Whispering Gallery Research Articles

Diffraction of a Whispering Gallery Mode at a Jumply Straightening of the Boundary

Diffraction of a high-frequency small-number whispering gallery mode running along a concave boundary, which turns into a flat one so that its curvature experiences a jump, is studied. The cases of rigid (Neumann) and soft (Dirichlet) boundary conditions are considered. Within the framework of the parabolic equation method, a mathematically correct scattering problem is obtained which is solved explicitly and investigated asymptotically in detail. Analytic expressions are found for all emerging wavefields. In particular, an edge wave diverging from the point of non-smoothness of the boundary is described. For the rigid condition, its amplitude is proportional to the magnitude of curvature jump, but not for the soft condition.

Sandwich structure magnetometer with a high sensitivity and signal-to-noise ratio based on an ultrahigh-Q CaF2 resonator.

Whispering gallery mode (WGM) resonators with an ultrahigh quality (Q) factor provide an extremely high resolution for high-precision sensing. However, it is difficult to use them directly in magnetic sensors because of the transparency to the magnetic field. In this paper, a sandwich structure consisting of a neodymium iron boron magnet and calcium fluoride resonator with a Q factor of 109 is proposed. The experimental results show that, compared with the conventional magnetometer, the signal-to-noise ratio of the optical WGM magnetometer reaches 62dB, with the direct current sensitivity of 42.59MHz/mT and the AC sensitivity of 794p T H z -1/2 at 42kHz.

Excitonic Mechanisms of Stimulated Emission in Low-Threshold ZnO Microrod Lasers with Whispering Gallery Modes.

Whispering gallery mode (WGM) ZnO microlasers gain attention due to their high Q-factors and ability to provide low-threshold near-UV lasing. However, a detailed understanding of the optical gain mechanisms in such structures has not yet been achieved. In this work, we study the mechanisms of stimulated emission (SE) in hexagonal ZnO microrods, demonstrating high-performance WGM lasing with thresholds down to 10-20 kW/cm2 and Q-factors up to ~3500. The observed SE with a maximum in the range of 3.11-3.17 eV at room temperature exhibits a characteristic redshift upon increasing photoexcitation intensity, which is often attributed to direct recombination in the inverted electron-hole plasma (EHP). We show that the main contribution to room-temperature SE in the microrods studied, at least for near-threshold excitation intensities, is made by inelastic exciton-electron scattering rather than EHP. The shape and perfection of crystals play an important role in the excitation of this emission. At lower temperatures, two competing gain mechanisms take place: exciton-electron scattering and two-phonon assisted exciton recombination. The latter forms emission with a maximum in the region near ~3.17 eV at room temperature without a significant spectral shift, which was observed only from weakly faceted ZnO microcrystals in this study.

Fractional Optical Angular Momentum and Multi-Defect-Mediated Mode Renormalization and Orientation Control in Photonic Crystal Microring Resonators.

Whispering gallery modes (WGMs) in circularly symmetric optical microresonators exhibit integer quantized angular momentum numbers due to the boundary condition imposed by the geometry. Here, we show that incorporating a photonic crystal pattern in an integrated microring can result in WGMs with fractional optical angular momentum. By choosing the photonic crystal periodicity to open a photonic band gap with a band-edge momentum lying between that of two WGMs of the unperturbed ring, we observe hybridized WGMs with half-integer quantized angular momentum numbers (m∈Z+1/2). Moreover, we show that these modes with fractional angular momenta exhibit high optical quality factors with good cavity-waveguide coupling and an order of magnitude reduced group velocity. Additionally, by introducing multiple artificial defects, multiple modes can be localized to small volumes within the ring, while the relative orientation of the delocalized band-edge states can be well controlled. Our Letter unveils the renormalization of WGMs by the photonic crystal, demonstrating novel fractional angular momentum states and nontrivial multimode orientation control arising from continuous rotational symmetry breaking. The findings are expected to be useful for sensing and metrology, nonlinear optics, and cavity quantum electrodynamics.

Large-scale flexible-resonators with temperature insensitivity employing superoleophobic substrates.

Whispering gallery mode polymer resonators are becoming competitive with devices made of other materials, however, the inherent thermal sensitivity of the materials and the small size limit their applications, such as high-precision optical gyroscope. Here, a method is proposed for fabricating large-scale NOA65 resonators with quality factors greater than 105 on a chip employing superoleophobic. The sandwich structure as the core layer of resonator is used to present the flexible remodeling characteristics, the surface roughness remains below 1 nm when the diameter changes by more than 25%. Importantly, theoretical and experimental results show that under the tuning action of external pressure, the equivalent thermal expansion coefficient of the resonator gradually approaches the glass sheet on both sides with the variation of 2 × 10-4 /°C∼0.9 × 10-4 /°C, and the corresponding temperature response range of 0.12 nm/°C∼-0.056 nm/°C shows the promise of temperature insensitivity resonators on a chip.

High-precision whispering gallery microsensors with ergodic spectra empowered by machine learning

Whispering gallery mode (WGM) microcavities provide increasing opportunities for precision measurement due to their ultrahigh sensitivity, compact size, and fast response. However, the conventional WGM sensors rely on monitoring the changes of a single mode, and the abundant sensing information in WGM transmission spectra has not been fully utilized. Here, empowered by machine learning (ML), we propose and demonstrate an ergodic spectra sensing method in an optofluidic microcavity for high-precision pressure measurement. The developed ML method realizes the analysis of the full features of optical spectra. The prediction accuracy of 99.97% is obtained with the average error as low as 0.32 kPa in the pressure range of 100 kPa via the training and testing stages. We further achieve the real-time readout of arbitrary unknown pressure within the range of measurement, and a prediction accuracy of 99.51% is obtained. Moreover, we demonstrate that the ergodic spectra sensing accuracy is∼11.5%higher than that of simply extracting resonating modes’ wavelength. With the high sensitivity and prediction accuracy, this work opens up a new avenue for integrated intelligent optical sensing.

Scattering-Assisted and Logic-Controllable WGM Laser in Liquid Crystal Micropillar

Whispering gallery mode (WGM) microcavities can efficiently store and manipulate light with strong light confinement and long photon lifetime, while coupling light into and from WGMs is intrinsically hindered by their unique feature of rotational symmetry. Here, a scattering-assisted liquid crystal (LC) micropillar WGM laser is proposed. WGM lasing at the surface of the micropillar is obviously enhanced by fluorescence scattering in the core of the micropillar. Besides, weak scattering of LC molecules also builds efficient coupling channels between the laser modes and the axial transmission modes of the micropillar-based waveguide, providing an all-in-one liquid WGM laser with functions of self-seeding and self-guiding. Furthermore, based on the hysteresis characteristics of the electrically anchored LC molecules under the interaction of thermal force, an erasable read-write liquid memory device is proposed, paving the way for the application of logic-controllable WGM lasers in optical storage and optical control.

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.

The Distribution and Evolution of Refractive Index in a Polystyrene Whispering Gallery Microcavity during Glass Transition

AbstractThere is limited knowledge on the evolution of physical and chemical parameters near the surface of polymer materials at the microscopic level during glass transition, owing to the complexity of this process. For example, the distribution and evolution of the refractive index near the surface of polymer materials during glass transition are thus far unknown; however, this information is necessary for the design of optical micro‐elements based on polymer materials. Herein, a strategy is proposed to achieve the expression of the refractive index as a function of depth and temperature inside a polystyrene microsphere. The difference in the wavelength shifts between two whispering gallery modes with different radial orders is employed to detect the evolution of the refractive index during the glass transition. A model is established based on experimental observations and further confirmed by numerical simulations. It is believed that the results obtained can assist with the design of polymer optical micro‐elements and systems as well as sensing applications of whispering gallery microcavities.

Low fabrication cost wavelength tunable WG-FP hybrid-cavity laser working over 1.7 μm

A wide wavelength tuning range and single-mode hybrid cavity laser consists of a square Whispering-Gallery (WG) microcavity and a Fabry–Pérot (FP) was introduced and demonstrated. A wavelength tuning range over 12.5 nm from 1760.87 to 1773.39 nm which was single-mode emitting was obtained with the side-mode suppression ratio over 30 dB. The hybrid cavity laser does not need grating etching and special epitaxial structure, which reduces the fabrication difficulty and cost, and shows the potential for gas sensing with absorption lines in this range.

Near-field electrospinning of polymer/phage whispering gallery mode microfiber resonators for label-free biosensing

Whispering gallery mode (WGM) resonators have attracted attention as optical biosensors due to their capacity for rapid label-free detection. Separately, the M13 bacteriophage has been investigated as an alternative biorecognition element. In this work, a novel polymer WGM biosensor functionalized with filamentous virus biocapture agents was fabricated using near-field electrospinning (NFES) in a single step. Polyvinyl alcohol (PVA) was mixed with streptavidin-binding M13 bacteriophage and rhodamine 6 G in aqueous solution for blend electrospinning of active cavity microfiber-based resonators. Confocal fluorescence microscopy revealed alignment of M13 bioreceptors along the fiber axis, while x-ray photoelectron spectroscopy (XPS) and streptavidin-conjugated nanoparticle binding studies indicated that bioactive M13 bacteriophage were on the fiber surface. Dye-doped, crosslinked PVA/M13 fibers supported moderate quality (Q) factor first order WGM resonances within their cross-sections that exhibited a spectral shift in response to streptavidin binding. Composite PVA/M13 microfibers demonstrated specific label-free biosensing of streptavidin with a theoretical limit of detection of 3 nM and a maximum surface coverage of 21 ± 5%. These results demonstrate the potential of NFES to fabricate WGM resonators from PVA/M13 microfibers and use them as a platform for rapid, sensitive label-free sensing.

Cascade Brillouin Lasing in a Tellurite-Glass Microsphere Resonator with Whispering Gallery Modes.

Brillouin microlasers based on microresonators with whispering gallery modes (WGMs) are in high demand for different applications including sensing and biosensing. We fabricated a microsphere resonator with WGMs from a synthesized high-quality tellurite glass with record high Q-factors for tellurite microresonators (Q ≥ 2.5 × 107), a high Brillouin gain coefficient (compared to standard materials, e.g., silica glasses), and a Brillouin frequency shift of 9 ± 0.5 GHz. The high density of excited resonance modes and high loaded Q-factors allowed us to achieve experimentally cascade Stokes-Brillouin lasing up to the 4th order inclusive. The experimental results are supported by the results of the theoretical analysis. We also theoretically obtained the dependences of the output Brillouin powers on the pump power and found the pump-power thresholds for the first five Brillouin orders at different values of pump frequency detuning and Q-factors, and showed a significant effect of these parameters on the processes under consideration.

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.

Magnetic sensor based on a Whispering Gallery Mode double-tailed silica microsphere

A vector magnetometer based on a stretchable Whispering Gallery Mode (WGM) silica Microsphere with a Q-factor of 108 is reported. The used microsphere has two fiber tails enabling it to serve as a strain sensor. The Double-Tailed Microsphere (DTM) is mechanically coupled to a Terfenol-D Rod (TDR), which undergoes longitudinal strain in response to a change in the ambient magnetic field. The strain of the magnetostrictive rod induces mechanical stress on the DTM resulting in a spectral shift of the WGMs of the microsphere. The strong dependence of the magnetometer sensitivity on the sensor geometry is studied, and geometry considerations for optimizing sensor performance are discussed. By introducing and employing geometrical adjustments, applicable due to the unique structure of the magnetometer, the sensitivity improved by a factor of 33. A sensitivity of 7.73 pm/mT is reported with a detection limit of ∼1 µT. The conditions required to achieve a considerably higher sensitivity of ∼200 pm/mT are presented, suggesting an expected detection limit under 40 nT. The simplicity, low cost and lack of electrical wiring make it attractive for remote magnetic sensing. In addition, the possibility to concatenate multiple sensors to a single optical fiber makes the suggested magnetometer highly suitable for quasi-distributed vector magnetic sensing.

Femtosecond Dynamics of a Polariton Bosonic Cascade at Room Temperature.

Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. Here, by using a femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO whispering gallery microcavity are explicitly visualized. Clear ladder-form build-up processes from higher to lower energy branches of the polariton condensates are observed, which are well reproduced by modeling using rate equations. Remarkably, a pronounced superbunching feature, which could serve as solid evidence for bosonic cascades, is demonstrated by the measured second-order time correlation factor. In addition, the nonlinear polariton parametric scattering dynamics on a time scale of hundreds of femtoseconds are revealed. Our understandings pave the way toward ultrafast coherent control of polaritons at room temperature.

Spatial and polarization-resolved exciton–photon coupling in quasi-whispering-gallery mode ZnO microrod cavity

Whisper gallery mode (WGM) resonances and the coupling of excitons and WGMs are measured from ZnO microrod cavity in the visible and ultraviolet spectral ranges. The coupling of excitons and WGMs leads to split modes which exhibit a clear spatial dependence due to the uneven field distribution of WGM on ZnO microrod surface. Polarization dependent behavior together with a blueshift of the upper polariton branch mode are observed owing to the transition polarization of different types of excitons in the ZnO microcavity. Numerical simulations are carried out using finite element method to support the experimental results. This work reveals the spatial and polarization dependent exciton–photon coupling in a ZnO microrod which implies promising applications in photonic polariton field ranging from ultraviolet lasers, high-sensitivity detectors to optoelectronic devices

Graphene-coupled silica microsphere polarizer

Polarization-selective devices can control the polarization of the optical system. We develop a compact hybrid structure based on silica microsphere and monolayer graphene to tune the polarization of the optical system, which the working principle is polarization-dependent absorption of graphene combined with silica microsphere through whispering-gallery (WG) TE mode and TM mode. Because of different field distributions and polarization states of resonant modes in microsphere, the gap distance between microsphere and graphene can modify the quality factor and resonant wavelength of microsphere to realize the different variations for TE and TM WG modes, respectively. As gap distance is reduced from 2.2 µm to 0.3 µm, 11 dB polarization extinction ratio is realized between resonance polarized mode in the 90° polarization direction and that in the 0° polarization direction. Furthermore, for different original gap distance between microsphere and graphene, corresponding to different preconditions of quality factors of microsphere, one can obtain a polarization-selective device with the adjustable bandwidth. This results offer an attractive and effective photonic platform to achieve high-performance polarization-selective devices.

Graphene Whisperitronics: Transducing Whispering Gallery Modes into Electronic Transport.

When confined in circular cavities, graphene relativistic charge carriers occupy whispering gallery modes (WGMs) in analogy to classical acoustic and optical fields. The rich geometrical patterns of the WGMs decorating the local density of states offer promising perspectives to devise new disruptive quantum devices. However, exploiting these highly sensitive resonances requires the transduction of the WGMs to the outside world through source and drain electrodes, a yet unreported configuration. Here, we create a circular p-n island in a graphene device using a polarized scanning gate microscope tip and probe the resulting WGM signatures in in-plane electronic transport through the p-n island. Combining tight-binding simulations and the exact solution of the Dirac equation, we assign the measured device conductance features to WGMs and demonstrate mode selectivity by displacing the p-n island with respect to a constriction. This work therefore constitutes a proof of concept for graphene whisperitronic devices.

Whispering gallery modes in bowl-shaped stilbene microresonators

Whispering gallery mode (WGM) resonance was investigated in bowl-shaped microresonators. These resonators were fabricated on 20-μm-diameter silica microspheres from the stilbene derivative 4,4′-bis[(N-carbazole)styryl]biphenyl. Photoluminescence (PL) spectra showed periodic peaks that resulted from amplified spontaneous emission (ASE) at the eigen-wavelength of the WGM. The ASE threshold (Ith) was dependent on the peak wavelength and had a minimum value of 4 μJ mm−2 at 482 nm. When the microbowls were fabricated on randomly stacked silica microspheres, no mode coupling was observed among the WGMs. This is a major difference from the behavior of microdisks, which exhibit long-range mode coupling. The microdisks and microbowls were both fabricated from the same materials, but the stilbene in the microdisks was deposited on substrates with surfaces that were decorated with silica microdisks. The main structural difference between the microbowls and the microdisks concerned the presence or lack of a planar stilbene thin film between the WGM resonators; the thin film mode is expected to act as the mode coupler between adjacent WGMs. Therefore, the lack of mode coupling among the microbowls supports our model that long-range mode coupling in the microdisks was realized by crosslinking of the WGMs via the thin film mode.

Photonic thermometer by silicon nitride microring resonator with milli-kelvin self-heating effect

Whispering gallery mode (WGM) photonic thermometer had achieved ultra-high sensitivity and resolution. However, while pursuing extremely high quality factor and thereafter high precision, power-induced self-heating within microcavity can bring a significant systematic error. We propose a photonic sensor with less than milli-kelvin self-heating effect utilizing silicon nitride (Si3N4) microring resonator with a loaded quality factor of 4.75 × 105. By investigating thermal broadening transmission spectra under various probing powers, effective absorption coefficient and thermal relaxation constant of the device were obtained where thermo-refractive and Kerr effect were accounted. Self-heating temperature rise was predicted to be mitigated to 245 μK under a proposed measurement condition, and this ultra-low self-heating effect was experimentally proved. The proposed approach can be used to establish metrological standards for photonic thermometry and various sensing applications.