Whispering-gallery Microresonator Research Articles

Low-Threshold Anti-Stokes Raman Microlaser on Thin-Film Lithium Niobate Chip.

Raman microlasers form on-chip versatile light sources by optical pumping, enabling numerical applications ranging from telecommunications to biological detection. Stimulated Raman scattering (SRS) lasing has been demonstrated in optical microresonators, leveraging high Q factors and small mode volume to generate downconverted photons based on the interaction of light with the Stokes vibrational mode. Unlike redshifted SRS, stimulated anti-Stokes Raman scattering (SARS) further involves the interplay between the pump photon and the SRS photon to generate an upconverted photon, depending on a highly efficient SRS signal as an essential prerequisite. Therefore, achieving SARS in microresonators is challenging due to the low lasing efficiencies of integrated Raman lasers caused by intrinsically low Raman gain. In this work, high-Q whispering gallery microresonators were fabricated by femtosecond laser photolithography assisted chemo-mechanical etching on thin-film lithium niobate (TFLN), which is a strong Raman-gain photonic platform. The high Q factor reached 4.42 × 106, which dramatically increased the circulating light intensity within a small volume. And a strong Stokes vibrational frequency of 264 cm-1 of lithium niobate was selectively excited, leading to a highly efficient SRS lasing signal with a conversion efficiency of 40.6%. And the threshold for SRS was only 0.33 mW, which is about half the best record previously reported on a TFLN platform. The combination of high Q factors, a small cavity size of 120 μm, and the excitation of a strong Raman mode allowed the formation of SARS lasing with only a 0.46 mW pump threshold.

Asymmetrical Cross-Polarization Coupling in a Whispering-Gallery Microresonator

Cross-polarization coupling between transverse electric (TE) and transverse magnetic (TM) whispering-gallery modes in an optical microresonator produces effects such as coupled-mode induced transparency (CMIT). The detailed analytical theory of this coupling indicates that the TE-to-TM and TM-to-TE couplings may have different strengths. Using an experimental setup centered around a hollow bottle resonator and polarization-sensitive throughput detection, that had been used in previous CMIT experiments, this asymmetry was confirmed and studied. By fitting the throughput spectra of both polarizations to the numerical output of a basic model, the asymmetry parameter defined as the ratio of the coupling amplitudes was determined from the output power in the polarization orthogonal to that of the input. The results of many experiments give a range for this ratio, roughly from 0.2 to 4, that agrees with the range predicted by the detailed theory. An analytical approximation of this ratio shows that the main reason for the asymmetry is a difference in the axial orders of the coupled modes. In some experimental cases, the orthogonal output is not well fitted by the model that assumes a single mode of each polarization, and we demonstrate that this fitting discrepancy can be the result of additional mode interactions.

Modes Trimming and Clustering in a Weakly Perturbed High‐Q Whispering Gallery Microresonator

AbstractIn general, a high‐quality (Q) microresonator can accommodate abundant whispering gallery modes (WGMs) with the mode number increasing with the dimensional sizes of the microresonator. Removing the unnecessary modes while reorganizing the remaining modes is of vital importance, which, however, has been proved challenging and usually results in a tradeoff with the Q of the microresonator. Here, an effective and controllable mode trimming and clustering mechanism is revealed underlying the generation of polygon and star modes in weakly perturbed tapered fiber‐coupled lithium niobate whispering gallery microresonators. Experimentally, various polygon and star modes are observed in sequence within a single microresonator by tuning the excitation wavelength or varying the coupling position between a tapered fiber and the circular microresonator, which can be well reproduced with the theoretical model. The finding offers a ubiquitous solution for a broad range of applications requiring elaborate selection and organization of the high‐Q WGMs.

Squeezing-induced nonreciprocal photon blockade in an optomechanical microresonator.

We propose a scheme to generate nonreciprocal photon blockade in a stationary whispering gallery microresonator system based on two physical mechanisms. One of the two mechanisms is inspired by recent work [Phys. Rev. Lett.128, 083604 (2022)10.1103/PhysRevLett.128.083604], where the quantum squeezing caused by parametric interaction not only shifts the optical frequency of propagating mode but also enhances its optomechanical coupling, resulting in a nonreciprocal conventional photon blockade phenomenon. On the other hand, we also give another mechanism to generate stronger nonreciprocity of photon correlation according to the destructive quantum interference. Comparing these two strategies, the required nonlinear strength of parametric interaction in the second one is smaller, and the broadband squeezed vacuum field used to eliminate thermalization noise is no longer needed. All analyses and optimal parameter relations are further verified by numerically simulating the quantum master equation. Our proposed scheme opens a new avenue for achieving the nonreciprocal single photon source without stringent requirements, which may have critical applications in quantum communication, quantum information processing, and topological photonics.

Enhancement of Dissipative Sensing in a Microresonator Using Multimode Input.

Optical whispering-gallery microresonators have proven to be especially useful as chemical sensors. Most applications involve dispersive sensing, such as the frequency shift of resonator modes in response to a change in the ambient index of refraction. However, the response to dissipative interaction can be even more sensitive than the dispersive response. Dissipative sensing is most often conducted via a change in the mode linewidth owing to absorption in the analyte, but the change in the throughput dip depth of a mode can provide better sensitivity. Dispersive sensing can be enhanced when the input to the microresonator consists of multiple fiber or waveguide modes. Here, we show that multimode input can enhance dip-depth dissipative sensing by an even greater factor. We demonstrate that the multimode-input response relative to single-mode-input response using the same fiber or waveguide can be enhanced by a factor of more than one thousand, independent of the mode linewidth, or quality factor (Q), of the mode. We also show that multimode input makes the dip-depth response nearly one hundred times more sensitive than the linewidth-change response. These enhancement factors are predicted by making only two measurements of dip depth in the absence of an analyte: one with the two input modes in phase with each other, and one with them out of phase.

Optomechanically induced thermal bistability in an optical microresonator

Optomechanical systems have attracted considerable attention due to their wide applications in hybrid transduction systems, sensors, and quantum measurements. Classical optomechanical systems have various dynamical regimes that depend on the parameters, and the situation can be even more complicated when incorporated with other nonlinearities, such as the thermal nonlinearity that is common in optical resonators. Here, we experimentally and theoretically study the thermal bistability induced by optomechanical effect in a whispering-gallery mode microresonator. The mechanical oscillation modulates the intracavity photon number and modifies the thermal stability of states under a blue-detuned pump laser. Utilizing a silica microtip to suppress the mechanical motion, we identify the regions where the optical cavity temperature and detuning change due to the onset of oscillation and characterize the shifts in the detuning quantitatively. Hysteretic response in the mechanical frequency shifts (optical spring effect) to the pump wavelength is also observed to confirm the presence of bistability. The thermal dynamics discussed here is essentially different from the optomechanical heating and cooling of the effective temperature of the mechanical oscillator. This discovery shows that the thermal nonlinearity, which has been used for locking the pump frequency in the optomechanical systems, may also make some detuning states thermally unstable. Our study demonstrates an approach to characterize the detuning of the pump laser in an optomechanical resonator with thermal nonlinearity and also lays the groundwork for further understanding of dynamical effects induced by optomechanical effects.

Nanocrystal Superparticles with Whispering-Gallery Modes Tunable through Chemical and Optical Triggers.

Whispering-gallery microresonators have the potential to become the building blocks for optical circuits. However, encoding information in an optical signal requires on-demand tuning of optical resonances. Tuning is achieved by modifying the cavity length or the refractive index of the microresonator. Due to their solid, nondeformable structure, conventional microresonators based on bulk materials are inherently difficult to tune. In this work, we fabricate irreversibly tunable optical microresonators by using semiconductor nanocrystals. These nanocrystals are first assembled into colloidal spherical superparticles featuring whispering-gallery modes. Exposing the superparticles to shorter ligands changes the nanocrystal surface chemistry, decreasing the cavity length of the microresonator by 20% and increasing the refractive index by 8.2%. Illuminating the superparticles with ultraviolet light initiates nanocrystal photo-oxidation, providing an orthogonal channel to decrease the refractive index of the microresonator in a continuous fashion. Through these approaches, we demonstrate optical microresonators tunable by several times their free spectral range.

Three methods for the single-photon transport in a chiral cavity quantum electrodynamics system

We investigate the single-photon transport problem in the system of a Whispering-Gallery mode microresonator directionally coupled with a two-level quantum emitter (QE). This QE-microresonator coupling system can usually be studied by cavity quantum electrodynamics and the single-photon transport methods. However, we find that if we treat a two-level QE as a single-photon phase-amplitude modulator, we can also deal with such systems using the transfer matrix method. Further, in theory, we prove that these three methods are equivalent. The corresponding relations of respective parameters among these approaches are precisely deduced. Our work can be extended to a multiple-resonator system interacting with two-level QEs in a chiral way. Therefore, the transfer matrix method may provide a convenient and intuitive form for exploring more complex chiral QE-resonator interaction systems.

Dynamical determination of the strength of cross-polarization coupling in a whispering-gallery microresonator

Cross-polarization coupling (CPC) between coresonant orthogonally polarized modes in a single whispering-gallery microresonator can lead to induced transparency effects. Depending on the CPC strength, coupled-mode-induced transparency (CMIT) or Autler-Townes splitting (ATS) can be observed. Determining the CPC strength is crucial in locating the exceptional point, where CMIT evolves into ATS and an enhancement in sensitivity to perturbations is expected. The CPC strength can be calculated theoretically based on the transverse structure of whispering-gallery modes. Experimentally, however, CPC strength has been inferred only indirectly by fitting the experimental throughput spectrum to a steady-state model. In this work, we propose and demonstrate a direct determination of the CPC strength based on the response of the throughput to a sinusoidal amplitude modulation of the input light. Our experimental results agree with numerical fitting values and are in the same range as predicted by theoretical calculations.

Dynamic regulating of lasing mode in a whispering-gallery microresonator by thermo-optic effect

Realizing dynamic regulation of specific optical components while being easy to integrate with photonic circuits will have a revolutionary impact on tunable laser sources, active filters, and all-optical switching/integration. Here, we demonstrate a temperature tunable Ga-doped ZnO microresonator with a conductivity of ∼1.75 S/cm controlled by the injection current and realize point-to-point heating. The shift in the resonant wavelength with a tuning range of 2 nm for TE66 mode is achieved by the effect of current-induced temperature rising on the refractive index of the microresonator crystal, namely, the thermo-optic effect. Meanwhile, the thermo-optic coefficient of 16.7 × 10−4/K around the bandgap energy of ZnO is also obtained. This operating mode of electronically controlled temperature establishes a solid foundation for the practicality and integration of tunable lasers.

Single-Phonon Addition and Subtraction to a Mechanical Thermal State.

Adding or subtracting a single quantum of excitation to a thermal state of a bosonic system has the counter-intuitive effect of approximately doubling its mean occupation. We perform the first experimental demonstration of this effect outside optics by implementing single-phonon addition and subtraction to a thermal state of a mechanical oscillator via Brillouin optomechanics in an optical whispering-gallery microresonator. Using a detection scheme that combines single-photon counting and optical heterodyne detection, we observe this doubling of the mechanical thermal fluctuations to a high precision. The capabilities of this joint click-dyne detection scheme adds a significant new dimension for optomechanical quantum science and applications.

Frequency comb generation threshold via second-harmonic excitation in χ (2) optical microresonators

We investigate the threshold of χ(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 χ(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.

Polygon Coherent Modes in a Weakly Perturbed Whispering Gallery Microresonator for Efficient Second Harmonic, Optomechanical, and Frequency Comb Generations.

We observe high optical quality factor (Q) polygonal and star coherent optical modes in a lithium niobate microdisk. In contrast to the previous polygon modes achieved by deformed microcavities at lower mechanical and optical Q, we adopt weak perturbation from a tapered fiber for the polygon mode formation. The resulting high intracavity optical power of the polygon modes triggers second harmonic generation at high efficiency. With the combined advantages of a high mechanical Q cavity, we observe optomechanical oscillation in polygon modes for the first time. Finally, we observe frequency microcomb generation from the polygon modes with an ultrastable taper-on-disk coupling mechanism.

Perpendicular coupler for standing wave excitation and wavelength selection in high-Q silicon microresonators.

High quality factor (Q) whispering gallery mode (WGM) resonators have been widely applied in photonics, while the excitation and collection of WGMs are mostly restricted to traveling wave coupler. Here, we experimentally demonstrate a novel on-chip perpendicular coupler (PC) for high-Q (∼1.1 × 105) silicon whispering gallery microresonators. The PC is compact and allows efficiently tunneling coupling between the waveguide and the microresonator, hence it holds great potential for fan-out photonic devices. Drastically different from the traveling wave couplers, standing wave mode can be excited through the PC. In addition, a PC working as an output coupler can also selectively collect the resonance of different wavelengths by locating on different azimuth angles. Our results show the feasibility of such novel coupler for WGM resonators and its potential use in future applications of integrated high Q microresonators.

Coupled-mode-induced transparency and attenuation resulting from cross-polarization coupling

Induced transparency and attenuation effects are observed in the throughput of a single whispering-gallery microresonator due to mode coupling between two coresonant orthogonally polarized whispering-gallery modes of very different quality factors. Intracavity cross-polarization coupling, occurring when either the transverse electric (TE) mode or the transverse magnetic (TM) mode is driven, results in coupled-mode induced transparency or coupled-mode induced attenuation. Coresonance between the TE and TM modes is obtained by strain tuning, and the cross-polarization coupling is produced by polarization rotation due to optical spin-orbit interaction in a slightly asymmetric resonator. The observed behavior enables slow light and fast light, i.e., the delay or advancement of an incident resonant pulse. Experimental results representative of several different types of behavior are presented here. Induced transparency is seen to be accompanied by pulse delay, whereas induced attenuation can involve pulse advancement or delay. The results are analyzed and explained by analytical modeling and by comparison to the output of a more detailed numerical model describing these effects. Delays of up to 170 ns and advancements of up to 14 ns are found. The observed range of cross-polarization coupling strengths (probability of polarization change per round trip), namely, ${10}^{\ensuremath{-}10}--{10}^{\ensuremath{-}7}$, is in agreement with theory.

Symmetry-breaking-induced dynamics in a nonlinear microresonator.

Optically induced symmetry-breaking plays a key role in nonlinear photonics. Recently, the experiment has successfully observed the Kerr-nonlinearity-induced chiral symmetry breaking in a single ultrahigh-Q whispering-gallery microresonator. Here, we show this symmetry-breaking can generate exotic dynamics between two counter-propagating modes. In particular, we predict two kinds of self-trappings, in which the corresponding relative phase oscillates around π or runs without bound although they have both the nonzero mean energy imbalance. Finally, we also clarify the impacts of the mode loss, finding a dynamical transition from self-trappings to an anharmonic oscillation. The presented scheme offers a new route to understanding the nonlinear dynamics and wave chaos in the microresonator.

Electrically pumped 1.5 μm InP-based quantum dot microring lasers directly grown on (001) Si.

Directly grown quantum dot (QD) lasers on silicon are appealing for monolithic integration of photonic circuits from a technoeconomic perspective. In this Letter, we report miniaturization of these Si-based lasers employing high-quality whispering-gallery mode microresonators. Based on previously developed InAs/InAlGaAs QDs on the complementary metal-oxide-semiconductor-standard (001) Si platform and optimized device implementation techniques, on-chip electrically pumped InP-based QD microring lasers (MRLs) on Si are successfully realized for the first time. Room-temperature pulsed lasing in the 1.5μm wavelength band, with a threshold of 50mA, is measured for 50-μm-diameter MRLs. Lasing up to 70°C is achieved with a characteristic temperature of 51.5K.

Scully-Lamb quantum laser model for parity-time-symmetric whispering-gallery microcavities: Gain saturation effects and nonreciprocity

We use a non-Lindbladian master equation of the Scully-Lamb laser model for the analysis of light propagation in a parity-time symmetric photonic system composed of coupled active and passive whispering-gallery microresonators. Performing the semiclassical approximation, we obtain a set of two nonlinear coupled differential equations describing the time evolution of intracavity fields. These coupled equations are able to explain the experimentally-observed light non-reciprocity [Peng {\em et al.}, Nature Physics {\bf 10}, 394 (2014), Chang {\em et al.}, Nature Photonics {\bf 8}, 524 (2014)]. We show that this effect arises from the interplay between gain saturation in the active microcavity, intercavity coupling, and losses in the cavities. Additionally, using this approach, we study the effect of the gain saturation on exceptional points, i.e., exotic degeneracies in non-Hermitian systems. Namely, we demonstrate that the inclusion of gain saturation leads to a modification of the exceptional points in the presence of intense intracavity fields. The Scully-Lamb master equation for systems of coupled optical structures, as proposed and applied here, constitutes a promising tool for the study of quantum optical effects in coupled systems with losses, gain, and gain saturation.

Integrated Thermal Silica Micro-Resonator Waveguide System With Ultra-Low Fluorescence

Thermally grown silica is one of the most desirable materials for the realization of optical waveguides and whispering-gallery mode micro-resonators due to the ultra-low propagation loss and broad transparency window from UV to mid-IR. Here, we present the design, fabrication, and characterization of a high-Q ring-resonator device with a monolithically integrated evanescent rib waveguide coupler made solely from silica thermally grown on a Si substrate. The device delivered an intrinsic Q-factor of $3.7\,\times \, 10^{6}$ and operated close to the critical coupling regime for both TE and TM polarizations. The achieved Q-factors were more than three orders of magnitude higher than those of comparable silica resonator devices. Furthermore, we showcase the ultra-low parasitic fluorescence unique to thermally grown silica and compare it to a device fabricated in Si3N4.

Photonic-chip-on-tip: compound photonic devices fabricated on optical fibers.

Just like nanometer-scale conductive paths in an electronic chip at some point end up connected to macroscopic wires of the printed circuit board, photonic integrated circuits often need light in/out coupling from/to external devices, such as light sources or detectors. In the optical domain, these connections are challenging due to the scale mismatch and alignment precision required. At the same time, there is more than 24,500 μm2 of space available on two cleaved single mode optical fiber tips. We demonstrate that this space can be used to fabricate compound photonic assembly - Photonic-chip-on-tip - directly integrated with the fibers. As an example, we present a simple setup consisting of in- and out-coupling prisms, tapered waveguide, and a whispering gallery micro-resonator, all made in a single process with two-photon laser photolithography. Temperature sensing is demonstrated as an example of application. This approach to photonic circuit design intrinsically addresses the problems of scale mismatch, fiber alignment, light coupling, and packaging.