Publikationsliste Dr. Ingo Breunig

Originalarbeiten in wissenschaftlichen Fachzeitschriften
Reviews/Übersichtsartikel in wissenschaftlichen Fachzeitschriften
Konferenzbeiträge

Originalarbeiten in wissenschaftlichen Fachzeitschriften

Jahre: 2020 | 2019 | 2018 | 2017 | 2016 | 2015 | 2013 | 2012 | 2011 | 2010 | 2009 | 2008 | 2007 | 2006 | 2005

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2020

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

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

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

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.
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.

2016

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.
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

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.

2013

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.
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.
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

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.
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.

2011

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.
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.

2010

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.

2009

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.
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.
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.
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.

2008

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.

2007

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.
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.

2006

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.

2005

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.

Reviews/Übersichtsartikel in wissenschaftlichen Fachzeitschriften

Jahre: 2016 | 2011

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2016

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

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.

Konferenzbeiträge

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

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2020

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

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

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

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.
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

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.

2015

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

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

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

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

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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