Whispering Gallery Mode Cavities Research Articles

Simulation and theoretical study of high Q gradient refractive index microresonators

Whispering gallery mode (WGM) microresonators are widely used because of their high Q factors. However, high Q factors cannot be consistently attained, as WGM cavities continue to be affected by environmental contamination and roughness. To overcome this limitation, the gradient refractive index (GRIN) has been suggested to shifting the mode field inward and preventing total internal reflection at the surface. This study proves the following two results using the simulation and potential functions. First, after ion-exchange, the refractive index fluctuations at the internal mode positions are quite small, even with roughness. As a result, the Q factor is always greater than that before ion-exchange. Second, the GRIN microresonator have a high Q factor (7.5×107) in the external high refractive index (nex=1.5) environment because the GRIN material itself creates a potential barrier, which means that it is significantly more resistant to environmental disturbances than the WGM.

Auxiliary-Cavity-Induced Ultrasensitive and Ultrahigh-Resolution Biomolecule Mass Sensing in a Hybrid Spinning Resonator System

We theoretically demonstrate an ultrasensitive and ultrahigh-resolution biomolecule mass sensing based on a spinning resonator system, which includes a spinning whispering-gallery-mode (WGM) optomechanical cavity and an auxiliary optical cavity. When the WGM cavity is spinning along the clockwise or counterclockwise direction, the cavity field will undergo different Sagnac effect. If the Sagnac effect and the auxiliary optical cavity are simultaneously taken into consideration, not only the line width of the transmission spectrum is significantly squeezed, but also the transmission intensity will be extremely enhanced, which indicates an ultrasensitive and ultrahigh-resolution mass sensor. When the mass of external biomolecules (such as baculovirus or coronavirus) is deposited on the resonator, their mass can be determined by tracking the resonance frequency shifts in the transmission spectrum. Thus, our research can provide a method to classify kinds of viruses, especially can be used to identify 2019-nCoV.

A Lever-Type PDMS Flexible Cavity for Acoustic Vector Sensor With High Sensitivity

The whispering gallery mode cavities with high quality factor as highly sensitive acoustic sensors, due to narrow linewidth, are capable of detecting slight external disturbances. However, spherical cavities pose a challenge in the directivity of acoustic detection caused by structure symmetry. Here, a lever acoustic sensor combined with flexible polymer cavity is proposed. The copper sheet as the acoustic signal receiver provides a good directivity. As a signal readout unit, the flexible characteristics of polymer cavity with the quality factor of 105 increase the signal response, which is fabricated by superoleophobic substrates. An optical fiber acts as a cantilever to transfer energy between them. Frequency responses and acoustic vibrations are collected between 200 Hz and 10 kHz in the experiment. The maximum sensitivity and the minimum detectable acoustic pressure at 4 kHz are 23.58 mV/Pa and 1.38 mPa/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text {1/2}}$ </tex-math></inline-formula> , respectively. The good directivity with a notch depth of 16 dB in the directional map is obtained, which shows a promising candidate for applications.

Rod and slit photonic crystal microrings for on-chip cavity quantum electrodynamics

Abstract Micro-/nanocavities that combine high quality factor (Q) and small mode volume (V) have been used to enhance light–matter interactions for cavity quantum electrodynamics (cQED). Whispering gallery mode (WGM) geometries such as microdisks and microrings support high-Q and are design- and fabrication-friendly, but V is often limited to tens of cubic wavelengths to avoid WGM radiation. The stronger modal confinement provided by either one-dimensional or two-dimensional photonic crystal defect geometries can yield sub-cubic-wavelength V, yet the requirements on precise design and dimensional control are typically much more stringent to ensure high-Q. Given their complementary features, there has been sustained interest in geometries that combine the advantages of WGM and photonic crystal cavities. Recently, a “microgear” photonic crystal ring (MPhCR) has shown promise in enabling additional defect localization ( &gt; $ &gt; $ 10× reduction of V) of a WGM, while maintaining high-Q ( ≈ 1 0 6 ) $(\approx 1{0}^{6})$ and other WGM characteristics in ease of coupling and design. However, the unit cell geometry used is unlike traditional PhC cavities, and etched surfaces may be too close to embedded quantum nodes (quantum dots, atomic defect spins, etc.) for cQED applications. Here, we report two novel PhCR designs with “rod” and “slit” unit cells, whose geometries are more traditional and suitable for solid-state cQED. Both rod and slit PhCRs have high-Q ( &gt; 1 0 6 ) $( &gt; 1{0}^{6})$ with WGM coupling properties preserved. A further ≈10× reduction of V by defect localization is observed in rod PhCRs. Moreover, both fundamental and 2nd-order PhC modes co-exist in slit PhCRs with high Qs and good coupling. Our work showcases that high-Q/V PhCRs are in general straightforward to design and fabricate and are a promising platform to explore for cQED.

High-specificity molecular sensing on an individual whispering-gallery-mode cavity: coupling-enhanced Raman scattering by photoinduced charge transfer and cavity effects.

Optical whispering-gallery-mode (WGM) cavities have gained considerable interest because of their unique properties of enhanced light-matter interactions. Conventional WGM sensing is based on the mechanisms of mode shift, mode broadening, or mode splitting, which requires a small mode volume and an ultrahigh Q-factor. Besides, WGM sensing suffers from a lack of specificity in identifying substances, and additional chemical functionalization or incorporation of plasmonic materials is required for achieving good specificity. Herein, we propose a new sensing method based on an individual WGM cavity to achieve ultrasensitive and high-specificity molecular sensing, which combines the features of enhanced light-matter interactions on the WGM cavity and the "fingerprint spectrum" of surface-enhanced Raman scattering (SERS). This method identifies the substance by monitoring the Raman signal enhanced by the WGM cavity rather than monitoring the variation of the WGM itself. Therefore, ultrasensitive and high-specificity molecular sensing can be accomplished even on a low-Q cavity. The working principles of the proposed sensing method were also systematically investigated in terms of photoinduced charge transfer, Purcell effect, and optical resonance coupling. This work provides a new WGM sensing approach as well as a strategy for the design of a high-performance SERS substrate by creating an optical resonance mode.

The nonlinear effects and applications of gain doped whispering-gallery mode cavities

Whispering-gallery mode (WGM) cavities formed by dielectric structures have attracted intensive interest in various fields. The high-quality factor and smaller mode volume associated with the optical modes have inspired experiments in nonlinear optics, nanophotonics, and quantum information science. Moreover, they are also used in optical biosensors and other significant applications. To further reduce the material loss of the resonator, optical gain materials, such as erbium and ytterbium, are doped into the dielectric structure to increase the nonlinear effect and enhance the interaction between light and matter. Here in this review, we outline the most recent advancements in gain-doped optical WGM microcavities. Moreover, we introduce the dynamics of the gain in WGM resonators, the integration of gain media into WGM microcavities with various shapes, and the fabrication and applications of the gain microcavities. Also, the applications of the gain cavity based on the whispering-gallery mode have been introduced, e.g., ultra-sensitive sensors, low-threshold lasers, and high-performance optical systems.

Exceptional Points in a Spiral Ring Cavity for Enhanced Biosensing

The unique point of non-Hermitian degeneracy in an open system, known as exceptional point (EP), holds great potential in cavity quantum electrodynamics, optical signal processing, and sensing applications. For on-chip miniaturized optical cavities, auxiliary means have been employed for steering a non-Hermitian system to an EP, which typically requires either precision post-fabrication tuning or external off-chip components. Here we report a route of engineering non-Hermitian degeneracies in spiral ring microcavities without any external or temporary assistance. In such a deformed whispering gallery mode cavity which can be readily obtained through the self-rolling of pre-strained nanomembranes, the coupling strength of original non-degenerate modes can be fine tailored by spiral edge-induced backscatterings. Our theoretical results suggest that an EP can be reached by adjusting the physical scales and relative positions of inner and outer spiral edges. For EP-enhanced sensing, a functional regime upon a tiny perturbation where splitting scales as the square root of the perturbation and a saturated regime upon a sufficiently large perturbation are distinguished. For EPs in such a single-resonator system, the enhanced sensitivity towards localized perturbations and robustness towards homogenous perturbations are desirable for single-molecule detections. We expect that this work will serve as a generic guideline for realizing compact EP-empowered sensing systems and provide new insights into ultra-sensitive bioanalyses.

Flexible colloidal quantum dot lasers enabled by self-assembly

Colloidal quantum dot (CQD) lasers show promising applications in flexible optoelectronic devices, due to their tunable emission wavelength, narrow spectrum bandwidth and high power intensity. However, fabricating a flexible CQD laser is challenging because of the difficulties in fabricating optical cavities on flexible substrates using traditional microfabrication technologies. Herein, we propose a one-step self-assembly approach to fabricate flexible CQD supraparticle lasers. The whole assembly approach is processed in a liquid environment without surfactants, and the formed spherical CQD supraparticles are featured with smooth surfaces, serving as high-quality-factor whispering-gallery mode cavities to support laser oscillation. A low lasing threshold of 54 µJ/cm2 is observed while exciting a CQD supraparticle with pulsed femtosecond lasers. The calculated cavity quality factor of 963 for CQD supraparticle lasers is twofold larger than that of CQD lasers assembled with surfactants. Moreover, the CQD supraparticles can serve as free-standing lasers, which allows them to be deposited on flexible substrates such as paper and cloth. Furthermore, our CQD lasers show high stability, after being continuously photoexcited above the threshold for 400 min, their lasing intensity remains at 85.7% of the initial value. As bright, free-standing and long-term stable light sources, the assembled CQD lasers proposed in this work show potential applications in wearable devices and medical diagnosis.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

Active Control of Plasmonic-Photonic Interactions in a Microbubble Cavity.

Active control of light-matter interactions using nanophotonic structures is critical for new modalities for solar energy production, cavity quantum electrodynamics (QED), and sensing, particularly at the single-particle level, where it underpins the creation of tunable nanophotonic networks. Coupled plasmonic-photonic systems show great promise toward these goals because of their subwavelength spatial confinement and ultrahigh-quality factors inherited from their respective components. Here, we present a microfluidic approach using microbubble whispering-gallery mode cavities to actively control plasmonic-photonic interactions at the single-particle level. By changing the solvent in the interior of the microbubble, control can be exerted on the interior dielectric constant and, thus, on the spatial overlap between the photonic and plasmonic modes. Qualitative agreement between experiment and simulation reveals the competing roles mode overlap and mode volume play in altering coupling strengths.

The fast and slow light in a hybrid spinning optomechanical system mediated by a two-level system

We present a hybrid spinning whispering-gallery-mode (WGM) optomechanical system, where the mechanical resonator is not only coupled to the WGM cavity via radiation pressure but also coupled to a two-level system (TLS) via the Jaynes–Cummings coupling. If the WGM cavity is spinning along the clockwise or counterclockwise direction, the cavity field can undergo different Sagnac effect. The transmission spectra experience the conversion from optomechanically induced transparency (OMIT) to Fano resonance by controlling the resonator-TLS coupling, the rotational direction of the WGM cavity, and the cavity-pump detuning. Furthermore, both the OMIT and Fano resonance in the transmission spectra can induce the transparency windows accompanied by the rapid normal phase dispersion, which results in the tunable delayed or accelerated probe light transmission. The numerical results indicate that the OMIT leads to slow light, while Fano resonances generate the conversion from fast to slow light (or slow to fast light), and the parameters of the resonator-TLS coupling, the Sagnac frequency shift, the pump field frequency, and the cavity-pump detuning together determine the fast and slow light effect based on the hybrid spinning optomechanical system.

III–V Semiconductor Whispering-Gallery Mode Micro-Cavity Lasers: Advances and Prospects

III-V semiconductor lasers are ubiquitous in modern optoelectronic devices, with applications ranging from telecommunication to general lighting. Among the different kinds of laser cavity designs, whispering-gallery mode (WGM) micro-cavity lasers boast outstanding optical performance due to advantages such as ultra-high Q-factor, compact mode volume, and narrow emission linewidth. Over the past decades, research in III-V WGM micro-cavity lasers has progressed rapidly in various aspects, including the fabrication techniques, emission outcoupling methods, and practical applications. In this paper, a comprehensive review is performed on the advances in these aspects. Although III-V WGM lasers are conventionally fabricated with top-down approaches, recent reports have demonstrated the potential of highly-scalable bottom-up methods in fabricating low-loss WGM lasers. Despite the strong optical confinement in III-V WGM cavities, various techniques have been developed to either outcouple WGM emission into waveguides or direct it into free-space with small beam divergence. Finally, recent developments in different applications of III-V WGM micro-cavity lasers are discussed. Other than serving as integrated photonic components, III-V WGM lasers have also displayed exciting potential in other applications such as label-free sensing and the study of cavity quantum electrodynamics (cQED).

Whispering Gallery Mode Resonances in Thermally Poled Borosilicate Glass Hetero-Fibers

Whispering gallery mode (WGM) resonances in radially, thermally poled glass hetero-fibers, are investigated for the first time. Upon radially oriented thermal poling, both TE and TM polarized responses undergoing a spectral “cleaning” process, namely higher-order radial modes are being suppressed, or even fully annihilated, compared to the spectral behavior of the pristine fibers. Second-harmonic generation (SHG) microscopy, Energy Dispersive X-Ray Spectroscopy (EDX) and micro-Raman (μ-Raman) measurements were conducted in order to reveal the spatial profile of the structural and optical changes introduced to the thermally poled hetero-fibers, and correlate them with the WGM spectral measurements. The specific selective, mode cleaning behavior is attributed to refractive index changes generated in proximity to the fiber's circumference, which are associated with radially-symmetric ionic re-distribution. The thermally poled hetero-fiber WGM cavities have been rigorously simulated using the finite element method, the calculated modal eigenstates and transmission spectra, further confirm the specific mode selection mechanism, introduced by the azimuthally symmetric thermal, poling process.

Electrically driven whispering-gallery-mode microlasers in an n-MgO@ZnO:Ga microwire/p-GaN heterojunction.

In emerging miniaturized applications, semiconductor micro/nanostructures laser devices have drawn great public attentions of late years. The device performances of micro/nanostructured microlasers are highly restricted to the different reflective conditions at various side surfaces of microresonators and junction interface quality. In this study, an electrically driven whispering-gallery-mode (WGM) microlaser composed of a Ga-doped ZnO microwire covered by a MgO layer (MgO@ZnO:Ga MW) and a p-type GaN substrate is illustrated experimentally. Incorporating a MgO layer on the side surfaces of ZnO:Ga MWs can be used to reduce light leakage along the sharp edges and the ZnO:Ga/GaN interface. This buffer layer incorporation also enables engineering the energy band alignment of n-ZnO:Ga/p-GaN heterojunction and manipulating the current transport properties. The as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device can emit at an ultraviolet wavelength of 375.5 nm and a linewidth of about 25.5 nm, achieving the excitonic-related recombination in the ZnO:Ga MW. The broadband spectrum collapsed into a series of sharp peaks upon continuous-wave (CW) operation of electrical pumping, especially for operating current above 15.2 mA. The dominant emission line was centered at 378.5 nm, and the line width narrowed to approximately 0.95 nm. These sharp peaks emerged from the spontaneous emission spectrum and had an average spacing of approximately 5.5 nm, following the WGM cavity modes. The results highlight the significance of interfacial engineering for optimizing the performance of low-dimensional heterostructured devices and shed light on developing future miniaturized microlasers.

Research Progress of Gallium Nitride Microdisk Cavity Laser

Whispering gallery mode (WGM) cavities provide resonance configurations for light propagation through internal reflection, achieving high Q factors, low thresholds, and small mode volumes. GaN-based materials exhibit high freedom in band engineering and are highly compatible with contemporary semiconductor processing technology. Recently, lasers from artificial GaN microdisks, obtained by combining the excellent material properties of GaN with the advantages of WGM, have attracted considerable research attention. These have a wide application scope in optical communication, display, and optoelectronic integration. In this review, we summarize the recent advances in GaN-based WGM microlasers, including the fabrication methods for GaN microcavities, observations of optical pumped GaN microdisk lasing, lasing mechanisms, comparison of Q factors, lasing modes, and threshold properties, commonly used light field control techniques, and mode clipping methods. Furthermore, we introduce the recent advances in electrically driven GaN-based laser diodes, followed by research challenges and strategies for promising applications, such as electrically pumped lasers and optoelectronic chip integration.

Single-Mode Emission in InP Microdisks on Si Using Au Antenna.

An important building block for on-chip photonic applications is a scaled emitter. Whispering gallery mode cavities based on III–Vs on Si allow for small device footprints and lasing with low thresholds. However, multimodal emission and wavelength stability over a wider range of temperature can be challenging. Here, we explore the use of Au nanorod antennae on InP whispering gallery mode lasers on Si for single-mode emission. We show that by proper choice of the antenna size and positioning, we can suppress the side modes of a cavity and achieve single-mode emission over a wide excitation range. We establish emission trends by varying the size of the antenna and show that the far-field radiation pattern differs significantly for devices with and without antenna. Furthermore, the antenna-induced single-mode emission is dominant from room temperature (300 K) down to 200 K, whereas the cavity without an antenna is multimodal and its dominant emission wavelength is highly temperature-dependent.

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide.

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Control of polarization switching in a VCSEL via resonant feedback from a whispering-gallery-mode cavity.

We report a method for flexibly switching the dominant polarization of a vertical-cavity surface-emitting laser (VCSEL) by introducing polarization-resolved resonant optical feedback from a whispering-gallery-mode (WGM) cavity to the lasing cavity. Switching between the originally dominant mode and a side mode is experimentally demonstrated under different bias currents once one of them is locked to the resonance mode of the WGM cavity. In addition to a controllable polarization state, the reported VCSEL also demonstrates a linewidth as narrow as tens of kilohertz, which is highly desirable for many applications, including high-speed data communication, light detection and ranging (lidar), and absorption spectroscopy.

Optical isolator based on backward Brillouin scattering

We propose to realize an optical isolator based on backward Brillouin scattering in a whispering-gallery-mode (WGM) cavity that supports both traveling optical and acoustic waves. When the WGM cavity is optically driven in one direction, the time-reversal symmetry for light transmission is broken due to the unidirectional photon–phonon coupling. We show that the signal light is either transmitted or completely absorbed depending on whether it propagates in the same or opposite direction as the strong pump laser. The perfect absorption can be interpreted as a phase transition in a parity–time–symmetric Hamiltonian with the balanced gain and loss. The present work paves a way for the realization of an on-chip optical isolator with the high degree of isolation, large bandwidth, and low insertion loss simultaneously.

High-Q slow light and its localization in a photonic crystal microring

We introduce a photonic crystal ring cavity that resembles an internal gear and unites photonic crystal (PhC) and whispering gallery mode (WGM) concepts. This `microgear' photonic crystal ring (MPhCR) is created by applying a periodic modulation to the inside boundary of a microring resonator to open a large bandgap, as in a PhC cavity, while maintaining the ring's circularly symmetric outside boundary and high quality factor ($Q$), as in a WGM cavity. The MPhCR targets a specific WGM to open a large PhC bandgap up to tens of free spectral ranges, compressing the mode spectrum while maintaining the high-$Q$, angular momenta, and waveguide coupling properties of the WGM modes. In particular, near the dielectric band-edge, we observe modes whose group velocity is slowed down by 10 times relative to conventional microring modes while supporting $Q~=~(1.1\pm0.1)\times10^6$. This $Q$ is $\approx$50$\times$ that of the previous record in slow light devices. Using the slow light design as a starting point, we further demonstrate the ability to localize WGMs into photonic crystal defect (dPhC) modes for the first time, enabling a more than 10$\times$ reduction of mode volume compared to conventional WGMs while maintaining high-$Q$ up to (5.6$\pm$0.1)$\times$10$^5$. Importantly, this additional dPhC localization is achievable without requiring detailed electromagnetic design. Moreover, controlling their frequencies and waveguide coupling is straightforward in the MPhCR, thanks to its WGM heritage. By using a PhC to strongly modify fundamental properties of WGMs, such as group velocity and localization, the MPhCR provides an exciting platform for a broad range of photonics applications, including sensing/metrology, nonlinear optics, and cavity quantum electrodynamics.