization of the multiplexers for up to sixteen WDM channels (1-dB bandwidth up to 0.1 nm, channel spacing down to 0.4 nm), and (3) construction of a two-channel multiplexer device.
Andreas B, Peithmann K, Soergel E, Buse K
Photorefractive lithography with synchrotron light in poly(methyl methacrylate) (PMMA)
2003 Appl Phys Lett, Band: 83, Seite: 1116
KurzfassungSynchrotron light changes the refractive index of poly(methyl methacrylate) (PMMA). Refractive-index enhancements as well as reductions depending on dose and energy of the radiation used can be observed. This effect allows the manufacturing of, for example integrated-optical components, as is demonstrated by realization of waveguides in PMMA.
Yang Y, Psaltis D, Luennemann M, Berben D, Hartwig U, Buse K
Photorefractive properties of lithium niobate crystals doped with manganese
2003 J Opt Soc Am B, Band: 20, Seite: 1491
KurzfassungThe photorefractive properties of lithium niobate crystals doped with manganese (Mn) have been investigated. It is found that the effect of dark decay due to electron tunneling, which is the limiting factor of the highest practical doping level, is less in LiNbO3 :Mn than in LiNbO3 :Fe, and higher doping levels can be used in LiNbO3 :Mn to achieve larger dynamic range and sensitivity for holographic applications. The highest practical doping level in LiNbO3 :Mn has been found to be ;0.5 wt.% MnCO3 , and refractive-index changes and sensitivities up to 1.5 3 1023 and 1.3 cm/J are measured for extraordinarily polarized light of the wavelength 458 nm. It has been found that, in terms of both dynamic range (or refractive-index change) and sensitivity, the optimal oxidation state is highly oxidized. The distribution coefficient of Mn has been determined to be ;1. Absorption measurements are used to obtain more information about charge-transport parameters. The material is excellently suited for holographic recording with blue light. The hologram quality is outstanding because holographic scattering is much weaker compared with that in, e.g., iron-doped lithium niobate. Thermal fixing has been successfully demonstrated in LiNbO3:Mn crystals.
Wengler M C, Müller M, Soergel E, Buse K
Poling dynamics of lithium niobate crystals
2003 Appl Phys B-lasers O, Band: 76, Seite: 393
KurzfassungFerroelectric domain reversal via electric field poling of congruently melting lithium niobate (LiNbO3) crystals is investigated. An electro-optic interferometric observation technique reveals spatial and temporal dynamics of the poling process. Starting from seeds, the domains grow until the entire crystal has a switched polarization. During the switching process the boundaries are preferentially aligned along the crystallographic axes. The coercive field between two sequenced domain inversions is transiently reduced after a poling event, and recovers exponentially with a time constant of about half a minute. No light-induced change of the recovery time constant, neither with green nor with ultraviolet light, is observed. The results are of relevance for domain engineering of LiNbO3 crystals.
Bertram R P, Soergel E, Blank H, Benter N, Buse K, Hagen R, Kostromine S G
Strong electro-optic effect in electrically-poled photoaddressable polymers
2003 J Appl Phys, Band: 94, Seite: 6208
KurzfassungElectrically poled photoaddressable polymers, originally designed for optical data storage, are investigated in an interferometric setup. After optimization of the corona poling conditions, we obtain Pockels coefficients r33 of 44 pm/V at 1555 nm. Even at nonoptimum temperatures, the material can be electrically poled quite efficiently using light to trigger trans–cis isomerization cycles. At room temperature, the degree of poling is stable over months. At elevated temperatures, after a fast initial decay, again stable or very slowly decaying electro-optic coefficients are observed.
Müller M, Soergel E, Buse K
Visualization of ferroelectric domains with coherent light
2003 Opt Lett, Band: 28, Seite: 2515
KurzfassungIllumination of a lithium niobate or lithium tantalate crystal along the crystallographic c axis with coherent light while simultaneously applying an external electrical field through transparent electrodes allows real-time, in situ, nondestructive monitoring of ferroelectric domain patterns. Imaging of the optical near-field through a lens directly visualizes the domain walls, whereas the far-field yields averaged information about the spatial orientation of the domain boundaries.
Berben D, Sturman B, Freschi A A, Buse K
X-Ray-induced conductivity in iron-doped lithium-niobate crystals
2003 Phys Rev B, Band: 68, Seite: 035120
KurzfassungIllumination of photorefractive, iron-doped lithium niobate crystals (LiNbO3 :Fe) with x-rays generates a conductivity that we determine from the speed of hologram erasure. The doping levels of the crystals and the acceleration voltage of our x-ray tube are varied. A theoretical model is presented, which describes the obtained results. A decrease of the conductivity with increasing Fe2+ concentration can be explained by assuming that holes are the dominant charge carriers for this short-wavelength illumination.

2002

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Mützel M, Tandler S, Haubrich D, Meschede D, Peithmann K, Flaspöhler M, Buse K
Atom lithography with a holographic light Mask
2002 Phys Rev Lett, Band: 88, Seite: 083601
KurzfassungIn atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and nonperiodic patterns at nanometer scales.
Peithmann K, Buse K, Krätzig E
Dark conductivity in copper-doped lithium-niobate crystals
2002 Appl Phys B-lasers O, Band: 74, Seite: 549
KurzfassungThe impact of protons and copper on the dark conductivity of copper-doped photorefractive lithium niobate crystals is investigated by observation of the dark decay of holograms. The dark conductivity is significantly smaller than that of iron-doped lithium niobate. A non-mono-exponential hologram decay is observed, which needs to be considered for multiplexing of holograms.
Berben D, Andreas B, Buse K
Photorefractive X-Ray imaging
2002 Appl Phys Lett, Band: 81, Seite: 1567
KurzfassungPhotorefractive effects that are affected by x-rays are used to design an x-ray imaging system with outstanding spatial resolution. An elementary holographic reflection grating is recorded in a photorefractive iron-doped lithium niobate crystal. The x-rays yield a conductivity pattern that erases the hologram locally. The diffracted visible light now bears the x-ray image. In first experiments, structures of 25 mm size are easily resolved with a dose of 90 Gray and a signal-to-noise ratio of 6. The detector is fully reversible, has a logarithmic response, and is able to operate for hard x rays (.100 keV).
Yang Y, Buse K, Psaltis D
Photorefractive recording in LiNbO3:Mn
2002 Opt Lett, Band: 27, Seite: 158
KurzfassungThe dynamic range, sensitivity, and dark decay of holographic recording of wavelength 458 nm in LiNbO3 crystals doped with 0.2-at.% Mn with different oxidation states have been measured. The measured sensitivity is 0.5 cm/J and is found to be independent of the oxidation state, and the largest M/# obtained is 12/mm (extraordinary light polarization; light wavelength, 458 nm). This combination of very large M/# and high sensitivity is in strong contrast with results for LiNbO3:Fe for which a direct trade-off exists between M/# and sensitivity. The activation energy of the dark decay of holograms stored in these LiNbO3:Mn crystals is ~1.0 eV, which is characteristic of proton compensation and leads to a projected lifetime of holograms of three years at room temperature.
Korneev N, Flores Ramirez O, Bertram R P, Benter N, Soergel E, Buse K, Hagen R, Kostromine S G
Pyroelectric properties of electrically poled photoaddressable polymers
2002 J Appl Phys, Band: 92, Seite: 1500
KurzfassungIn this article, we present pyroelectric investigations of electrically poled side-chain polymers containing mesogenic disazobenzene chromophores using a very simple setup. Heating above the glass temperature ~about 104°C! usually eliminates the pyroelectric effect of the sample. However, in samples with an indium tin oxide/aluminum electrode design, a self-poling effect is observed. The pyroelectric coefficients of electrically poled samples reach values of up to 0.2 µC/(m2K). From this, we conclude that the strongly anisometric side chains can be reoriented, and that their degree of alignment is about 3%.
Panotopoulos G, Luennemann M, Buse K, Psaltis D
Temperature dependence of absorption in photorefractive iron-doped lithium niobate crystals
2002 J Appl Phys, Band: 92, Seite: 793
KurzfassungWe present experimental data showing a significant dependence of light absorption on temperature in photorefractive LiNbO3 :Fe crystals. The results are successfully explained by assuming that the widths of the Fe2+ absorption bands in the visible and in the infrared spectral region depend on temperature. The findings are of relevance for thermal fixing of holograms. Furthermore, a temperature-induced increase of the infrared absorption is promising for improved infrared recording.

2001

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Nee I, Müller M, Buse K
Development of thermally fixed photorefractive holograms without light
2001 Appl Phys B-lasers O, Band: 72, Seite: 195
KurzfassungHolographic gratings are recorded and thermally fixed in iron-doped photorefractive lithium-niobate crystals. A spatially modulated concentration of filled and empty electron traps (Fe2+ and Fe3+) yields a modulated dark conductivity. As a consequence, dark diffusion currents and dark drift currents arise. Space–charge fields and electro-optic refractive-index changes build up. An additional refractive index grating, which may originate from a modulated proton concentration, as well as a pronounced absorption grating arising from the modulated Fe2+ concentration are also observed. The dark development has practical advantages: thermally fixed holograms can be used in devices without the need to develop them freshly from time to time by illumination. Although with dark development diffraction efficiencies up to 50% are demonstrated, in general the efficiencies are smaller compared to those achieved by development with light.
Vogt H, Buse K, Hesse H, Krätzig E, García R R
Growth and holographic characterization of nonstoichiometric sillenite-type crystals
2001 J Appl Phys, Band: 90, Seite: 3167
KurzfassungWe have grown Bi12GeO20 and Bi12SiO20 single crystals from melts with different GeO2 and SiO2 contents, respectively, to investigate the influence of an intrinsic defect, the antisite defect ~Bi on Ge or Si site!, on the light-induced charge transport. The optical absorption and the effective trap density of the crystals increase with decreasing GeO2 and SiO2 content in the melt. Furthermore, a variation of the photoconductivity is observed. Our results can be described by a one-center model with the antisite defect Bi3+/4+ on Ge or Si site as dominant photorefractive center.
Yang Y, Nee I, Buse K, Psalti D
Ionic and electronic dark decay of holograms in LiNbO3:Fe crystals
2001 Appl Phys Lett, Band: 78, Seite: 4067
KurzfassungThe lifetimes of nonfixed holograms in LiNbO3:Fe crystals with doping levels of 0.05, 0.138, and 0.25 wt% Fe2O3 have been measured in the temperature range from 30 to 180 °C. The time constants of the dark decay of holograms stored in crystals with doping levels of 0.05 and 0.25 wt% Fe2O3 obey an Arrhenius-type dependence on absolute temperature T, but yield two activation energies: 1.0 and 0.28 eV, respectively. For these crystals, two different dark decay mechanisms are prevailing, one of which is identified as proton compensation and the other is due to electron tunneling between sites of Fe21 and Fe31. The dark decay of holograms stored in crystals with the doping level of 0.138 wt% Fe2O3 is the result of a combination of both effects.
Woike Th, Berben D, Imlau M, Buse K, Pankrath R, Krätzig E
Lifetime of small polarons in strontium-barium-niobate crystals doped with cerium or chromium
2001 J Appl Phys, Band: 89, Seite: 5663
KurzfassungSmall polarons are excited by pulsed illumination (l=532 nm! of strontium–barium–niobate single crystals doped with 0.66 mol% cerium or with 0.025 mol% chromium. The dark decay of the polarons is observed by monitoring the light-induced absorption at l=632.8 nm and at l=785 nm. The relaxation fulfills a stretched-exponential behavior over at least five decades. Using the Arrhenius law the temperature dependence yields the activation energy EA=(0.58+/-0.02) eV and a frequency factor of Z=(5+/-3)x10^13 s-1. Delayed double-pulse technique (l=532 nm pulse followed by a l=1064 nm pulse!)reveals a distance dependent recombination rate of the small polarons, i.e., the lifetime of a created polaron is a function of the distance to the next available deep electron trap. In SBN:Cr the R branch can be excited by pulses of l=1064 nm with energy transfer via an unknown X center. Excitation and recombination show a single-exponential temporal evolution without any significant temperature dependence.
Adibi A, Buse K, Psaltis D
System measure for persistence in holographic recording and application to singly- and doubly-doped lithium niobate
2001 Appl Optics, Band: 40, Seite: 5175
KurzfassungWe define a measure for persistence in holographic recording. Using this measure and the known measures for dynamic range and sensitivity, we compare the performance of singly-doped and doublydoped LiNbO3 crystals. We show that the range of performance that can be obtained using doubly-doped crystals is much larger than that obtained using singly-doped ones.
Adibi A, Buse K, Psaltis D
The role of carrier mobility in holographic recording in LiNbO3
2001 Appl Phys B-lasers O, Band: 72, Seite: 653
KurzfassungWe investigate the role of carrier mobility in holographic recording in LiNbO3 crystals. Both normal holographic recording (single wavelength, single trap) and twocenter recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude.
Adibi A, Buse K, Psaltis D
Theoretical analysis of two-step holographic recording with high-intensity pulses
2001 Phys Rev A, Band: 63, Seite: 023813
KurzfassungWe develop a full numerical as well as an approximate analytic solution for two-step holographic recording with high intensity pulses in LiNbO3 :Fe crystals. We find the unknown material parameters by fitting the numerical solution to the experimental results. The two important parameters that were unknown so far and found in this work are the bulk photovoltaic coefficient and absorption cross section for the excitation of the electrons from small polarons in LiNbO3 with infrared light. We show that the approximate analytic solution agrees very well with the numerical solution ~as well as the experimental results! for most practical applications. We use the analytic solution to explain the experimental observations that were not understood before.
Korneev N, Veenhuis H, Buse K, Krätzig E
Thermal fixing of holograms and their electrically assisted development in barium-calcium-titanate crystals
2001 J Opt Soc Am B, Band: 18, Seite: 1570
KurzfassungPhotorefractive space-charge gratings in barium calcium titanate crystals are studied with non-steady-state photoelectromotive force and holographic techniques in the temperature range from 20 to 160 °C. Although the crystals undergo a ferroelectric–paraelectric phase transition at 100 °C, no sharp discontinuity of the space–charge field is observed. However, a slowly compensating grating is found, and thermal fixing is demonstrated. The behavior of the fixed grating is well described by a theory that was developed for lithium niobate. The lifetime of the fixed grating depends strongly on fringe spacing and temperature, and the diffraction efficiency of the fixed grating can be drastically enhanced by application of an external electric field.
Adibi A, Buse K, Psaltis D
Two-Center holographic recording
2001 J Opt Soc Am B, Band: 18, Seite: 584
KurzfassungWe describe a two-center holographic recording method for the storage of persistent holograms in doubly doped lithium niobate crystals. We use a two-center model, and we show that our experimental observations can be explained by the model. We describe experimental methods for finding the unknown material parameters of LiNbO3:Fe:Mn crystals for the two-center model, and we discuss the optimization of two-center recording.
Berben D, Andreas B, Buse K
X-Ray-induced photochromic effects in copper-doped lithium-niobate crystals
2001 Appl Phys B-lasers O, Band: 72, Seite: 729
KurzfassungX-Ray-induced absorption changes in Cu-doped lithium niobate crystals (LiNbO3) are investigated. All induced photochromic effects are fully reversible, i.e., illumination with white light annihilates the absorption changes. With increasing doping level, a decrease in the saturation value of the absorption changes is found. Changes of the Li concentration of the crystal to a more stoichiometric composition via Li indiffusion further decreases the photochromic effect. For sufficient X-ray doses, the saturation value is independent of the radiation energy in the measured range from 50–150 kV. The spectral distribution of the X-ray-induced absorption changes and the slow dark decay indicate that the additional absorption results from an increase of the Cu+ concentration.

2000

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Buse K, Luennemann M
3D imaging: wave front sensing utilizing a birefringent crystal
2000 Phys Rev Lett, Band: 85, Seite: 3385
KurzfassungUtilizing a rotatable thin birefringent crystal and two polarizers, the three-dimensional (3D) wave front of any light wave can be measured conveniently. In first experiments phase-front distortions as small as 15 mm are detected with a dynamic range of 3 mm and a spatial resolution of 50 mm. Such a dynamic range and spatial resolution exceed the performance of conventional wave front sensors of, e.g., the Shack-Hartmann type, significantly. Furthermore, the new system is rather insensitive against mechanical instabilities in opposition to interferometric and holographic techniques.
Peithmann K, Korneev N, Flaspöhler M, Buse K, Krätzig E
Investigation of small polarons in reduced iron-doped lithium-niobate crystals by non-steady-state photocurrent techniques
2000 Phys Status Solidi A, Band: 178, Seite: R1
Berben D, Buse K, Wevering S, Herth P, Imlau M, Woike Th
Lifetime of small polarons in iron-doped lithium-niobate crystals
2000 J Appl Phys, Band: 87, Seite: 1034
KurzfassungPulsed illumination of lithium–niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi 5+ (Nb on Li site in the valence state 5+) and creates NbLi 4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium–niobate crystals with different concentrations of NbLi are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to the next available deep electron trap. Based on the new insights, tailoring of lithium–niobate crystals for nonvolatile holographic storage becomes possible.
Veenhuis H, Börger T, Peithmann K, Flaspöhler M, Buse K, Pankrath R, Hesse H, Krätzig E
Light-induced charge transport properties of photorefractive barium-calcium-titanate crystals doped with rhodium
2000 Appl Phys B-lasers O, Band: 70, Seite: 797
KurzfassungNominally pure and rhodium-doped barium-calcium- titanate crystals of the congruently melting composition Ba0.77Ca0.23TiO3 (BCT) are grown by the Czochralski technique. These crystals are characterized by holographic and conventional electrical experiments. The doped samples have an absorption maximum at 630 nm. They are hole conductive, and for intensities between 0.1 and1Wcm-2 all properties are explained within a one-center charge-transportmodel. Two-beam-coupling gain factors up to 8 cm-1 are obtained, and doping with rhodium increases the effective trap density. Although diffusion is the dominant charge-driving force, bulk photovoltaic currents also occur which are especially pronounced in the range between 460 and 640 nm.
Veenhuis H, Börger T, Buse K, Kuper C, Hesse H, Krätzig E
Light-induced charge-transport properties of photorefractive barium-calcium-titanate crystals doped with iron
2000 J Appl Phys, Band: 88, Seite: 1042
KurzfassungNominally pure and iron doped, as-grown, and thermally annealed photorefractive barium– calcium–titanate crystals of the congruently melting composition Ba0.23Ca0.77TiO3 (BCT) are investigated by holographic and conventional electrical techniques. Refractive-index changes, two-beam-coupling gains, photoconductivities, dark conductivities, and bulk-photovoltaic current densities are measured. As-grown and oxidized crystals are hole conductive and at usual illumination conditions (light wavelength 514.5 nm, light intensity between 0.1 and 1 W/cm2) all measured properties are excellently described by an one-center charge-transport model. The effective electrooptic coefficient r333 is only about 30 pm/V and thus much smaller than the value obtained from interferometric measurements. Two-beam-coupling gains as high as 7 cm-1 are achieved. Doping with iron increases considerably the effective trap density, and bulk-photovoltaic fields of the order of some kilovolts per centimeter are observed in iron-doped crystals. Typical response times of iron-doped, as-grown, or oxidized crystals are about 0.5 s at 1 W/cm2. Reduction yields electron-conductive BCT. The dark storage time increases from 6 min in the as-grown state to 3 h upon a slight reduction treatment, but decreases for strongly reduced samples. The investigation reveals that BCT will become a very promising alternative to barium–titanate crystals (BaTiO3) for many applications.
Peithmann K, Wiebrock A, Buse K, Krätzig E
Low-spatial-frequency refractive-index changes in iron-doped lithium-niobate crystals upon illumination with a focused cw laser beam
2000 J Opt Soc Am B, Band: 17, Seite: 586
KurzfassungIron-doped lithium niobate crystals are illuminated with a single continuous-wave (cw) focused green laser beam. Surface deformations, temperature distributions, and changes of the refractive index of the material are investigated by means of interferometric techniques. It turns out that light absorption causes pronounced temperature profiles in the samples, which induce pyroelectric fields. Electronic space-charge fields that compensate these pyroelectric fields remain in the crystals after the focused light is switched off and modulate, together with bulk-photovoltaic fields, the refractive index by means of the electro-optic effect. These low-spatial-frequency effects must be taken into account when focused light beams are utilized, e.g., for high-speed holographic data storage or two-beam coupling, because the effects determine an upper limit of the highest usable cw light intensities.
Peithmann K, Hukriede J, Buse K, Krätzig E
Photorefractive properties of lithium niobate volume crystals doped by copper diffusion
2000 Phys Rev B, Band: 61, Seite: 4615
KurzfassungLithium niobate volume crystals (up to 1 mm thick) are doped by indiffusion of thin layers of evaporated copper. The obtained samples are investigated by conventional electrical and holographic methods. Saturation values Dns of the refractive-index changes, bulk-photovoltaic current densities jphv , and photoconductivities sph are measured utilizing samples with copper concentrations in the range (2.2-145)x10^24 m-3. Comparison with experimental data for melt-doped lithium niobate shows that the photorefractive properties do not depend on the doping technique. Another important outcome is that the refractive-index changes saturate at 7 x 10^24 (ordinary light polarization, green light) for copper concentrations larger than 60x10^24 m-3. This is caused by a strong increase of the photoconductivity for high doping levels.
Yue X, Adibi A, Hudson T, Buse K, Psaltis D
Role of cerium in lithium niobate for holographic recording
2000 J Appl Phys, Band: 87, Seite: 4051
KurzfassungCerium-doped lithium niobate crystals are tested for holographic recording. A photochromic effect is observed in crystals doped with cerium and manganese. But two-center recording in the sample is not as effective as in iron and manganese doubly doped crystals. Photocurrent measurements in cerium and iron singly doped crystals indicate that the photovoltaic constant in the cerium-doped crystal is only one third of that of the iron-doped one. This is the main reason accounting for the low sensitivity of cerium-doped lithium niobate crystals. However, in the diffusion dominated case, i.e., for reflection geometry, cerium-doped lithium niobate may give a strong effect.
Nee I, Müller M, Buse K
Role of iron in lithium-niobate crystals for the dark-storage-time of holograms
2000 J Appl Phys, Band: 88, Seite: 4282
KurzfassungThe dark decay of holograms stored in iron-doped photorefractive lithium-niobate crystals is studied for samples containing up to 0.25 wt% Fe2O3 ~iron concentration 71x10^18 cm-3). The oxidation/ reduction state of the crystals, i.e., the concentration ratio of Fe21 and Fe31 ions, is changed in a wide range by thermal annealing. The dark decay is attributed to two effects: An ionic dark conductivity arising from mobile protons and an electronic dark conductivity caused by tunneling of electrons between iron sites. The latter is proportional to the effective trap density, i.e., to the density of charge carriers which can be moved between the iron sites. The proportionality factor is the specific dark conductivity which increases exponentially with the third root of the entire iron concentration.
Adibi A, Buse K, Psaltis D
Sensitivity improvement in two-center holographic recording
2000 Opt Lett, Band: 25, Seite: 539
KurzfassungPersistent holograms are recorded with green light in LiNbO3 crystals doped with Mn and Fe. The recording sensitivity is 20 times better than that obtained by recording with red light. Partial loss of persistence is caused by using green light for recording.

1999

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Korneev N, Mayorga D, Veenhuis H, Buse K, Krätzig E
Dynamic bulk photovoltaic effect in photorefractive barium-calcium titanate crystals
1999 J Opt Soc Am B, Band: 16, Seite: 1725
KurzfassungOscillating photocurrents are generated in a photorefractive iron-doped barium calcium titanate crystal (Ba0.77Ca0.23TiO3; BCT) by illumination with an oscillating interference pattern. Non-steady-state photocurrents arise usually from the interaction of a space-charge field with an oscillating photoconductivity pattern. However, a second-harmonic signal of the oscillating photocurrents is observed in BCT, which is present without any space-charge field. The main features of the new effect can be explained by consideration of a dynamic, i.e., time-dependent, bulk photovoltaic effect. The effect might be useful for optically addressed memories or optical correlators.
Adibi A, Buse K, Psaltis D
Effect of annealing in two-center holographic recording
1999 Appl Phys Lett, Band: 74, Seite: 3767
KurzfassungPersistent holograms are recorded with red light in lithium niobate crystals doped with manganese and iron. We find that the oxidation/reduction state of the crystal has a profound impact on the recording and readout performance. The underlying physical processes are investigated and the recording and readout responses are explained and optimized.
Buse K, Adibi A, Psaltis D
Efficient non-volatile holographic recording in doubly-doped lithium niobate
1999 J Opt A, Band: 1, Seite: 237
KurzfassungWe present an efficient, non-volatile holographic storage scheme in doubly doped lithium niobate, with red recording light and simultaneous illumination by homogeneous ultraviolet light.
Korneev N, Mayorga D, Veenhuis H, Buse K, Kuper C, Hesse H, Krätzig E
Holographic and non-steady-state photocurrent characterization of photorefractive barium-calcium titanate
1999 Opt Commun, Band: 160, Seite: 98
KurzfassungMeasurements of non-steady-statephotocurrents and of time-modulated conductivities for homogeneous illumination of varying intensity are performed with a photorefractive iron-doped barium–calciumtitanate crystal of the congruently melting composition (Ba0.77Ca0.23TiO3, BCT). The diffraction efficiency and the two-beam coupling gain are determined from conventional holographic experiments. Debye screening and diffusion lengths are found to be of the order of 0.1 μm. The response-time of the material is 2 s (light intensity 0.2 W/cm2, wavelength 514 nm) and the dark storage time is 6 s. The measured refractive index changes are only one third of the values, which are expected from the known electrooptic coefficients and space-charge fields.
Adibi A, Buse K, Psaltis D
Multiplexing holograms in LiNbO3:Fe:Mn crystals
1999 Opt Lett, Band: 24, Seite: 652
KurzfassungPersistent holograms are recorded with red light in lithium niobate crystals doped with manganese and iron. Different erasure mechanisms are investigated, and a recording schedule for multiplexing holograms with equal diffraction efficiencies is proposed. To test the recording schedule experimentally, we multiplex 50 plane-wave holograms with the proposed recording schedule.
Nee I, Buse K, Havermeyer F, Rupp R A, Fally M, May R P
Neutron diffraction from thermally fixed gratings in photorefractive lithium niobate crystals
1999 Phys Rev B, Band: 60, Seite: R9896
KurzfassungIron-doped lithium niobate crystals are illuminated with a sinusoidal light pattern (period length 374 nm) ata temperature of about 180 °C. Electrons are redistributed and ions drift in the electronic space-charge pattern (‘‘thermal fixing’’). At room temperature the ions are almost immobile. A density grating ~space-charge fieldplus inverse-piezoelectric effect! and the ionic grating yield a refractive-index modulation for neutrons (‘‘photorefractive effect’’). Neutrons (wavelength 1.39 nm) are diffracted from this grating with an efficiency up to 1.2x10-3. Usually hydrogen ions form the ionic grating. The ions responsible for charge compensation in dehydrated crystals have a much smaller coherent neutron-scattering length and might be identified as lithium.
Veenhuis H, Buse K, Krätzig E, Korneev N, Mayorga D
Non-steady-state photoelectromotive force in reduced lithium niobate crystals
1999 J Appl Phys, Band: 86, Seite: 2389
KurzfassungNon-steady-state photocurrents are generated in reduced iron-doped lithium niobate crystals. Dependences on spatial frequency and on modulation amplitude are well described by the existing theory and a Debye screening length of 19 nm is determined. The non-steady-state photocurrents strongly decrease for frequencies above 2 Hz. The technique might be useful for material characterization and for convenient active stabilization of holographic setups against mechanical vibrations.
Imbrock J, Wevering S, Buse K, Krätzig E
Nonvolatile holographic storage in photorefractive lithium tantalate crystals with laser pulses
1999 J Opt Soc Am B, Band: 16, Seite: 1392
KurzfassungNonvolatile holograms are recorded in photorefractive LiTaO3 :Fe with laser pulses by use of two-step excitation. Ultraviolet laser pulses (wavelength l = 355 nm) yield a strong increase of absorption (as much as 600 m-1 at l = 633 nm) and sensitize the crystals for subsequent infrared (l = 1064 nm) holographic recording. Refractive-index changes of as much as 1.6 x 10^24 are achieved for intensities of the infrared light of 10^11 W/m^2. The saturation values are proportional to the concentration of Fe3+ ions. Nondestructive readout with infrared light is possible, and the holograms remain erasable for ultraviolet light. Typical time constants of recording and erasure are 0.5 µs for intensities of the ultraviolet light of 10^11 W/m^2. The results can be explained with a two-level charge-transport model.
van Stevendaal U, Buse K, Malz H, Krätzig E
Persistent light-induced absorption in oxidized iron-doped barium titanate crystals
1999 Opt Lett, Band: 24, Seite: 908
KurzfassungIllumination of oxidized iron-doped BaTiO3 crystals with visible or ultraviolet light yields absorption changes of as much as 1300 m-1 that are stable at room temperature. These photochromic effects cannot be erased by light, but heating the crystal to a moderate temperature (100 °C) can switch it back to its initial state. These effects, in particular the optical irreversibility, might be of interest, e.g., for optical control of charge-transport properties and for persistent data storage.
Peithmann K, Wiebrock A, Buse K
Photorefractive properties of highly doped lithium niobate crystals in the visible and near-Infrared
1999 Appl Phys B-lasers O, Band: 68, Seite: 777
KurzfassungLight-induced refractive-index changes, bulkphotovoltaic current densities, and photoconductivities of photorefractive iron-doped lithium niobate crystals (iron concentrations between 0.02 and 0.17 wt: %) are investigated in detail using visible and near-infrared light. It turns out that the one-center model predicts the material performance correctly for small iron concentrations (cFe < 0.06 wt:%Fe2O3), only. A strong increase of the photoconductivity for higher doping levels limits the space-charge fields. Refractive-index changes up to 7x10−4 for green and 2.8x10−4 for nearinfrared ordinarily polarized light are obtained. The corresponding hologram multiplexing numbers are 11 for green and 5 for near-infrared light.

1998

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Korneev N, Mayorga D, Stepanov S, Gerwens A, Buse K, Krätzig E
Characterization of photorefractive strontium-barium niobate with non-steady-state holographic photocurrents
1998 Opt Commun, Band: 146, Seite: 215
KurzfassungMeasurements of non-steady-stateholographicphotocurrents and conventional modulated photocurrents are performed with a n-type photorefractivestrontium-bariumniobate crystal at light wavelength of 532 nm. The effective charge carrier density and the photocarrier diffusion lengths for two orthogonal directions in the crystal are deduced. A ratio of the photoelectron mobilities along the c and a axis of 2.4 is obtained. The dependence of the photocurrent signals on the modulation frequency points to an essential contribution of shallow traps to the charge transport.
Buse K, Gerwens A, Wevering S, Krätzig E
Charge transport parameters of photorefractive strontium-barium niobate crystals doped with cerium
1998 J Opt Soc Am B, Band: 15, Seite: 1674
KurzfassungA complete set of charge-transport parameters of photorefractive cerium-doped strontium-barium niobate crystals is determined: the concentrations of filled and empty traps, the photon-absorption cross section, the quantum efficiency for excitation of an electron upon absorption of a photon, the recombination coefficient, and the charge-carrier mobility. These parameters are obtained evaluating optical absorption, conductivity, nonsteady- state photocurrents, neutron-activation analysis, and x-ray photoelectron spectra. The effective absorption radius of the filled electron traps is small and the effective trapping radius is large compared with the ionic radii. Before recombination with deep levels the excited electrons are trapped several times by shallow levels. A detailed description of the charge transport is obtained, and the properties of optimized crystals are deduced.
Yue X, Buse K, Mersch F, Krätzig E, Rupp R A
Diffraction-efficiency enhancement of photorefractive gratings in Bi4Ti3O12 at low temperatures
1998 J Opt Soc Am B, Band: 15, Seite: 142
KurzfassungPreillumination of Bi4Ti3O12 at low temperature causes a transient enhancement of diffraction efficiency for subsequent holographic recording. Diffraction efficiencies are measured that are up to ten times larger than the stationary value. The enhancement depends on the intensity of the preillumination, on the dark delay time, and on the crystal temperature. The effect can be explained considering electron–hole competition and contributions of different levels to the charge transport.
Kuper Ch, Buse K, van Stevendaal U, Weber M, Leidlo T, Hesse H, Krätzig E
Electrooptic and photorefractive properties of ferroelectric barium-calcium titanate crystals
1998 Ferroelectrics, Band: 208, Seite: 213
KurzfassungBarium-calcium titanate of congruently melting composition Ba0·77Ca0·23TiO3 is a novel photorefractive crystal. The refractive, electrooptic and photorefractive properties of an undoped as grown sample are investigated: The refractive indices are slightly smaller than those of BaTiO3, but the electrooptic coefficients r 113 = 36 ± 3 pm V−1 and r 333 = 140 ± 5 pm V−1 (light wavelength 514·5 nm) are larger. Light-induced absorption and a sublinear photoconductivity indicate the participation of a shallow level in the charge transport. There are no space charge limiting effects; bulk photovoltaic fields of the order of 15 kV m−1 are present. The obtained two-beam coupling gain coefficients for ordinarily and extraordinarily polarized light are 160 m−1 and 425 m−1, respectively. In summary, barium-calcium titanate is a very promising crystal which will compete with barium titanate.
Korneev N, Mayorga D, Stepanov S, Gerwens A, Buse K, Krätzig E
Enhancement of the photorefractive effect by homogeneous pyroelectric fields
1998 Appl Phys B-lasers O, Band: 66, Seite: 393
KurzfassungSpatially uniform heating or cooling of a photorefractive strontium–barium niobate crystal yields large pyroelectric fields. Two-beam coupling measurements and observation of beam intensity profiles behind the crystal reveal that drift of charge carriers in the pyroelectric field strongly enhances the photorefractive effect. Illumination by a continuous wave laser beam causes large pyroelectric fields via absorption and heating. The drift in pyroelectric fields must be taken into account as a transport mechanism if pyroelectric crystals are used. This mechanism is promising for novel photorefractive devices.
Gerwens A, Buse K, Krätzig E, Korneev N, Stepanov S
Hall measurements with photorefractive strontium-barium niobate by application of a non-steady-state photocurrent method
1998 J Opt Soc Am B, Band: 15, Seite: 2143
KurzfassungPhotoinduced Hall currents are generated in a photorefractive strontium barium niobate crystal by excitation of non-steady-state holographic currents and by application of a modulated external magnetic field. Electrons are the dominant photoinduced charge carriers, and the Hall mobilities m for migration along and perpendicular to the c axis are determined as mic = (4 +/- 1) x 10-6 m^2 V-1 s-1 and m'c = (1.3 +/- 0.3) x 10-6 m^2 V-1 s-1.
Breer S, Buse K, Rickermann F
Improved development of thermally fixed holograms in photorefractive LiNbO3 crystals with high-intensity laser pulses
1998 Opt Lett, Band: 23, Seite: 73
KurzfassungHolographic gratings were thermally fixed in iron-doped photorefractive LiNbO3 crystals and developed with frequency-doubled Q-switched pulses of a Nd:YAG laser (light wavelength, 532 nm; pulse duration, 3 ns). The saturation values of the diffraction efficiency increased with increasing pulse light intensity. Compared with development with cw light, high-intensity laser pulse development resulted in an enlargement of the diffraction efficiency of a factor of ,2. A contribution of the intrinsic defect Nb on Li site to the charge transport is most probably the origin of this effect. The results are useful, e.g., for improvements of volume holographic memories and wavelength filters.
Peithmann K, Wiebrock A, Buse K
Incremental holographic recording in lithium niobate with active phase locking
1998 Opt Lett, Band: 23, Seite: 1927
KurzfassungAngular-multiplexed hologram recording in iron-doped lithium niobate crystals was carried out with nearinfrared light. An incremental recording schedule with active phase locking of the light pattern onto the hologram was used. Continuous and reproducible recording of holograms of equal efficiency was achieved, and a hologram multiplexing number, M/# 2, for a 5-mm-thick crystal was obtained at a 760-nm wavelength of light.
Breer S, Vogt H, Nee I, Buse K
Low crosstalk WDM by Bragg diffraction from thermally fixed reflection holograms in lithium niobate crystals
1998 Electron Lett, Band: 34, Seite: 2419
KurzfassungDense wavelength-division-multiplexing (WDM) systems require narrowband wavelength filters for simultaneous multiplexing/demultiplexing of several channels. As a proof of concept the authors report the demultiplexing of two channels (channel spacing 0.8 nm) by Bragg diffraction from two thermally fixed volume phase reflection holograms in photorefractive lithium niobate crystals doped with iron. More than 25 dB crosstalk suppression is achieved, and intensity losses are only between 2.3 and 5.2 dB
Buse K, Adibi A, Psaltis D
Non-volatile holographic storage in doubly-doped lithium niobate
1998 Nature, Band: 393, Seite: 665
KurzfassungPhotorefractive materials are being widely investigated for applications in holographic data storage1. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known2, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed 3–6, but they have practical disadvantages and only laboratory demonstrators have been built7–10. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate—available both in large size and with excellent homogeneity—that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.
van Stevendaal U, Buse K, Malz H, Veenhuis H, Krätzig E
Reduction of light-induced refractive-index changes by a decreased modulation of light patterns in photorefractive crystals
1998 J Opt Soc Am B, Band: 15, Seite: 2868
KurzfassungIn holographic experiments many photorefractive crystals show refractive-index changes much smaller than one would expect from the known electro-optic coefficients and space-charge fields. We show that degradation of the interference pattern is the origin of this effect. The degree of modulation of a light grating in photorefractive crystals is measured by three methods and compared with the modulation of the grating of the incident light. All methods, measurement of the amplitudes of fundamental and second-order gratings, of the grating amplitudes in a crystal with and without another crystal in front of it, and of the drift currents through an inhomogeneously illuminated sample, yield consistent results: In lithium niobate there is almost no degradation of the light pattern. However, in our barium titanate and potassium tantalate–niobate samples the degree of modulation is smaller, and the reduction factor is 0.55 and 0.60, respectively. Inhomogeneities of refractive-index changes are shown to be the origin of the effect.
Stepanov S, Korneev N, Gerwens A, Buse K
Self-diffraction from free surface relief gratings in a photorefractive Bi12TiO20 crystal
1998 Appl Phys Lett, Band: 72, Seite: 879
KurzfassungExperimental results on self-diffraction of periodically phase-modulated laser beams from a relief grating formed on a free surface of a cubic photorefractive Bi12TiO20 crystal due to the piezoelectric effect are reported. Periodic intensity modulations of the reflected beams up to 10-2 due to energy exchange at this unshifted phase grating are measured. The modulation depends linearly on the fringe spacing and on the amplitude of an externally applied electric dc field. The observed self-diffraction process is independent of the light polarization.
Breer S, Buse K, Peithmann K, Vogt H, Krätzig E
Stabilized recording and thermal fixing of holograms in photorefractive lithium niobate crystals
1998 Rev Sci Instrum, Band: 69, Seite: 1591
KurzfassungHolograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 °C. The setup is actively stabilized during recording against movements or vibrations of the interference pattern which especially occur during long-period writing at enhanced temperatures. Two different techniques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilization. (2) Alternatively, one of the beams is periodically phase modulated and the beam-coupling signal is used for stabilization. Reproducible refractive index changes of thermally fixed holograms up to 7.5x10-4 are obtained with both stabilization techniques. However, the second method is advantageous for multiplexing experiments, because no readjustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed.
Wevering S, Buse K, Simon M, Pankrath R, Krätzig E
Time-resolved measurements of photoconductivity in cerium-doped photorefractive strontium-barium niobate using nanosecond light pulses
1998 Opt Commun, Band: 148, Seite: 85
KurzfassungTime-resolved measurements of photoconductivity in photorefractive strontium–barium niobate crystals doped with cerium are performed. The crystals are illuminated with nanosecond light pulses of a Q-switched frequency-doubled Nd:YAG laser and an external electric field is applied to the samples. Light-induced drift current is amplified and detected with a fast oscilloscope. The relaxation time of photoconductivity yields the lifetime of electrons in the conduction band. The carrier lifetime decreases and the photoconductivity increases with increasing cerium concentration in the crystals. The observed dependences and previous measurements of integrated conductivity are explained considering the participation of shallow levels in the charge transport.
Breer S, Buse K
Wavelength demultiplexing with volume phase holograms in photorefractive lithium niobate
1998 Appl Phys B-lasers O, Band: 66, Seite: 339
KurzfassungThe concept is discussed of wavelength demultiplexing with volume-phase holograms that are superimposed in a photorefractive lithium niobate crystal, and first experimental results are presented. From theoretical considerations, it turns out that in the telecommunication wavelength region around 1550 nm, simultaneous and polarization-independentdemultiplexing of 40 channels can be achieved with losses of less than 1 dB, a channel spacing of 0.8 nm and a cross-talk of less than -35 dB. Superposition of many holograms with high diffraction efficiency requires thermal fixing. Proof-of-concept experiments at room temperature are performed where holographic gratings are written by plane waves of green light. Rotation stages enable precise adjustment of orientation and spatial frequency of the gratings. Demultiplexing of a beam with light of the wavelengths 1558.0 and 1558.8 nm is demonstrated, where the losses are about 15 dB and cross-talk suppression up to 20 dB is achieved.

1997

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Rickermann F, Riehemann S, von Bally G, Breer S, Buse K
A high resolution real-time temporal heterodyne interferometer for refractive index topography
1997 Opt Commun, Band: 144, Seite: 173
KurzfassungA Mach-Zehnder interferometer for two-dimensional refractiveindextopography is presented which utilizes the temporalheterodyne technique. High scanning rates and excellent phase resolutions are obtained using an image dissector camera and a lock-in amplifier. The setup is applied for investigation of the photorefractive effect. A macroscopic light pattern is recorded and thermally fixed in iron-doped LiNbO3. The evolution of the refractiveindex pattern during development by homogeneous illumination is observed with the interferometer. The measured refractiveindex profiles are in good agreement with the results of computer simulations.
Gerwens A, Simon M, Buse K, Krätzig E
Activation of cerium-doped strontium-barium niobate for infrared holographic recording
1997 Opt Commun, Band: 135, Seite: 347
KurzfassungHomogeneous illumination of Sr1−xBaxNb2O6:Ce (0.25 ≤ x ≤ 0.75; SBN) crystals with a green light pulse allows subsequent recording of a holographic grating with infrared light pulses. The grating can be read nondestructively with infrared light and can be erased by illumination with green light. The results indicate that the charge transport in Sr1−xBaxNb2O6:Ce must be described by a two-level model.
Gerwens A, Buse K, Krätzig E
Alternating photocurrents and modulated photoconductivities in photorefractive sillenite crystals
1997 Ferroelectrics, Band: 202, Seite: 203
KurzfassungThe techniques of alternating photocurrents and modulated photo-conductivities are utilized for the characterization of BTO and BGO crystals. A vibrating interference pattern creates alternating photocurrents. Homogeneous illumination with a varying intensity yields a modulated photoconductivity. Frequency dependent measurements are performed. The results of both techniques agree rather well and charge carrier mobilities are estimated.
Mazur A, van Stevendaal U, Buse K, Weber M, Schirmer O F, Hesse H, Krätzig E
Light-induced charge transport processes in photorefractive barium titanate crystals doped with iron
1997 Appl Phys B-lasers O, Band: 65, Seite: 481
KurzfassungIron–doped photorefractive barium titanate crystals (BaTiO3) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe2+/Fe3+. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm−2. A relation between the absorption constant and the Fe2C concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO3:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm−2 are estimated.
Buse K, Breer S, Peithmann K, Kapphan S, Gao M, Krätzig E
Origin of thermal fixing in photorefractive lithium niobate crystals
1997 Phys Rev B, Band: 56, Seite: 1225
KurzfassungMacroscopic light patterns are thermally fixed in photorefractive iron- or copper-doped LiNbO3 crystals with different hydrogen concentrations. Spatially resolved absorption measurements reveal unambiguously that migrating hydrogen ions compensate for the space-charge fields at enhanced temperatures if the hydrogen concentration is large enough. However, in crystals with small hydrogen concentrations thermal fixing is also possible and another compensation mechanism appears. Partial depolarization of the crystals does not contribute to the charge compensation processes. The refractive-index pattern is developed by homogeneous illumination and measured by an interferometric technique. The development process is described considering the spatially modulated concentrations of filled and empty electron traps, which cause spatially modulated currents, even for homogeneous illumination.
Simon M, Wevering St, Buse K, Krätzig E
The bulk photovoltaic effect of photorefractive LiNbO3:Fe crystals at high light Intensities
1997 J Phys D Appl Phys, Band: 30, Seite: 144
KurzfassungDirect measurements of the bulk photovoltaic current density jphv in photorefractive LiNbO3:Fe,LiNbO3:Mg,Fe and LiNbO3:Zn,Fe crystals were performed at high light intensities. Illumination of the crystals with light pulses of a Q-switched frequency-doubled Nd:YAG laser (pulse duration 20 ns; light wavelength 532 nm) yielded a photovoltaic current through the crystals which charged a capacitor. Afterwards an electrometer measured the voltage of the charged capacitor. From this voltage and from capacity and exposure time the current density jphv was deduced. For intensities above 10^7 W m−2 a contribution to jphv which increased quadratically with light intensity I appeared for LiNbO3:Fe crystals. Co-doping with Mg and Zn lowered the quadratic component. Our results are in good qualitative and quantitative agreement with previous holographic measurements and support the claim that the intrinsic defect Nb4+/5+ Li acts as a secondary photorefractive centre in LiNbO3 at high light intensities.

1996

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Freschi A A, Garcia P M, Rasnik I, Frejlich J, Buse K
Avoiding hologram bending in photorefractive crystals
1996 Opt Lett, Band: 21, Seite: 152
KurzfassungDynamic coupled-wave theory predicts the bending of recorded hologram phase planes in most photorefractive crystals. Bent holograms occur in LiNbO3 and other photovoltaic crystals that are particularly interesting as holographic storage media and result in a reduced overall diffraction eff iciency. We show that hologram bending in LiNbO3 can be avoided or at least sensibly reduced by use of an actively stabilized recording technique.
Rickermann F, Riehemann S, Buse K, Dirksen D, von Bally G
Diffraction efficiency enhancement of holographic gratings in Bi12Ti0.76V0.24O20 crystals after recording
1996 J Opt Soc Am B, Band: 13, Seite: 2299
KurzfassungAn enhancement of the diffraction efficiency of holographic gratings in Bi12Ti0.76V0.24O20 crystals by a factor of almost 40 is obtained after recording by application of an external dc electric field and illumination with a spatially homogeneous light beam. The enhancement increases with increasing applied field, larger spatial frequency, and longer writing time of the holographic grating, and with intensity of the homogeneous illumination. A theoretical description of the enhancement effect is proposed, assuming a modulated photoconductivity, which is generated by a strongly modulated concentration of photorefractive traps or donors.
van Stevendaal U, Buse K, Kämper S, Hesse H, Krätzig E
Light-induced charge transport processes in photorefractive barium titanate doped with rhodium and iron
1996 Appl Phys B-lasers O, Band: 63, Seite: 315
KurzfassungAbsorption, light-induced absorption changes, photo and dark conductivities, bulk photovoltaic currents and two-beam coupling gain coefficients are measured for oxidized, as grown and reduced BaTiO3:Rh,Fe crystals. The oxidized sample shows hole conductivity and three photorefractive levels are present: Rh4+/5+, Fe4+/5+ and Rh3+/4+. The measurements indicate that Rh3+/4+ is responsible for the photo conductivity and that its energy level is located (0.9 +/- 0.1) eV above the valence band edge. The as grown sample also shows hole conductivity and two photorefractive levels are involved: Rh3+/4+ and Fe3+/4+. The results yield that the Fe3+/4+ level is located (0.95 +/- 0.05) eV above the valence band edge. In the reduced sample electrons are the dominant charge carriers and two levels are of importance. The shallow one is located (0.39 +/- 0.05) eV below the conduction band edge. Here the performance of this crystal is strongly impaired by a pronounced sublinear photo conductivity.
Buse K, van Stevendaal U, Pankrath R, Krätzig E
Light-induced charge transport properties of Sr0.61Ba0.39Nb2O6:Ce crystals
1996 J Opt Soc Am B, Band: 13, Seite: 1461
KurzfassungMeasurements of absorption, photoconductivity, and dark conductivity and of light-induced absorption changes in photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals with Ce concentrations from 0.025 wt% to 1.6 wt% are carried out for different light wavelengths, intensities, and polarizations. The experimental findings are interpreted alternatively by the two-center or by the three-valence model. The results yield that the concentrations of the deep and of the more shallow photorefractive levels increase linearly with the Ce concentration and show that for SBN:Ce the two-center model is probably more appropriate.
Weber M, van Stevendaal U, Buse K, Zhang Z G, Yin A M, Fu P M, Ding Y, Eichler H J
Light-induced charge transport properties of potassium niobate crystals doped with nickel
1996 Phys Status Solidi A, Band: 156, Seite: 433
KurzfassungMeasurements of absorption, light-induced absorption changes, photoconductivity, and bulk photovoltaic currents are carried out for different light wavelengths and intensities with a potassium niobate crystal (KNbO3) containing 90 mol ppm Ni. Absorption bands around 2.1 and 2.7 eV are present. The band around 2.1 eV yields no significant contribution to the photoconductivity but causes appreciable bulk photovoltaic currents. The photoconductivity is larger than in KNb03 : Fe which makes KNb03 : Ni promising for holographic applications. A nonlinear intensity dependence of the photoconductivity and of the bulk photovoltaic current density indicates that more than one photorefractive level contribute to the charge transport processes. The bulk photovoltaic currents are negligible in usual holographic experiments.
Simon M, Buse K, Pankrath R, Krätzig E, Freschi A A
Photoconductivity of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals at high light intensities
1996 J Appl Phys, Band: 80, Seite: 251
KurzfassungPhotorefractive Sr0.61Ba0.39Nb2O6 (SBN) is a promising optical material.1 Recently SBN crystals were investigated for various applications, e.g. for volume holographic storage,2 self- pumped phase conjugation,3,4 light modulation, 5 spatial filtering,6 and for photoassisted switching of ferroelectric domains.7 Photorefractive properties and lightinduced charge transport are of special interest. At pulse laser intensities new effects arise, e.g. an enhancement of the photorefractive sensitivity by acceleration of the charge carriers in pyroelectric fields.8,9 Furthermore, high intensities as an extreme case are useful to obtain detailed information on the light-induced charge transport properties. The photoconductivity plays a crucial role in the photorefractive effect. In this article we develop a special method for direct photoconductivity measurements and investigate SBN crystals doped with Ce under the influence of high intensity light pulses.
Buse K, Loheide S, Sabbert D, Krätzig E
Photorefractive properties of tetragonal KTa0.52Nb0.48O3:Fe crystals and explanation with the three-valence charge-transport model
1996 J Opt Soc Am B, Band: 13, Seite: 2644
KurzfassungPhotoconductivities, dynamics of light-induced absorption changes, and dark relaxations of holographic gratings are measured in as-grown and oxidized tetragonal potassium tantalate niobate crystals (KTa0.52Nb0.48O3) doped with iron. A sublinear intensity dependence of photoconductivity, buildup, steady-state value, and relaxation of the induced absorption changes, nonexponential dark relaxations of holographic gratings, and further holographic measurements are well explained, qualitatively and quantitatively, in terms of the threevalence charge-transport model, i.e., considering one impurity center occurring in three different valence states. Most probably the responsible photorefractive center is Fe3+,4+,5+.
Buse K, Havermeyer F, Glabasnia L, Schlömp K, Krätzig E
Quadratic polarization-optic coefficients of cubic KTa1-xNbxO3 crystals
1996 Opt Commun, Band: 131, Seite: 339
KurzfassungQuadraticpolarization-opticcoefficients of paraelectric KTa1−xNbxO3crystals with different mole fractions x are measured by an interferometric technique. The coefficient g33 is within ± 10% independent of temperature in the range 10°C < T − TC < 60°C, but shows a linear dependence on the mole fraction x: in the investigated range 0.35 < x < 0.41, the relation g33 = (−1.72 x + 0.80) m4A−2 s−2 is valid.
Kip D, Aulkemeyer S, Buse K, Mersch F, Pankrath R, Krätzig E
Refractive indices of Sr0.61Ba0.39Nb2O6 single crystals
1996 Phys Status Solidi A, Band: 154, Seite: K5
Niemann R, Buse K, Pankrath R, Neumann M
XPS study of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals
1996 Solid State Commun, Band: 98, Seite: 209
KurzfassungX-ray photoelectron spectroscopy (XPS) measurements show that Ce occurs in photorefractive Sr0.61Ba0.39Nb2O6 (SBN) crystals mainly in the valence state 3+. We give arguments that Ce3+/4+ source is the photorefractive center in SBN:Ce with a typical concentration ratio c_Ce3+/c_Ce4+=600. Quantitative analysis of the XPS data yields a Ce distribution coefficient of about 0.6 ± 0.3.

1995

nach oben zur Jahresübersicht

Buse K, Bierwirth T
Dynamics of light-induced absorption in BaTiO3 and application for intensity stabilization
1995 J Opt Soc Am B, Band: 12, Seite: 629
KurzfassungIllumination of a photorefractive BaTiO3 crystal provides an induced absorption increase, which is explained by the two-center charge transport model. Detailed investigations of buildup and decay of light-induced absorption are performed. Intensity changes of the incoming wave are attenuated by these absorption changes. The requirements for distinct intensity stabilization are theoretically estimated. Intensity stabilization is experimentally demonstrated, and a decrease of the intensity fluctuations of as much as 30% has been achieved.
Buse K, Jermann F, Krätzig E
Infrared holographic recording in LiNbO3:Fe and LiNbO3:Cu
1995 Opt Mater, Band: 4, Seite: 237
KurzfassungHolograms may be recorded in photorefractive LiNbO3:Fe and LiNbO3:Cu with infrared light pulses (wavelength λ = 1064 nm, pulse duration tp = 20 ns), if the crystals are previously or simultaneously illuminated with green light pulses (λ = 532 nm, tp = 15 ns). Refractive index changes and time constants are studied for crystals with different iron and copper concentrations after thermal reduction and oxidation treatments. A model explaining this effect is discussed.
Jermann F, Buse K, Simon M, Krätzig E
Laser-induced transient gratings in LiNbO3:Fe
1995 Opt Mater, Band: 4, Seite: 318
KurzfassungHolographic gratings are recorded in LiNbO3:Fe using frequency-doubled pulses of a Q-switched Nd:YAG laser (light wavelength λ = 532 nm, pulse duration tp = 20 ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe2+ concentration, light intensity, and fringe spacing show that different types of gratings are involved: amplitude, thermo-optic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect partially compensate each other. In LiNbO3:Fe the photovoltaic effect is the dominant charge driving force for our recording conditions.
Sochava S L, Buse K, Krätzig E
Photoinduced hall-current measurements in photorefractive sillenites
1995 Phys Rev B, Band: 51, Seite: 4684
KurzfassungHall currents are excited in the photorefractive sillenite crystals BSO, BGO, and BTO [Bi12Si(Ge,Ti)O20] using an externally applied magnetic field and an internal electric field of a space charge grating. We measure Hall mobilities of photoexcited electrons and obtain the values μ = (5.5±1.0) cm2V-1s-1 for BSO and BGO and μ = (2.5±0.5) cm2V-1s-1 for BTO.
Garcia P M, Buse K, Kip D, Frejlich J
Self-stabilized holographic recording in LiNbO3:Fe crystals
1995 Opt Commun, Band: 117, Seite: 235
KurzfassungWe describe the self-stabilizedholographicrecording in Fe-doped LiNbO3crystals reaching almost 100% diffraction efficiency using weak interfering beams of 514.5 nm wavelength, even for relatively low light pattern modulation values. The evolution of the recording process is shown to be adequately described by a simple mathematical formulation based on Kogelnik's coupled-wave theory in spite of the dynamic nature of the recording process. Crystal parameters computed from this experiment are compared to the available data from literature.
Buse K, Krätzig E
Three-valence charge-transport model for explanation of the photorefractive effect
1995 Appl Phys B-lasers O, Band: 61, Seite: 27
KurzfassungA new charge-transport model assuming one center in three different valence states is discussed for explanation of the photorefractive effect. Quantitative description of experimental results in KNb03:Fe by the model is demonstrated. Many similarities with the so-called ldquotwo-centerrdquo model are found although the microscopic explanation of the light-induced charge transport is rather different.
Buse K, Kämper S, Frejlich J, Pankrath R, Ringhofer K H
Tilting of holograms in photorefractive Sr0.61Ba0.39Nb2O6 crystals by self-diffraction
1995 Opt Lett, Band: 20, Seite: 2249
KurzfassungHolographic gratings are written in photorefractive Sr0.61Ba0.39Nb2O6 crystals by two interfering light beams. Angular-dependent readout of the holograms shows that applying an external electric f ield to the crystal produces a pronounced tilting of the holograms. The results are in good agreement with theoretical predictions considering self-diffraction of the recording beams.

1994

nach oben zur Jahresübersicht

Sochava S, Buse K, Krätzig E
Characterization of photorefractive KNbO3:Fe by non-steady-state photocurrent techniques
1994 Opt Commun, Band: 105, Seite: 315
KurzfassungNon-steady-state photocurrent measurements are carried out with a n-type photorefractive KNbO3:Fe crystal at the light wavelength 488 nm. Spatial frequency dependent measurements yield the effective trap density. From the dielectric relaxation time for different crystal orientations we deduce the mobility ratios for charge carrier migration parallel to the a-, b- and c-axis.
Buse K, Jermann F, Krätzig E
Infrared holographic recording in LiNbO3:Cu
1994 Appl Phys A-mater, Band: 58, Seite: 191
KurzfassungHolograms may be recorded in photorefractive LiNbO3:Cu with pulsed infrared light (wavelength lambda=1064 nm, Q-switched Nd:YAG laser), if the crystals are previously or simultaneously illuminated with a green (lambda=532 nm) light pulse. We study refractive index changes and time constants of as-grown and thermally treated crystals with different copper concentrations. A model explaining this effect is discussed.
Jermann F, Buse K
Light-induced Thermal Gratings in LiNbO3:Fe
1994 Appl Phys B-lasers O, Band: 59, Seite: 437
KurzfassungHolographic gratings are recorded in LiNbO3: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelengthlambda = 532 nm, pulse durationt p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe2+ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO3: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.
Buse K, Krätzig E
Photoconductivity of KNbO3 and BaTiO3 at high light Intensities
1994 Phys Status Solidi A, Band: 146, Seite: K37
Buse K, Hesse H, van Stevendaal U, Loheide S, Sabbert D, Krätzig E
Photorefractive properties of ruthenium-doped potassium niobate
1994 Appl Phys A-mater, Band: 59, Seite: 563
KurzfassungPhotoconductivities and photovoltaic currents of ruthenium-doped KNbO3 are orders of magnitude larger than that of undoped and ion-doped crystals. KNbO3:Ru is very sensitive for holographic recording with red light and the photovoltaic current increases sublinearly with light intensity.
Buse K, Pankrath R, Krätzig E
Photorefractive properties of tetragonal potassium tantalate-niobate crystals at high light Intensities
1994 Opt Commun, Band: 104, Seite: 325
KurzfassungPhotorefractive properties of an iron-doped tetragonal KTa0.52Nb0.48O3 single crystal are investigated with Q-switched light pulses (wavelength 532 nm). Pyro-electric fields significantly enhance light-induced refractive index changes.
Bartholomäus T, Buse K, Deuper C, Krätzig E
Pyroelectric coefficients of LiNbO3 crystals of different compositions
1994 Phys Status Solidi A, Band: 142, Seite: K55
Buse K, Pankrath R, Krätzig E
Pyroelectrically-induced photorefractive effect in Sr0.61Ba0.39Nb2O6:Ce
1994 Opt Lett, Band: 19, Seite: 260
KurzfassungPhotorefractive properties of a cerium-doped strontium barium niobate single crystal (Sr0.61Ba0.39Nb2O6) are investigated with Q-switched light pulses (wavelength 532 nm). Pyroelectric fields are the dominant charge driving force and significantly enhance light-induced refractive-index changes.

1993

nach oben zur Jahresübersicht

Buse K, Frejlich J, Kuper G, Krätzig E
Dark build-up of holograms in BaTiO3 after recording
1993 Appl Phys A-mater, Band: 57, Seite: 437
KurzfassungHolograms recorded under suitable conditions in photorefractive BaTiO3 exhibit an unusual dark build-up. The diffraction efficiency increases by some orders of magnitude after the recording beams are switched off, and then steadily decreases afterwards. An interpretation of this effect in terms of a two-center charge transport model is given.
Mersch F, Buse K, Sauf W, Hesse H, Krätzig E
Growth and characterization of undoped and doped Bi12TiO20 crystals
1993 Phys Status Solidi A, Band: 140, Seite: 273
KurzfassungSingle crystals of B12 TiO20 are grown from solutions containing an excess of Bi2O3 by the TSSG (“top-seeded solution growth”) technique. Doping with iron, manganese, aluminium, and chromium is performed by adding the oxides to the melt. Measurements of dielectric constant, refractive indices, optical activity, absorption, light-induced absorption, and dark conductivity are performed. Bi12TiO20-Einkristalle werden aus Lösungen mit einem Überschuß an Bi2O3 unter Verwendung der TSSG (“top-seeded solution growth”)-Technik gezüchtet. Dotierung mit Eisen, Mangan, Aluminium und Chrom erfolgt durch Beimischung der entsprechenden Oxide zur Schmelze. Messungen der Dielektrizitätskonstanten, der Brechungsindizes, der optischen Aktivität, der Absorption, der lichtinduzierten Absorption sowie der Dunkelleitfähigkeit werden durchgeführt.
Sochava S, Buse K, Krätzig E
Non-steady-state photocurrent technique for the characterization of photorefractive BaTiO3
1993 Opt Commun, Band: 98, Seite: 265
KurzfassungNon-steady-statephotocurrent measurements are carried out with a photorefractive BaTiO3 crystal at the light wavelength 514 nm. We obtain the diffusion length for different crystal orientations, and derive the mobility ratio for charge carrier migration parallel and perpendicular to the c-axis. The frequency dependence of the photocurrent reveals two characteristic times.
Buse K, Baller F, Pankrath R, Hesse H, Krätzig E
Photorefractive and related properties of Ba0.984Sr0.016TiO3 crystals
1993 Solid State Commun, Band: 88, Seite: 587
KurzfassungMeasurements of static dielectric constants, electrooptic coefficients, absorption, light-induced absorption, photo and dark conductivity, holographic diffraction efficiency and photorefractive two-beam coupling gain are performed with a single- crystalline Ba0.984Sr0.016TiO3 sample. Crystals of this composition melt congruently and can be grown by the Czochralski technique. They are very attractive for photorefractive applications.
Buse K, Ringhofer K H
Pyroelectric drive for light-induced charge transport in the photorefractive process
1993 Appl Phys A-mater, Band: 57, Seite: 161
KurzfassungWe calculate the influence of thermal patterns and pyroelectric fields on photorefractive holographic recording in terms of a one center charge transport model, taking into account diffusion, photovoltaic current and space charge limitation. At pulse laser intensities the pyroelectric effect can enhance the holographic sensitivity and the saturation value of refractive index changes significantly.
Buse K, Riehemann S, Loheide S, Hesse H, Mersch F, Krätzig E
Refractive indices of single domain BaTiO3 for different wavelengths and temperatures
1993 Phys Status Solidi A, Band: 135, Seite: K87
Buse K
Thermal gratings and pyroelectrically produced charge redistribution in BaTiO3 and KNbO3
1993 J Opt Soc Am B, Band: 10, Seite: 1266
KurzfassungThe influence of a pyroelectric charge-driving force on photorefractive currents is demonstrated experimentally for the first time to my knowledge. Illumination of photorefractive BaTiO3 and KNbO3 crystals by two intersecting coherent light pulses (light wavelength λ1 = 532 nm) induces thermal gratings. A spatially modulated pyroelectric field arises. Photoexcited carriers move in this field and tend to compensate for it. The pyroelectric field vanishes after termination of the pulse with a characteristic decay time, and redistributed charge remains. Measurements demonstrate that this pyroelectric charge-transport driving force can be much stronger than diffusion for fringe spacings greater than 2 µm. From this new charge-transport mechanism I derive a promising proposal for hologram writing and reading at the same wavelength without erasure.
Buse K, Jermann F, Krätzig E
Two-step photorefractive hologram recording in LiNbO3:Fe
1993 Ferroelectrics, Band: 141, Seite: 197
KurzfassungHolographic recording experiments are performed in iron-doped photorefractivc LiNbO3 using twostep (light wavelengths λ = 1064 nm and λ = 532 nm) excitations. Variations of Fe2+ concentration and light intensity show that two-step excitations via an excited Fe2 state are responsible for the large holographic sensitivity. The experimental results indicate that the dominant charge driving force is the photovoltaic effect.

1992

nach oben zur Jahresübersicht

Buse K, Krätzig E
Light-induced absorption in BaTiO3 and KNbO3 generated with high intensity laser pulses
1992 Opt Mater, Band: 1, Seite: 165
KurzfassungLight-induced absorption generated with nanosecond laser pulses (wavelength 532 nm, intensities up to 600 GW m-2) is investigated in BaTiO3 and KNbO3. The measurements strongly support the two-center model for absorption processes in these materials. Comparison with the results of cw experiments clearly indicates that at high light intensities additional shallow traps become active.

1991

nach oben zur Jahresübersicht

Buse K, Holtmann L, Krätzig E
Activation of BaTiO3 for infrared holographic recording
1991 Opt Commun, Band: 85, Seite: 183
KurzfassungHomogeneous illumination of a BaTiO3 crystal with a green light pulse allows subsequent recording of a holographic grating with infrared light pulses. The grating can be read nondestructively with infrared light and can be erased by illumination with green light. The results strongly support the two-center model recently proposed for the charge transport in BaTiO3.
Holtmann L, Buse K, Kuper G, Groll A, Hesse H, Krätzig E
Photoconductivity and light-induced absorption in KNbO3:Fe
1991 Appl Phys A-mater, Band: 53, Seite: 81
KurzfassungMeasurements of photoconductivity and light-induced absorption in KNbO3:Fe are performed at different light intensities and crystal temperatures. The results are interpreted in terms of a two-center charge transport model. Different model parameters may be evaluated from the experimental data. A complete set of parameters is suggested explaining the dependences of photoconductivity and light-induced absorption on light intensity and temerature for the KNbO3:Fe crystal investigated.

Reviews/Übersichtsartikel in wissenschaftlichen Fachzeitschriften

Jahre: 2016 | 2011 | 2003 | 1997 | 1996

zurück zur Übersicht aller Publikationen

2016

nach oben zur Jahresübersicht

Breunig I
Three-wave mixing in whispering gallery resonators
2016 Laser Photonics Rev, Band: 10, Seite: 569
KurzfassungWe review the progress in the development of frequency converters based on three-wave mixing in whispering gallery resonators (WGRs). The theoretical description, given in a unified notation for all basic processes, reveals that the phase-matching condition known from conventional devices is replaced by several selection rules and, furthermore, the fact that conversion efficiencies of more than 25% can be reached in the overcoupled regime only. Experimentally, the conversion efficiencies exceed 50% already at milliwatt input powers. This is achieved, however, so far in bulk resonators only since today the on-chip devices have two orders of magnitude lower quality factors. Regarding the stability of the conversion process, one has to consider impurities left from the crystal growth and material specific effects like photoconductivity, photorefractivity, and pyroelectricity. The impressive experimental progress paves the way that micrometer-sized frequency converters based on WGRs will find the way out of the lab into real-world applications.

2011

nach oben zur Jahresübersicht

Breunig I, Haertle D, Buse K
Continuous-wave optical parametric oscillators: recent developments and prospects
2011 Appl Phys B-lasers O, Band: 105, Seite: 99
KurzfassungWe review the progress in the development of continuous-wave optical parametric oscillators over the last decade. A recently developed theoretical analysis shows that their stability strongly depends on the group velocity dispersion of the nonlinear material used. Now, these devices generate not only near- and mid-infrared radiation, but also visible and terahertz light. Active locking to external references like atom transitions, resonators, or frequency combs enables mode-hop-free operation up to days. Furthermore, whispering-gallery-resonator-based devices enable the realization of millimeter-sized monolithic resonators with microwatt oscillation thresholds.

2003

nach oben zur Jahresübersicht

Buse K, Soergel E
Holographie in Wissenschaft und Technik
2003 Physik Journal, Band: 2, Seite: 37
KurzfassungHolographie ist mehr als nur eine faszinierende Methode, wirklichkeitsgetreue und dreidimensionale Abbilder von Gegenständen herzustellen. Davon zeugt die breite Palette von Anwendungen in Wissenschaft in Technik. Zudem führt die Entwicklung holographischer Aufzeichnungsmaterialien zu interessanten physikalischen Fragen.

1997

nach oben zur Jahresübersicht

Buse K
Light-induced charge transport processes in photorefractive crystals I: models and experimental methods
1997 Appl Phys B-lasers O, Band: 64, Seite: 273
KurzfassungLight-induced charge transport processes are essential for photorefractive effects in electrooptic crystals. Charge driving forces, charge transport models and experimental methods for investigation of charge transport processes are reviewed. – Drift, bulk photovoltaic currents and diffusion are the charge driving forces. The one-center model without and with electron-hole competition and two-levelmodels, i.e., the two-center model and the three-valence model, are described. The special features of the different models in the case of homogeneous and inhomogeneous illumination are pointed out.Methods for reliablemeasurements of absorption, light-induced absorption changes, dark and photoconductivities, amplitude and phase of refractive index and absorption gratings, and of charge carrier mobilities are presented.
Buse K
Light-induced charge transport processes in photorefractive crystals II: materials
1997 Appl Phys B-lasers O, Band: 64, Seite: 391
KurzfassungThe charge transport processes in different photorefractive oxide crystals are reviewed and consequences for applications are outlined. – The lightinduced charge transport processes in LiNbO3, LiTaO3, BaTiO3, Ba1−xSrxTiO3 (0<x<1, BST), Ba1−xCaxTiO3 (0<x<1, BCT), KNbO3, KTa1−xNbxO3 (0<x<1, KTN), Sr1−xBaxNb2O6 (0<x<1, SBN) and Bi12(Si,Ti,Ge)O20 (BSO, BTO, BGO) are discussed. Utilizing the knowledge on the charge transport processes, consequences for applications are deduced; improved techniques for nondestructive readout of holograms with light of the recording wavelength are described.

1996

nach oben zur Jahresübersicht

Buse K
Pyroelektrische Felder und der photorefraktive Effekt
1996 Phys Bl, Band: 52, Seite: 881
KurzfassungPhotorefraktive Kristalle gewinnen u. a. für die holographische Datenspeicherung und für die Anwendung in der optischen nachrichtentechnik zunehmend an Bedeutung. Diese Kristalle setzen Lichtmuster über einen mehrstufigen, reversiblen Prozess in Brechungsindexmuster um. Dabei werden Ladungsträger umverteilt, Raumladungsfelder entstehen, und aufgrund des elektrooptischen Effekts ändert sich der Brechungsindex. Erst in jüngster Vergangenheit hat man entdeckt, daß bei hohen Lichtintensitäten ein zusätzlicher Mechanismus wirkt: Der Kristall erwärmt sich in den beleuchteten Bereichen merklich, und durch den pyroelektrischen Effekt entstehen Felder, die die Umverteilung der beweglichen Ladungsträger forcieren. Dadurch läßt sich die Photoempfindlichkeit um mehr als eine Größenordnung steigern.

Buchbeiträge

Jahre: 2007 | 2005 | 2002 | 2000 | 1999

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2007

nach oben zur Jahresübersicht

Buse K, Havermeyer F, Liu W, Moser Ch, Psaltis D
Holographic filters
In: Photorefractive materials and their applications 3
2007, Springer Verlag, Günter P, Huignard J P, Günter P, Huignard J P,
Buse K, Imbrock J, Krätzig E, Peithmann K
Photorefractive effects in LiNbO3 and LiTaO3
In: Photorefractive materials and their applications 2
2007, Springer Verlag, Günter P, Huignard J P, Günter P, Huignard J P,
Krätzig E, Buse K
Two-step recording in photorefractive crystals
In: Photorefractive materials and their applications 3
2007, Springer Verlag, Günter P, Huignard J P, Günter P, Huignard J P,

2005

nach oben zur Jahresübersicht

Buse K, Krätzig E
Photorefractive crystals
In: Springer Handbook of Lasers and Optics
2005, Springer Verlag, Träger F, Träger F,

2002

nach oben zur Jahresübersicht

Krätzig E, Buse K
Two-step processes and IR recording in photorefractive crystals
In: Infrared Holography for Optical Communications
2002, Springer Verlag, Ubalbi M, Piccinin D, Boffi P, Ubalbi M, Piccinin D, Boffi P,

2000

nach oben zur Jahresübersicht

Buse K, Krätzig E
Inorganic photorefractive materials
In: Holographic Data Storage
2000, Springer Verlag, Coufal H, Psaltis D, Sincerbox G, Coufal H, Psaltis D, Sincerbox G,

1999

nach oben zur Jahresübersicht

Buse K, Krätzig E
Light-induced charge-transport in photorefractive crystals
In: Photorefractive optics: materials, properties and applications
1999, Academic Press, Yu F, Yin S, Yu F, Yin S,

Konferenzbeiträge

Jahre: 2020 | 2019 | 2018 | 2017 | 2016 | 2015 | 2014 | 2013 | 2012 | 2010

zurück zur Übersicht aller Publikationen

2020

nach oben zur Jahresübersicht

Minet Y, Breunig I, Buse K
Electro-optic based adiabatic frequency conversion in a non-centrosymmetric microresonator
2020 Laser Resonators, Microresonators, and Beam Control XXII, San Francisco, CA, United States Proceedings of SPIE, Band: 11266, Seite: 1126606
KurzfassungA rather unknown method to perform optical frequency tuning is the adiabatic frequency conversion. But this method has some appealing advantages compared to conventional frequency conversion schemes, i. e. nonlinear- optical based ones: The internal conversion efficiency can reach unity even on a single-photon level. No threshold and no phase-matching conditions need to be fulfilled. Previous realizations of adiabatic frequency conversion suffer from short photon lifetimes, limited tuning range and challenging experimental setups. Here, we employ the Pockels effect for adiabatic frequency conversion (AFC) in a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. With a 70-μm-thick resonator we observe frequency shifts of more than 5 GHz by applying a moderate voltage of 20V. In contrast to former schemes our setup is considerably simplified and provides a linear electric-to-optical link that enables us to generate also arbitrary waveforms of frequency shifts. Furthermore, our presented conversion scheme is well-suited for on-chip fabrication. Volume fabrication and application of larger electric fields for reasonable voltages become possible. By doing this, it is feasible to achieve tuning on the order of hundreds of GHz.
Szabados J, Breunig I, Buse K
Frequency comb generation in non-centrosymmetric optical microresonators
2020 Laser Resonators, Microresonators, and Beam Control XXII, San Francisco, CA, United States Proceedings of SPIE, Band: 11266, Seite: 112660G
KurzfassungOptical frequency combs are a key technology for precision measurements. In the past years, microresonatorbased frequency combs based on third-order χ (3) (Kerr) nonlinearities have attracted significant attention thanks to their small footprint and their wide-ranging applications in fields such as telecommunications, molecular spectroscopy or ultrafast distance measurements. In this contribution, we present a frequency comb generated in a microresonator made of 5% MgO-doped congruent lithium niobate, a non-centrosymmetric crystalline material, employing the generally much stronger second-order χ (2)-nonlinearities of such a material via a scheme of cascaded nonlinear processes. This approach paves the way towards reduced pump thresholds for comb generation and comes with intrinsic suitability for self-referencing.
Breunig I, Buse K
Juggling with light: powerful second-order nonlinear optical effects in whispering gallery resonators
2020 Laser Resonators, Microresonators, and Beam Control XXII, San Francisco, CA, United States Proceedings of SPIE, Band: 11266, Seite: 1126609
KurzfassungWhispering gallery resonators made out of crystalline materials exhibiting second-order nonlinearities enable frequency mixing such as optical parametric oscillation with high efficiency at low optical input powers and are ideally suited to realize versatile and compact optical frequency converters. Recent achievements stimulate this field further: Frequency conversion not only at a single wavelength or with few wavelengths is possible, entire frequency combs can be transferred into different spectral domains, e.g., allowing the realization of frequency combs in spectral regions that are suitable for multicomponent analytics like the mid-infrared region. Furthermore, these resonators are also supposed to be the ideal host for cascaded nonlinearities allowing the build-up of frequency combs based on second-order nonlinearities. All this comes with new and better schemes to tune whispering gallery resonators, providing advanced opportunities to modulate the laser wavelengths with nanosecond speed employing the Pockels effect. Juggling with light: We will summarize in the presentation these recent achievements, demonstrating that in the field of whispering gallery resonators still many discoveries are ahead of us.
Breunig I, Herr S, Buse K
Tunable single-frequency lasing in whispering gallery resonators
2020 Laser Resonators, Microresonators, and Beam Control XXII, San Francisco, CA, United States Proceedings of SPIE, Band: 11266, Seite: 112660Q
KurzfassungWhispering gallery resonators (WGRs) are ideally suited for the realization of miniaturized lasers. Due to their high quality factor and small mode volume, they allow for low-threshold and narrow-linewidth emission from (sub)millimeter-sized cavities made of laser-active materials. However, so far the majority of experimental realizations relies on expensive pump light sources like narrow-linewidth or pulsed laser systems, impeding most applications. We demonstrate two whispering-gallery-based single-frequency lasers pumped by compact spectrally multimode low-cost laser diodes. The spheroidally-shaped millemeter-sized WGRs are made of Pr:LiLuF4 and Nd:YVO4. They provide quality factors beyond 107 at the lasing wavelengths (640 nm and 1064 nm, respectively). The pump light is focused onto the rim of the WGR. We observe single frequency emission at milliwatt output powers. The temporal stability of the output power and of the output frequency are determined to be ±1:5 % and ±30 MHz within 30 min, respectively. By changing the temperature of the cavity, we achieve mode-hop-free tuning exceeding 11 GHz.

2019

nach oben zur Jahresübersicht

Minet Y, Nuno dos Reis L, Buse K, Breunig I
Adiabatic Frequency Conversion in Non-Centrosymmetric High-Q Optical Microresonators
2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungMicroresonators are a versatile and efficient system platform for optical frequency conversion. Here, the non-linear response of material polarization is used for frequency synthesis. For example in WGRs made of non-centrosymmetric materials, tunable OPOs have been realized, and in WGRs made of centrosymmetric materials frequency combs were generated. Another way for frequency conversion is adiabatic tuning. Here, the optical length of the circumference of the resonator is changed during its ringdown time. This induces a frequency shift of the circulating light. So far different methods have been demonstrated. Free electrons generated by laser pulses, induce a change of the refractive index. By this method the wavelength can be tuned by some nanometers. The free electrons however lower the quality factor of the resonator. Moreover the repetition rate is limited by the time it takes for the free electrons to recombine. Another method is to change the frequency adiabatically via the AC-Kerr effect. Here, again second pump laser is needed. With this method the quality factor is not affected, but the achieved frequency shift is smaller compared to that for the free-electron method.
Roiz M, Werner C, Breunig I, Vaini M
Femtosecond-Driven Up-Conversion in a Radially Poled LiNbO3 Microresonator
2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungNowadays, there are plenty of different techniques developed for the generation of Optical Frequency Combs (OFCs), but Mode Locked Laser (MLL) OFC is still the most commonly used and commercially available one [1]. Although MLLs are capable of producing very high-peak power OFCs with femtosecond pulse durations, it is sometimes challenging to apply them directly for a number of reasons. First, MLLs typically produce OFCs with repetition frequencies (f r ) below 1 GHz, so additional filtering of the unwanted modes is required for the applications in telecommunications, astrocombs etc. Second, many spectral regions - namely mid-infrared, visible and ultraviolet - are difficult to access by MLLs, which implies the use of external parametric up- and down-conversion. One promising platform for such spectral transfer is high Q-factor optical microresonators based on second order (χ (2) ) nonlinear crystals, since not only do they provide the desired spectral transfer, but also open up opportunities for the miniaturization of the system. In addition, such microresonators support whispering gallery modes that possess very low mode volumes, making it possible to enhance nonlinear χ (2) processes. This is a significant advantage over the already existing bulk analogs.
Szabados J, Breunig I, Buse K
Frequency Comb Generation and Conversion in Non-Centrosymmetric Optical Microresonators
2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungOptical frequency combs have become the key technology in optical precision measurements [1], bear a high potential for broadband, high-resolution molecular spectroscopy [2] and are promising to advance future optical clocks [3]. For low-power, compact and mobile applications, high-repetition-rate combs such as continuous-wave-pumped Kerr combs [4] and soliton frequency combs based on a third-order nonlinearity [5] have turned out to be especially useful. Thus far, most of these frequency combs operate in the near-infrared (NIR) spectral regime around 1550 nm for reasons such as chromatic material dispersion. Many applications ranging from astronomical spectroscopy, optical clocks and quantum physics all the way to molecular sensing, however, require the frequency combs to be centered in the ultraviolet (UV), visible (VIS) and mid-infrared (MIR) spectral regimes [3,6-8].
Szabados J, Brasch V, Herr S, Obrzud E, Jia Y, Lecomte S, Buse K, Breunig I, Herr T
High repetition rate frequency comb up-and down-conversion in synchronously driven microresonators
2019 Laser Resonators, Microresonators, and Beam Control XXI, San Francisco, CA, United States Proceedings of SPIE, Band: 10904, Seite: 109040D
KurzfassungOptical frequency combs are a key technology for optical precision measurements. So far, most frequency combs operate in the near-infrared regime (NIR). Many applications, however, require combs in the ultraviolet (UV), visible (VIS) or mid-infrared (MIR) spectral ranges. This can be achieved by making use of nonlinear-optical processes. In this contribution, we demonstrate the efficient conversion of frequency combs with a repetition rate of 21 GHz to UV, VIS and MIR wavelengths in a synchronously driven high-Q microresonator with second-order optical nonlinearity. This opens up a new path for applications including, but not limited to, molecular sensing and quantum optics.
Breunig I, Buse K
Quasi-phase matching in integrated lithium-niobate whispering galleries
2019 Laser Resonators, Microresonators, and Beam Control XXI, San Francisco, CA, United States Proceedings of SPIE, Band: 10904, Seite: 1090403
KurzfassungWhispering-gallery resonators (WGRs) made of lithium niobate are very attractive for nonlinear-optical frequency conversion due to their small mode volumes and high Q-factors. To achieve phase matching, methods like birefringent phase matching and quasi phase matching (QPM) have been employed in millimeter-sized bulk WGRs. Among these, the latter provides ultimate flexibility in terms of wavelengths and polarization of the interacting waves. Integrated on-chip WGRs are in particular very appealing due to the possibility of building photonic circuits and the usage of highly-parallel and thus scalable semiconductor manufacturing techniques. Integrated WGRs are, however, fabricated on thin-film substrates. This leads to one major drawback: QPM is hard to achieve, since it is difficult to realize periodically-poled thin films by field-assisted domain inversion. We report on a method to resolve this issue. First, we do domain engineering in bulk material. Next, we bond this sample on a quartz substrate by direct wafer bonding and finally we polish the lithium niobate to a 2-μm-thick film. By lithography and reactive-ion etching we structure waveguide rings with 200 μm diameter into the thin film. Subsequent polishing of the waveguide sidewalls decreases surface-scattering losses and enables on-chip WGRs with quality factors exceeding one million. This allowed us to demonstrate for the first time quasi-phase matched second-harmonic generation in integrated WGRs, pumped by light with 1550 nm wavelength, obtaining a normalized conversion efficiency of 0.9 ‰/mW. Being now able to deploy type-0 and type-ii phase matching opens entirely new possibilities for frequency conversion with on-chip WGRs.
Uluda M, Szabados J, Breunig I, Buse K
Radially-Poled Stoichiometric Lithium Tantalate Microresonators for Nonlinear-Optical Applications
2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungWhispering gallery-mode microresonators guide light by total internal reflection and exhibit exceptionally high quality factors and small mode volumes. If such a microresonator is fabricated from a nonlinear-optical material, one can benefit from these properties for efficient optical frequency conversion. One of the well-known second-order nonlinear-optical materials for these microresonators is periodically-poled lithium niobate (PPLN) because of its high second-order susceptibility and wide transmission window. Thus PPLN-based whispering gallery-mode microresonators are efficient platforms for higher-harmonic generation of ultraviolet light using a near-infrared pump source in the telecom range [1], for low-input-power optical parametric oscillation [2] and for frequency comb up- and down-conversion from the near-infrared region to the visible, ultraviolet and mid-infrared regions [3].
Breunig I, Buse K
Whispering gallery optical parametric oscillators: Just a scientific oddity?
2019 Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII, San Francisco, CA, United States Proceedings of SPIE, Band: 10902, Seite: 10902T
KurzfassungOptical parametric oscillators (OPOs) convert the frequency of laser light to almost arbitrary values. Nowadays, they serve as wavelength-agile light sources for spectroscopic applications as well as photon generators for quantum-optical experiments. Conventional OPOs are based on a nonlinear-optical crystal surrounded by a mirror cavity. Regarding the cavity, one often finds the following rule of thumb: the more waves are resonant, the lower is the oscillation threshold and the more difficult is the wavelength tuning. In whispering-gallery-resonator-based OPOs (WGR OPOs), light is guided by total internal reflection in a millimeter-sized spheroidally-shaped nonlinear-optical crystal. Thus, these devices are intrinsically triply resonant. Indeed, they provide microwatt-level oscillation thresholds, i.e., the lowest values of all OPO configurations. However, following the abovementioned rule, their applicability in fields beyond fundamental science might be questionable, because the most striking feature of an OPO, i.e., the wavelength tunability, is hampered. Nevertheless, several experimental studies revealed that the output wavelengths of WGR OPOs could be tuned in well-defined steps over hundreds of nanometers by temperature variation. Combined with strategies for mode-hop free tuning, it is possible, e.g., to tune the output wavelength in a controlled way until it meets MHz-wide resonances. This is sufficient for high-resolution spectroscopy. WGR OPOs are nowadays operated around various center frequencies, covering the visible-to-mid-infrared spectral range. These light sources - despite of their intrinsic triple resonance - might serve as compact and wavelength-agile devices for various applications.

2018

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Szabados J, Kini Manjeshwar S, Breunig I, Buse K
Electro-optic tuning of potassium tantalate-niobate whispering gallery resonators
2018 Laser Resonators, Microresonators, and Beam Control XX, San Francisco, CA, United States Proceedings of SPIE, Band: 10518, Seite: 1051802
KurzfassungMost centrosymmetric materials exhibit a neglectable electro-optic response; the Pockels coefficients are zero, while the DC-Kerr coefficients are typically small. For a standard material such as silica they are on the order of 10−22 m2/V2 . 1 Thus, refractive index tuning is achieved by thermal or mechanical means. Potassium tantalateniobate (KTa1−xNbxO3 with 0 < x < 1, KTN) is a rare exception. It offers outstandingly high DC-Kerr coefficients in the 10−15 m2/V2 range.2 In this contribution, the first-ever KTN whispering gallery resonators (WGR) are demonstrated with quality factors Q > 107 and electro-optic eigenfrequency tuning of more than 100 GHz at λ = 1040 nm for moderate field strengths of 250 V/mm. These results potentially pave the way for applications such as electro-optically tunable Kerr frequency combs.
Herr S, Buse K, Breunig I
Incoherently pumped lasing and self-pumped three-wave mixing in laser-active whispering-gallery resonators
2018 Laser Resonators, Microresonators, and Beam Control XX, San Francisco, CA, United States Proceedings of SPIE, Band: 10518, Seite: 105180U
KurzfassungWe report lasing at 1064 nm in an LED-pumped high-Q whispering-gallery resonator (WGR) made from Nd:YVO4. A single LED is sufficient to achieve quasi-cw operation. Furthermore, self-pumped three-wave-mixing is demonstrated in high-Q WGRs made from laser-active lithium niobate. When pumping the resonator with a free-running laser diode without any external frequency stabilization, self-frequency doubling of the 1.08-μm laser line into the green spectral region is observed. With the application of quasi-phase matching techniques, the nonlinear optical processes can be selected at will, which enables compact and versatile self-pumped frequency synthesizers. As a proof-of-principle experiment, self-pumped optical parametric oscillation is demonstrated in a radially poled WGR made from neodymium-doped lithium niobate.
Jia Y, Hanka K, Breunig I, Zawilski K.T., Schunemann P.G., Buse K.
Mid-infrared whispering gallery resonators based on non-oxide nonlinear optical crystals
2018 Laser Resonators, Microresonators, and Beam Control XX, San Francisco, CA, United States Proceedings of SPIE, Band: 10518, Seite: 105180X
KurzfassungHigh-quality whispering gallery resonators (WGRs) made of AgGaSe2 and CdSiP2 bulk crystals are fabricated. With femtosecond laser matching and subsequent fine polishing, the intrinsic Q-factors of these resonators are approaching values of 106 to 107 . By adjusting the coupling condition, maximum coupling efficiencies of 60% and 30% for AgGaSe2 and CdSiP2 resonators could be obtained. In addition, thermal effects on the distortion of the mode spectra of these resonators are also investigated. The results in this work reveal great potentials of WGRs made of non-oxide crystals for mid infrared applications.
Breunig I, Hanka K, Jia Y, Zawilski K.T., Schunemann P.G., Buse K.
Whispering gallery optical parametric oscillators for the mid-infrared spectral range
2018 Nonlinear Frequency Generation and Conversion: Materials and Devices XVII, San Francisco, CA, United States Proceedings of SPIE, Band: 10516, Seite: 105160E
KurzfassungThe tunability of cw optical parametric oscillators can be extended in the mid-infrared spectral range by using whispering gallery resonators (WGRs) made of non-oxide crystals. We demonstrate the fabrication of WGRs from silver gallium selenide, cadmium silicon phosphide and orientation-patterned gallium phosphide with quality factors above 106 . The resonators made of the first two provide optical parametric oscillation in the mid-infrared pumped by a compact laser diode at 1.57 μm wavelength. The wavelength tunability of the device based on silver gallium selenide provides a tuning range beyond 8 μm wavelength. These achievements are considered as the first steps of cw optical parametric oscillators into this important spectral region.

2017

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Wolf R, Zappe H, Buse K, Breunig I
Batch-processed high-Q integrated lithium-niobate-on-insulator ridge waveguide whispering-gallery resonators
2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungLithium niobate whispering-gallery resonators (WGRs) show high intensity enhancement due to small mode volumes and high Q-factors. With bulk WGRs, shaped by diamond-blade-cutting tools followed by surface polishing, Q-factors in the range of 1011 have been demonstrated, which make them of significant interest for e.g. nonlinear-optical frequency conversion [1]. However, WGRs are still rare in commercial products due to the serial and thus expensive fabrication. Thus integration of WGRs on chip level became of great interest in the past years.
Breunig I, Fürst J, Hanka K, Buse K
Continuous-wave optical parametric oscillation tunable up to 8 μm wavelength
2017 Photorefractive Photonics, Qingdao, Shandong, China. Journal of Physics: Conference Series, Band: 867, Seite: 012010
KurzfassungWe demonstrate the first cw OPO emitting mid-infrared light at wavelengths up to 8 μm. This device is based on a 3.5-mm-diameter whispering gallery resonator made of silver gallium selenide (AgGaSe₂) pumped by a compact distributed feedback laser diode emitting light at 1.57 μm wavelength. Phase-matching is achieved for a c-cut resonator disk pumped with extraordinarily polarized light at this wavelength. The oscillation thresholds are in the mW region, while the output power ranges from 10 to 800 μW. Wavelength tuning is achieved via changing the radial mode number of the pump wave and by changing the resonator temperature. Simulations predict that whispering gallery OPOs based on AgGaSe₂ with diameters around 2 mm can generate idler waves exceeding 10 μm wavelength.
Veenhuizen K, Stone GA, Knabe B, Buse K, Dierolf V
Dependence of stoichiometry of lithium niobate nanocrystals on initial lithium to niobium ratios in the synthesis step
2017 International Conference on Defects in Insulating Materials, ICDIM 2016, Lyon, France IOP conference series. Materials science and engineering, Band: 169, Seite: 012022
KurzfassungFerroelectric nanocrystals show promise for application in forming hybridized nonlinear materials with liquid crystals. It is well known that bulk single crystals of lithium niobate (LiNbO₃) are most easily grown in a congruent (lithium-deficient) form but can also be grown in a stoichiometric form. This is controlled by the specific growth conditions and the stoichiometric ratio ρ = MLi/(MLi + MNb), where M is the molar fraction. This work explores the dependence of the stoichiometry of LiNbO₃ nanocrystals on the value of ρ in the synthesis step. Batches of LiNbO₃ nanocrystals were synthesized using a sol-gel method. The nanocrystals were analysed via SEM and Raman spectroscopy to gain information about their morphology, stoichiometry, defect content, and phase. For bulk crystals, previous work has demonstrated that the spectral widths of specific Raman modes strongly depend on ρ. For the nanocrystals, the Raman spectra indeed reveal that the resultant nanocrystal stoichiometry depends on the initial ρ used in the synthesis step. In addition, a close examination of the Raman spectra reveals the presence of an extra phase in batches with ρ ≥ 55%. Somewhat counterintuitively, this phase is identified by its Raman spectra to be LiNb₃O₈, a relatively lithium-poor phase compared to LiNbO₃. Avoiding this extra phase, we find that high quality, roughly spherical LiNbO₃ nanocrystals can be synthesized for ρ between 52 and 54%.
Schiller A, Beckmann T, Fratz M, Belzer D, Bertz A, Carl D, Buse K
Digital holography on moving objects: Multiwavelength height measurements on inclined surfaces
2017 Optical Measurement Systems for Industrial Inspection X, Munich, Germany Proceedings of SPIE, Band: 10329, Seite: 103290D
KurzfassungMultiwavelength digital holography on moving objects enables fast and precise inline-measurements of surface pro files. Due to the use of multiple wavelengths, optically rough surfaces with structure heights in the micrometer range can be mapped unambiguously. In this work we explore the influence of the object velocity on height measurements on inclined surfaces. We show measurements using spatial-phase-shifting holography employing two wavelengths and object velocities of up to 90 mm/s with eye-safe cw-lasers with less than 1 mW of laser light. Despite motion blur exceeding the mean speckle size, reliable height measurements can be conducted at these velocities. The height map of a metal cone with two different slope angles (1°,10°) is measured at an exposure time of 2 ms. Using line shaped illumination, each frame yields a height map of approximately 2 x 17 mm². The overlap between the frames allows averaging as the image is put together, improving data quality. The mean repeatability of the height information in the investigated setup is better than 4.5 µm at a synthetic wavelength of 214 µm.
Fratz M, Beckmann T, Schiller A, Seyler T, Bertz A, Carl D, Buse K
Digital holography: Evolution from a research topic to a versatile tool for the inline 100% 3D quality control in industry
2017 18th International Conference on Sensors and Measurement Technology (SENSOR), Nuremberg, Germany Proceedings SENSOR 2017, Band: 2017, Seiten: 286 - 289
KurzfassungDigital multiwavelength holography is a technique for precise 3D height measurements of optically rough surfaces. We demonstrate measurements on a milled surface, using four wavelengths between 514 and 532 nm, and achieve precision in the submicrometer range. Height structures of <400 nm can be resolved up to an unambiguous height of 370 μm and with a lateral resolution of 7 μm. Acquisition times of < 400 ms, including the time needed for parallel processing of the data, make our sensor a versatile tool for high-throughput 100% inspection in manufacturing environments.
Herr S, Buse K, Breunig I
Laser-active whispering-gallery resonators as versatile platform for optical three-wave mixing
2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungWhile laser sources are limited in their emission spectra by their gain-medium, non-linear frequency conversions allow for generation of coherent light in almost any spectral region. Optical whispering-gallery resonators (WGRs), fabricated from non-linear crystals, were successfully applied as miniaturized, high-efficiency frequency converting devices [1]. Frequency conversion in WGRs is usually induced by pumping optical resonances of the WGR by means of an external narrow-linewidth and frequency tunable laser. Hence, the effective technological overhead of the overall system is rather high, hindering this technology from spreading into everyday life's applications. One approach to overcome this major drawback is to incorporate laser-active ions into the resonator's material. This turns passive WGRs into narrow-linewidth lasers themselves, capable of inducing non-linear processes directly within the same crystal. With this, requirements on pump sources are lowered considerably, as laser-active ions can be excited even with incoherent light. The non-linear optical processes of interest can be selected by tailoring the phase-matching properties of the crystal by periodic domain inversion. With these tools at hand, laser-active whispering-gallery resonators may emerge as a versatile platform for continuous-wave frequency conversion. In principle, coherent light of any frequency within the transparency range of the non-linear crystal can be generated.
Werner C, Buse K, Breunig I
Piezo-tunable second-harmonic-generation in a whispering-gallery resonator
2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungWhispering-gallery resonators gain more and more interest in optical applications. Their combination of small dimensions, strong light confinement, and high quality factors make them ideally suited for optical frequency conversion. Here, their unique features not only allow operation with very small pump powers, they also allow covering the optical spectrum from the UV up to the mid-IR [1, 2, 3]. So far, a common way to change whispering-gallery resonance frequencies is to vary the resonator temperature. While this approach allows far tuning by altering the geometry and the refractive index at the same time, it is inherently slow. Further, due to dispersion, the tuning efficiency changes with wavelength and polarization. Especially in nonlinear-optical applications with many interacting waves, this is often a limiting factor as phase matching is destroyed.
Jia Y, Winkler M, Szabados J, Breunig I, Kirste L, Cimalla V, Zukauskaite A, Buse K
Potassium tantalate-niobate mixed crystal thin films for applications in nonlinear integrated optics
2017 Photorefractive Photonics, Qingdao, Shandong, China. Journal of Physics: Conference Series, Band: 867, Seite: 012020
KurzfassungPotassium tantalate-niobate mixed crystal (KTN) thin films are promising candidates to meet the needs of integrated nonlinear optical devices for electro-optic and frequency-conversion applications. In this contribution we report on pulsed-laser-deposition growth of ferroelectric KTN films on MgO substrates. It was shown that highly-oriented KTN films are epitaxially grown as revealed by X-ray diffraction analysis. Moreover, the thermal annealing treatment can be further optimized to obtain optically smooth KTN films with RMS surface roughness as low as 1 nm.
Jia Y, Szabados J, Winkler M, Breunig I, Cimalla V, Kirste L, Zukauskaite A, Buse K
Potassium-tantalate-niobate mixed crystal thin films for applications in nonlinear integrated optics
2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungPerovskite potassium tantalate-niobate mixed crystals (KTa1−xNbxO3 with 0 ≤ x ≤1, KTN) undergo a phase transition from a paraelectric cubic to a ferroelectric tetragonal structure with decreasing temperature [1]. By adjusting the Ta/Nb content one can tune the phase-transition temperature of KTN and thus also its main properties at a given temperature. For example, the phase transition occurs around room temperature for x = 0.4, accompanied by changes in the dielectric and electro-optic (EO) properties [2]. This extremely promising material is of great interest because of its large EO effect and excellent nonlinear optical performance [2,3]. However, due to the fact that the crystals grown have compositions being different from those of the molten ingredients, high-quality and homogeneous single-crystalline KTN is difficult to produce, which is limiting the application of this material [4,5]
Wolf R, Buse K, Breunig I, Zappe H
Q-factor enhancement of integrated lithium-niobate-on-insulator ridge waveguide whispering-gallery-mode resonators by surface polishing
2017 Laser Resonators, Microresonators, and Beam Control XIX, San Francisco, CA, United States Proceedings of SPIE, Band: 10090, Seite: 1009002
KurzfassungWhispering-gallery resonators (WGRs) are most promising for nonlinear-optical frequency-conversion due to their intensity enhancement by small mode volumes and high Q-factors. This has been shown frequently by millimeter-sized diamond-blade cut and polished bulk WGRs. For reproducible batch fabrication, however, the integration of WGRs into lithium-niobate-on-insulator (LNOI) substrates became of great interest. Here we report on integrated WGRs made by batch processes like lithography and reactive-ion etching. Since the Qfactor of integrated WGRs is limited by scattering losses, we focused on developing a polishing process for the waveguide sidewalls that allowed us to enhance the unloaded Q-factors already to more than 106 with room for further improvements. Furthermore we employ a coupling scheme with two waveguide chips, one comprising a linear coupling waveguide and one with the integrated WGR. By adjusting the distance between the coupling waveguide and the WGR, we can reproducibly and stably tune the coupling-efficiency between 0 and 95 %.
Breunig I
Whispering gallery optical parametric oscillators become mature
2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, IEEE
KurzfassungContinuous-wave optical parametric oscillators (cw OPOs) combine narrow-linewidth emission with a large wavelength tuning range far beyond that of lasers. Conventionally, these devices are based on a nonlinear-optical crystal surrounded by a mirror cavity. In this configuration, cw OPOs are nowadays operated from the visible to the mid-infrared with watt-level output powers. They are ideally suited not only for high-resolution spectroscopy. Enabled by the pioneering work [1] of researchers from the Jet Propulsion Lab, a new OPO configuration was demonstrated some years ago [2] — whispering gallery optical parametric oscillators (see Fig. 1a). Here, the light is guided by total internal reflection in a spheroidally-shaped nonlinear-optical crystal. These monolithic devices do not require any reflective or anti-reflection coating, and they are easily miniaturized down to sub-millimeter diameters. Furthermore, due their low oscillation thresholds, they can be pumped by compact laser diodes. However, they are intrinsically triply-resonant, i.e. all three interacting waves are simultaneously circulating the cavity. It might be assumed that the wavelength tuning of WGR OPOs is much more difficult to control compared with that of conventional mirror-based singly-resonant systems. Hence, it was questionable if they are applicable in fields beyond fundamental science.

2016

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Meisenheimer S, Fürst J, Schiller A, Buse K, Breunig I
Broadband wavelength control for optical parametric oscillation in radially-poled whispering gallery resonators
2016 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Proceedings of SPIE, Band: 9731, Seite: 97310U
KurzfassungBroadband infrared spectroscopy employing optical parametric oscillation in bow-tie cavities, including a periodically- poled lithium niobate (PPLN) crystal, is well known. We demonstrate, however, that such spectroscopy is also possible using 2-mm-size monolithic whispering gallery resonators (WGRs). This is achieved in a radially-poled WGR by controlling wavelength tuning despite triple resonance of pump, signal, and idler light. Simulated and measured tuning characteristics of the Type-0 OPOs, pumped at about 1 μm wavelength, coincide. Tuning branches, which are crossed or curved at degeneracy, are present over a spectral range of up to 0.9 µm. As a proof-of-principle experiment, we show that all spectroscopic features of ethanol can be resolved using the idler light between 2.2 and 2.55 μm.
Schiller A, Beckmann T, Fratz M, Belzer D, Bertz A, Carl D, Buse K
Multiwavelength digital holography with spatial phase shifting on moving objects
2016 Digital Holography and Three-Dimensional Imaging, Heidelberg, Germany OSA Technical digest series, Band: 2016, Seite: DM3I.6
KurzfassungIt was not possible so far to characterize moving objects using interferometric techniques. We present an implementation where the 3D profiles of objects moving at velocities up to 100 mm/s are obtained by digital holography.

2015

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Leidinger M, Buse K, Breunig I
Highly sensitive absorption measurements in lithium niobate using whispering gallery resonators
2015 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XIV Proceedings of SPIE, Band: 9347, Seite: 93471D
KurzfassungThe absorption coefficient of undoped, congruently grown lithium niobate (LiNbO3) for ordinarily and extraordinarily polarized light is measured in the wavelength range from 390 to 2600 nm using whispering gallery resonators (WGRs). These monolithic cavities guide light by total internal reflection. Their high Q-factor provides several hundred meters of propagation for the coupled light in millimetre size resonators allowing for the measurement of absorption coefficients below 10^−2 cm^−1, where standard methods such as Fourier-transform or grating spectroscopy meet their limit. In this work the lowest measured value is 10^−4 cm^−1 at 1700 nm wavelength. Furthermore, the known OH^− overtone at 1470 nm wavelength can be resolved clearly.
Fürst J U, Buse K, Breunig I, Becker P, Liebertz J, Bohaty L
Whispering gallery resonator from lithium tetraborate for nonlinear optics
2015 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XIV Proceedings of SPIE, Band: 9347, Seite: 93470A
KurzfassungFor second-order nonlinear-optical pro cesses in the ultraviolet, appropriate materials with a sufficiently large band-gap typically exhibit smaller nonlinear coefficients than materials with comparably smaller band-gap . Whisp ering gallery resonators, with their outstanding quality factors, provide field enhancement and can comp ensate for these small coefficients. We report on the successful fabrication of a whispering gallery resonator made of lithium tetraborate, a suitable material for ultraviolet applications with a small nonlinear coefficient of d31 = 0.073 pm/V. Quality factors of the order of 10^8 are observed from the ultraviolet to the near-infrared spectrum. The inferred absorption coefficients of lithium tetraborate are below 0.2 m^−1 in the visible and nearinfrared. Continuous-wave second harmonic generation from 490 nm light to 245 nm is observed with conversion efficiencies up to 2.2 %.

2014

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Kießling J, Buse K, Vodopyanov K L, Breunig I
Continuous-wave optical parametric source for terahertz waves tunable from 1 to 4.5 THz frequency
2014 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XIII Proceedings of SPIE, Band: 8964, Seite: 896408
KurzfassungWe demonstrate the continuous-wave operation of a cascade that has been successfully applied so far only for picosecond systems: A doubly-resonant optical-parametric oscillator (OPO) based on lithium niobate generates signal and idler waves close to degeneracy. Subsequently, these two light fields are converted to a terahertz field via difference frequency mixing in an orientation-patterned gallium arsenide crystal placed inside the OPO cavity. Using this scheme, we achieved tunability from 1 to 4.5 THz frequency, a linewidth smaller than 10 MHz, and a Gaussian beam profile. The output power is of the order of tens of μW, with a scalability into the milliwatt regime.
Breunig I, Bückle A, Werner C S, Buse K
Non-Lorentzian pump resonances in whispering gallery optical parametric oscillators
2014 Laser Resonators, Microresonators, and Beam Control XVI Proceedings of SPIE, Band: 8960, Seite: 896007
KurzfassungWhispering gallery optical parametric oscillators are millimeter-sized monolithic sources for tunable coherent light. Several experiments have revealed that during optical parametric oscillation the pump resonance strongly differs from a Lorentzian shape. We theoretically and experimentally analyze these line-shape distortions. It turns out that the line shape of the pump resonance strongly depends on the coupling strength of the pump light and on the loss ratio between generated light and pump light. The line-widths, i.e. the losses, for the light generated by the parametric process can be deduced without measuring them directly.

2013

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Breunig I, Buse K
Whispering gallery optical parametric oscillators
2013 Quantum Sensing and Nanophotonic Devices XI Proceedings of SPIE, Band: 8993, Seite: 89930B
KurzfassungWhispering gallery optical parametric oscillators (WGR OPOs) are monolithic sources for tunable coherent and non-classical light. They are based on total internal reflection. Since reflection losses are negligible, their oscillation threshold can be far below one milliwatt. With sub-millimeter diameters, they are the most compact OPOs demonstrated so far. Recent experimental results demonstrate that WGR OPOs emit coherent light tunable over hundreds of nanometers. Operation in the visible as well as in the near-infrared has been demonstrated with up to 30 % conversion efficiency. These results indicate a great potential of WGR OPOs for spectroscopic and sensing applications.

2012

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Breunig I, Beckmann T, Buse K
Monolithic optical parametric oscillators
2012 Laser Resonators, Microresonators, and Beam Control XIV Proceedings of SPIE, Band: 8236, Seite: 82360S
KurzfassungStability and footprint of optical parametric oscillators (OPOs) strongly depend on the cavity used. Monolithic OPOs tend to be most stable and compact since they do not require external mirrors that have to be aligned. The most straightforward way to get rid of the mirrors is to coat the end faces of the nonlinear crystal. Whispering gallery resonators (WGRs) are a more advanced solution since they provide ultra-high reflectivity over a wide spectral range without any coating. Furthermore, they can be fabricated out of nonlinear-optical materials like lithium niobate. Thus, they are ideally suited to serve as a monolithic OPO cavity. We present the experimental realization of optical parametric oscillators based on whispering gallery resonators. Pumped at 1 μm wavelength, they generate signal and idler fields tunable between 1.8 and 2.5 μm wavelength. We explore different schemes, how to phase match the nonlinear interaction in a WGR. In particular, we show improvements in the fabrication of quasi-phase-matching structures. They enable great flexibility for the tuning and for the choice of the pump laser.

2010

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Sowade R, Breunig I, Buse K
Continuous-wave optical parametric oscillators on their way to the terahertz range
2010 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications IX Proceedings of SPIE, Band: 7582, Seite: 75820O
KurzfassungContinuous-wave optical parametric oscillators (OPOs) are known to be working horses for spectroscopy in the near- and mid-infrared. However, strong absorption in nonlinear media like lithium niobate complicates the generation of far-infrared light. This absorption leads to pump thresholds vastly exceeding the power of standard pump lasers. Our first approach was, therefore, to combine the established technique of photomixing with optical parametric oscillators. Here, two OPOs provide one wave each, with a tunable difference frequency. These waves are combined to a beat signal as a source for photomixers. Terahertz radiation between 0.065 and 1.018 THz is generated with powers in the order of nanowatts. To overcome the upper frequency limit of the opto-electronic photomixers, terahertz generation has to rely entirely on optical methods. Our all-optical approach, getting around the high thresholds for terahertz generation, is based on cascaded nonlinear processes: the resonantly enhanced signal field, generated in the primary parametric process, is intense enough to act as the pump for a secondary process, creating idler waves with frequencies in the terahertz regime. The latter ones are monochromatic and tunable with detected powers of more than 2 μW at 1.35 THz. Thus, continuous-wave optical parametric oscillators have entered the field of terahertz photonics.
Breunig I, Sowade R, Kießling J, Beckmann T, Linnenbank H, Haertle D, Buse K
Intracavity frequency conversion: from bow-ties to whispering galleries
2010 Laser Resonators and Beam Control XII Proceedings of SPIE, Band: 7579, Seite: 75790V
KurzfassungProviding optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 μm wavelength to signal light being tunable from 1.25 to 1.85 μm and to corresponding idler light from 2.3 to 5.3 μm. We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of 1.35 THz and a power of 2 μW. Now, the straightforward next development step is to reduce the footprint of such devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity mode for 3 hours.

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