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To realize the refractive index dispersion accurately is essential for the design of optical devices. In this paper, we study hexagonal ZnO microrod cavities. We measure angle-resolved photolumines...

The strong confined electromagnetic field in the micro/nano photonic structures brings new opportunities for the research of interaction between light and matter. By designing optical modes, light can be controlled to realize reversible or inreversible energy exchange between photons and emitters at the micro/nano scale. Here, we review the basic principles of the interaction between light and matter in cavity quantum electrodynamics and their recent advances in strong coupling based on micro/nano photonic structures such as plasmon nanostructures, whispering gallery resonators and photonic crystals. These works opening the research of quantum nanophotonics will have great influence on the on-chip quantum devices and quantum informations.

Near-infrared micro/nanolaser devices utilizing low-dimensional semiconductors can provide essential building blocks to achieve integrated optoelectronic devices and circuitry for advanced functionalities and are compatible with on-chip technologies. Although significant progress has been made through using narrow-band semiconductor micro/nanostructures to realize near-infrared stimulated radiation at room temperature, severe challenges still remain involving much lower quantum efficiencies and higher auger recombination. Herein, we report an experimental realization of a current-injection semiconductor polariton device made of a ZnO microwire via Ga-doping (ZnO:Ga MW) and p-type GaAs template. The device can emit polaritonic illumination directly from sharp edges of the hexagonal MW. The experimental results of angle-resolved electroluminescence measurements reveal a typical anticrossing feature between excitons and cavity modes, unambiguous evidence of the strong exciton-polariton coupling, with corresponding Rabi splitting energy extracted to be about 195 meV. As the applied bias goes above a certain value, electrically driven whispering gallery lasing action was achieved in the near-infrared spectrum, and the lasing features can be assigned to the exciton-polariton effect. The results not only can afford insights into the development of low-threshold coherent light sources via the exciton-polariton effect, but also can expand the fabrication of low-dimensional, near-infrared microlaser devices.

We analytically investigate the performance of whispering gallery mode (WGM) spherical microlasers for gas sensing based on intra-cavity absorption. Using a simple model we estimate the response of a compact optical gas sensor based on these microlasers. We use detection of carbon dioxide by an Er:ZBLAN mid-IR microlaser with a relative intensity noise (RIN) of - 110 dB/Hz as an example, and show that using a commercial mid-IR photodetector, the limit of detection (LOD) for the resulting sensing system is about 5 ppm. We also investigate the impact of using a fiber optical amplifier on the LOD for different values of RIN. The numerical calculations and the presented approach can be applied to analyze the performance of any intracavity sensor based on WGM microlaser.

Summary Optical whispering-gallery mode (WGM) microresonators, confining resonant photons in a microscale volume for long periods of time, strongly enhance light-matter interactions, making them an ideal platform for photonic sensors. One of the features of WGM sensors is their capability to respond to environmental perturbations that influence the optical mode distribution. The exceptional sensitivity of WGM devices, coupled with the diversity in their structures and the ease of integration with existing infrastructures, such as conventional chip-based technologies, has catalyzed the development of WGM sensors for a broad range of analytes. WGM sensors have been developed for multiplexed detection of clinically relevant biomolecules while also being adapted for the analysis of single-protein interactions. They have been used for the detection of materials in different phases and forms, including gases, liquids, and chemicals. Furthermore, WGM sensors have been used for a wide variety of field-based sensing applications, including electric field, magnetic field, force, pressure, and temperature. WGM sensors hold great potential for applications in life and environmental sciences. They are expected to meet the ever-increasing demand in sensor networks, the Internet of Things, and real-time health monitoring. Here we review the mechanisms, structures, parameters, and recent advances of WGM microsensors and discuss the future of this exciting research field., Progress and Potential As a sensing platform that strongly enhances light-matter interactions, whispering-gallery mode (WGM) microresonators have been used for the detection of not only physical matters, such as nanoparticles, small bio/chemical molecules, or gases, but also physical parameters or fields surrounding the resonator that influence the optical modes, for example temperature, pressure, force, electric field, and magnetic field. During the last two decades, a variety of sensing mechanisms and enhancement techniques has been developed for WGM sensors. The future research efforts of WGM sensors will find numerous opportunities in life, health, and environmental sciences and are expected to meet the ever-increasing demand in sensor networks, the Internet of Things, and real-time health monitoring. Moreover, the developments of novel cavity structures and sensing enhancement mechanisms or techniques are always one of the research focuses in this area., Optical whispering-gallery mode (WGM) microresonators, confining resonant photons in a microscale volume for long periods of time, strongly enhance light-matter interactions, making them an ideal platform for photonic sensors. One of the features of WGM sensors is their capability to respond to environmental perturbations that influence the optical mode distribution. The exceptional sensitivity of WGM devices, coupled with the diversity in their structures and the ease of integration with existing infrastructures, such as conventional chip-based technologies, has catalyzed the development of WGM sensors for a broad range of analytes. WGM sensors have been developed for multiplexed detection of clinically relevant biomolecules while also being adapted for the analysis of single-protein interactions. They have been used for the detection of materials in different phases and forms, including gases, liquids, and chemicals. Furthermore, WGM sensors have been used for a wide variety of field-based sensing applications, including electric/magnetic fields, force/pressure, and temperature. WGM sensors hold great potential for applications in life and environmental sciences. They are expected to meet the ever-increasing demand in sensor networks, the Internet of Things, and real-time health monitoring. Here we review the mechanisms, structures, parameters, and recent advances of WGM microsensors and discuss the future of this exciting research field.

This study proposes a hybrid resonance structure of fiber Bragg grating (FBG) and whispering gallery resonator (WGR) in a single optical fiber to simultaneously measure temperature and strain. Bragg grating written in the fiber itself provides the first sensing signal in conjunction with the second signal produced by a whispering gallery mode (WGM) propagating along the circumferential cross-section of the FBG. Under changes in temperature and strain, variation in the refractive index or diameter (or length) of the fiber occurs, which also shifts the resonance frequency of each structure. Taking advantage of the difference in thermo- and strain-optic coefficients of FBG and WGR, such an integration of two different sensing techniques at an identical spot thus provides a $2\,\,{\times }\,\,2$ matrix to the characteristic equation of temperature–strain sensing that realizes a cross-sensitivity-free sensing with high accuracy. An uncertainty of ±1.91 °C and $\pm 36.2~{\mu \varepsilon }$ was achieved in the experimental verification.

The coherent light source is one of the most important foundations in both optical physics studies and applied photonic devices. However, the whispering gallery microcavity, as a prime platform for novel light sources, has the intrinsically chiral symmetry and severely rules out access to directional light output, all-optical flip-flops, efficient light extraction, etc. Here, we demonstrate a reconfigurable symmetry-broken microlaser in an ultrahigh-Q whispering gallery microcavity with the symmetric structure, in which a chirality of lasing field is empowered spontaneously by the optical nonlinear effect. Experimentally, the ratio of counter-propagating lasing intensities is found to exceed 160:1, and the chirality can be controlled dynamically and all-optically by the bias in the pump direction. This work not only presents a distinct recipe for coherent light sources with robust and reconfigurable performance, but also opens up an unexplored avenue to symmetry-broken physics in optical micro-structures., The directional lasing emission in whispering gallery microcavities typically resorts to breaking the structure symmetry. Here the authors demonstrate a reconfigurable symmetry-broken microlaser in a symmetric ultrahigh-Q whispering gallery microcavity, in which a chirality of lasing fields is empowered spontaneously by nonlinear effects.

Directional coupling of light in nanophotonic circuits has recently attracted increasing interest, with numerous experimental realizations based on broken rotational or mirror symmetries of the light-matter system. The most prominent underlying effect is the spin-orbit interaction of light in subwavelength structures. Unfortunately, coupling of light to such structures is, in general, very inefficient. In this work, we experimentally demonstrate an order of magnitude enhancement of the directional coupling between two nanowaveguides by means of a whispering gallery microcavity. We also show that both transversmagnetic and transverse electric modes can be used for the enhancement., 3 figures

Researchers in the field of whispering-gallery-mode (WGM) microresonators have proposed biointegrated low-threshold WGM lasers, to enable large-scale parallel single-cell tracking and barcoding. Although the reported devices have so far been primarily investigated in model applications, most recent results represent important steps towards the development of in vivo tags and sensors that utilize the unique and narrow spectral features of miniature WGM lasers., Omnidirectional lasing particles in living cells and line-by-line sweeping have revealed new prospects for cell tagging and tracking.

In spite of tremendous advancements in modern diagnostics, there is a dire need for reliable, label-free detection of highly contagious pathogens like viruses. In view of the limitations of existing diagnostic techniques, the present theoretical study proposes a novel scheme of detecting virus-like particles employing whispering gallery and quasi-whispering gallery resonant modes of a composite optical system. Whereas whispering gallery mode (WGM) resonators are conventionally realized using micro-disk, -ring, -toroid or spherical structures, the present study utilizes a rotationally symmetric array of silicon nanowires which offers higher sensitivity compared to the conventional WGM resonator while detecting virus-like particles. Notwithstanding the relatively low quality factor of the system, the underlying multiple-scattering mediated photon entrapment, coupled with peripheral total-internal reflection, results in high fidelity of the system against low signal-to-noise ratio. Finite difference time domain based numerical analysis has been performed to correlate resonant modes of the array with spatial location of the virus. The correlation has been subsequently utilized for statistical analysis of simulated test cases. Assuming detection to be limited by resolution of the measurement system, results of the analysis suggest that for only about 5% of the simulate test cases the resonant wavelength shift lies within the minimum detection range of 0.001-0.01 nm. For a single virus of 160 nm diameter, more than 8 nm shift of the resonant mode and nearly 100% change of quality factor are attained with the proposed nanowire array based photonic structure.

In 1878, Lord Rayleigh observed the highly celebrated phenomenon of sound waves that creep around the curved gallery of St Paul's Cathedral in London1,2. These whispering-gallery waves scatter efficiently with little diffraction around an enclosure and have since found applications in ultrasonic fatigue and crack testing, and in the optical sensing of nanoparticles or molecules using silica microscale toroids. Recently, intense research efforts have focused on exploring non-Hermitian systems with cleverly matched gain and loss, facilitating unidirectional invisibility and exotic characteristics of exceptional points3,4. Likewise, the surge in physics using topological insulators comprising non-trivial symmetry-protected phases has laid the groundwork in reshaping highly unconventional avenues for robust and reflection-free guiding and steering of both sound and light5,6. Here we construct a topological gallery insulator using sonic crystals made of thermoplastic rods that are decorated with carbon nanotube films, which act as a sonic gain medium by virtue of electro-thermoacoustic coupling. By engineering specific non-Hermiticity textures to the activated rods, we are able to break the chiral symmetry of the whispering-gallery modes, which enables the out-coupling of topological "audio lasing" modes with the desired handedness. We foresee that these findings will stimulate progress in non-destructive testing and acoustic sensing. This work was supported by the National Basic Research Program of China (2017YFA0303702), NSFC (12074183, 11922407, 11904035, 11834008, 11874215 and 12104226) and the Fundamental Research Funds for the Central Universities (020414380181). Z.Z. acknowledges the support from the China National Postdoctoral Program for Innovative Talents (BX20200165) and the China Postdoctoral Science Foundation (2020M681541). L.Z. acknowledges support from the CONEX-Plus programme funded by Universidad Carlos III de Madrid and the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement 801538. J.C. acknowledges support from the European Research Council (ERC) through the Starting Grant 714577 PHONOMETA and from the MINECO through a Ramón y Cajal grant (grant number RYC-2015-17156).

Ultra-stable lasers of high spectral purity are the technological basis for optical atomic clocks, quantum measurements, stable microwave signal sources and high-resolution optical spectroscopy. For these purposes, lasers with a linewidth of the order of hertz are used. Such lasers are usually stabilized with Fabry-Perot resonators made of ultra-low expansion materials, which are very fragile and bulky. Therefore, there is a clear demand for reliable miniature lasers with narrow linewidths. Optical resonators with a "whispering gallery" mode (WGM) make it possible to create narrow-band microlasers on their basis using self-injection locking method. But they have not yet reached the stability determined by their fundamental noise. Here we calculate the noise characteristics of lasers self-injection locked to WGM microresonators and estimate the linewidth limitations due to thermorefractive, thermoelastic, photoelastic and Brownian noise. We also calculated the total noise in an integrated waveguide and obtained an estimate of the linewidth of 0.5 kHz at an averaging time of 1 s.

In this work, we present a packaged whispering gallery mode (WGM) device based on an optical nanoantenna as the coupler and a glass microsphere as the resonator. The microspheres were fabricated from either SiO2 fiber or Er3+-doped fiber, the latter creating a WGM laser with a threshold of 93 µW at 1531 nm. The coupler-resonator WGM device was packaged in a glass capillary. The performance of the packaged microlaser was characterized, with lasing emission both excited in and collected from the WGM cavity via the nanoantenna. The packaged system provides isolation from environmental contamination, a small size, and unidirectional coupling while maintaining a high quality (Q-) factor (∼108).

Achieving strong coupling of electron beams with single photons and harnessing their potential entanglement promises advancemenets in quantum optics with free-electron, merging spectroscopy of individual quantum systems with the nanoscale spatial resolution available in electron microscopes. Recent experiments on inelastic electron-light scattering (IELS) [1] , [2] with dielectric microcavities such as microspheres [3] or photonic crystal cavities [4] have demonstrated the potential of using resonant field enhancement for increasing the interaction between electrons and photons. IELS with continuous-wave (CW) excitation is, however, challenging due to transition probabilities normally being far below unity [5] .

Optical frequency combs have become a key technology for optical precision measurements [1] and, moreover, bear potentials to boost the bandwidth of telecommunication networks [2] , to be applied for molecular spectroscopy [3] , and to be used in modern quantum information systems [4] . Owing to their feasibility for such a vast application range, frequency comb technology has seen major advances in terms of complexity, power requirements and size, leading to micro- to millimeter-sized frequency comb generators emitting continuous-wavepumped Kerr combs and, more lately, soliton frequency combs based on third-order ( χ (3) ) nonlinearities [2] .

Lithium niobate (LN) is a material with outstanding optical properties including large second-order nonlinear coefficients; strong linear electro-optic effects; and excellent transparency at visible and near infrared wavelengths. It is thus widely used for a range of optical frequency conversion techniques; recent examples comprise frequency combs [1] – [4] and tunable integrated optical parametric oscillators [5] . We recently demonstrated a novel frequency conversion method, Pockels-effect-based adiabatic frequency conversion (AFC), in a LN whispering gallery microresonator [6] . By applying a voltage pulse faster than the lifetime of the photons in the cavity, we change the resonator eigenfrequency and hence the frequency of the trapped light such that no threshold has to be overcome, no phase-matching conditions need to be fulfilled and no mode hopping can occur.

There have been impressive advances in the area of microresonator-based frequency combs (microcombs) during the last decade, making their way into applications in a wide variety of fields [1] . Although microcombs are commonly generated making use of third-order ( χ (3) ) nonlinear processes, several groups have recently started to investigate frequency comb generation based on the second-order ( χ (2) ) nonlinearity as it bears potential benefits like the linear electro-optic effect for either tuning or stabilization, or the possibility to convert light into more challenging spectral regions like the UV or mid-infrared [2] , [3] . For the latter, one approach is to start with optical parametric oscillation (OPO) and to operate it close to degeneracy. Then, a series of second-order nonlinear processes takes place, generating equidistant comb lines around both the pump frequency and the sub-harmonic frequency [2] , [4] . A promising χ (2) material to move into the mid-infrared region is cadmium silicon phosphide (CdSiP 2 ) which is transparent from 1 to 6.5 µm and has a high nonlinearity of 84.5 pm/V. Furthermore, OPO has already been shown for a CdSiP 2 mm-sized whispering gallery resonator pumped at telecom wavelength [5] . Based on these observations, we investigate OPO operation close to the point of degeneracy and this way, the possibility to generate frequency combs both around 1.5 µm and 3 µm. In this work, we have observed OPO with thresholds below 100 µW and conversion efficiencies of around 17 %. We could tune the OPO output wavelengths from 2.3 to 5 µm by coupling the pump light into different modes of the resonator and by controlling its temperature. One of the pump modes investigated enabled operation near degeneracy. Measured signal and idler wavelengths for this pump mode are plotted in figure 1a ) for different temperatures. However, OPO near degeneracy is prone to instabilities, as several parametric oscillations can be excited under similar conditions. Here, by tuning the temperature in the mK range and the pump detuning with respect to the resonance in the MHz range, the minimum obtained difference between signal and idler frequencies was 120 GHz (4 FSRs). Nonetheless, in this state we have already observed sidebands generated around the pump corresponding to SHG as shown in figure 1b ). Why operation closer to the point of degeneracy is hampered, is currently under investigation. A possibility to gain further insight and control on OPO operation might be to make use the linear electro-optic effect for fine tuning of the relative position of the resonances between the pump and the sub-harmonic frequencies.

Sound propagation in an area of shallow sea with circular isobaths and the bottom depth increasing towards their curvature center is considered. Such an area could be part of a bay, a lagoon, or a lake with bowl-like bottom relief. The possible formation of whispering gallery waves localized in the vicinity of curved isobaths is considered. Simple criteria for the formation of such whispering gallery due to horizontal refraction are derived; their transverse modal structure is described analytically in terms of Bessel functions and using the Wentzel–Kramers–Brillouin–Jeffreys approximation. The corresponding azimuthal wavenumbers and radial mode functions are computed. Dispersion of such waves is also investigated, and the possibility of their experimental observation is discussed.Sound propagation in an area of shallow sea with circular isobaths and the bottom depth increasing towards their curvature center is considered. Such an area could be part of a bay, a lagoon, or a lake with bowl-like bottom relief. The possible formation of whispering gallery waves localized in the vicinity of curved isobaths is considered. Simple criteria for the formation of such whispering gallery due to horizontal refraction are derived; their transverse modal structure is described analytically in terms of Bessel functions and using the Wentzel–Kramers–Brillouin–Jeffreys approximation. The corresponding azimuthal wavenumbers and radial mode functions are computed. Dispersion of such waves is also investigated, and the possibility of their experimental observation is discussed.

The effects of surface plasmon (SP) coupling with the excitation radiating dipole on the behaviors of the whispering-gallery resonance (WGR) modes in a hexagonal GaN nanowire cavity structure, either suspended or supported by a substrate, are numerically investigated by placing an Ag nanoparticle (NP) on one of six facets of the hexagonal structure. Through the Purcell effect, the intrinsic resonance behavior of a WGR mode can affect the radiation behavior of the source dipole and hence influence the SP-coupling behavior, which eventually determines the effects of SP coupling on the WGR behavior. The SP coupling can produce two competing effects, including the enhanced stored energy within the resonance cavity and the increased light extraction from the cavity into the air. Both of them are related to the Purcell factor and quality- (Q-) factor of cavity resonance. When the former (latter) dominates, the resonance intensity of a WGR mode can be enhanced (suppressed). It is observed that the WGR becomes stronger, i.e., the radiated power is enhanced, through SP coupling when the intrinsic resonance is not strong. The resonance intensities of those WGR modes with strong intrinsic resonances or high Q-factors can be reduced.

We study numerically a two-dimensional (2-D) elliptical microcavity laser, in the center of which a circular active region is located, modeling a focused pumped spot or an injection electrode. The object of the research is to study lasing spectra, gain-medium thresholds, and modal fields, found as solutions to a classical electromagnetic eigenvalue problem tailored to address the threshold conditions. The instrument of our study is the set of two coupled Muller boundary integral equations (BIE) on the boundaries of the cavity and the active region. These BIEs are discretized by the Nystrom technique that guarantees the convergence. Among the modes that co-exist in such a resonator, we demonstrate the whispering-gallery-like modes (WGM) and the bow-tie-like modes (BTM). We find that the threshold values of material gain of BTMs can become lower than those of nearby WGMs if the radius of the active region in the cavity center is getting sufficiently small.

We studied the condensation of exciton-polaritons in pump-induced potential by angle-resolved photoluminescence (PL) measurements at room temperature. The experimental dispersion spectrum shows that under the excitation of a small spot, polaritons in ZnO whispering gallery microcavity condense at a certain state where momentum is non-zero. This is different from the conventional Bose–Einstein condensation (BEC) in which case the polaritons condense at ground state with a zero momentum. The theoretical simulation of condensation shape given by Gross-Pitaevskii equation (GPE) is in good agreement with our experimental observations.

The integration of one more gain media in droplet microlasers with morphology-dependent modes, which can be employed in optofluidic systems as multi-wavelength lasing sources, is highly attractive and demands new cavity design and fabrication approaches. Here, cholesteric liquid crystal (CLC) droplets with an integrative triple-emulsion cavity are fabricated via glass-capillary-based microfluidic technologies and dual-gain lasing with variable modes, flexibly configured by the combination and incorporation of gain dyes and CLCs into both the core and shell. The distributed feedback (DFB) mode, formed by the feedback from the self-assembled helix periodic structure of CLCs, the whispering gallery (WG) mode, and the hybrid, is selectively excited by controlling the spatial coupling between the pump beam and the droplet with gain. With the merits of dual-gain and controllable lasing, a prototype dual-wavelength-ratiometric thermometer with self-calibration capability is expected to be developed. Furthermore, the anisotropic CLC core is substituted with an isotropic fluid and the gain from the CLC shell is additionally removed, DFB lasings in both shell and core are absent, and only Bragg-shell reflection-based hybrid modes are excited for lasing. The CLC droplet microlasers with an integrative cavity are expected to provide a new route to future lab-on-chip (LOC) applications.

Composite photoanode comprising mesoporous microspheres with a smooth spherical morphology exhibiting high light scattering due to optical whispering gallery modes (WGMs) is used to fabricate whisperonic solar cell (WSC) devices. The photoanode is sensitized with CdS–CuInS2 thin films (CdS–CIS-TF) of ∼5 nm thickness. CdS–CIS-TF-sensitized photoanodes exhibit strong coupling with WGM. These WSC devices show an average (ηavg) efficiency (η) of ≈3.2% in comparison with ηavg ≈ 1.9% for the nanoparticulate-based photoanode. The observed efficiency is the highest for CdS–CIS-TF-sensitized solar cells made using I–/I3– electrolyte. This remarkable increase in ηavg (∼60%) is attributed to increased photon absorption by the sensitizer films because of the presence of WGM scattering prevailing in smooth microspheres. Thus, WSC photoanode configuration is a promising approach to enhance the efficiency of TF-sensitized solar cells.

Lead halide perovskite-based lasers have attracted much research attention due to their tunable emission colors, high gain coefficient and low threshold. Here, we demonstrated a facile solution method via temperature modulation to synthesize CH3NH3PbBr3 perovskite crystals with sub-circular and quasi-cubic shapes for whispering gallery resonant mode and Fabry–Perot mode lasing emission, respectively. A low lasing threshold of 6.4 μJ cm−2 and a full width at half maximum of 0.7 nm in an individual sub-circular cavity can be achieved at room temperature. In the quasi-cubic cavities, the Fabry–Perot lasing oscillation modes not only exist in the horizontal directions, but also in the vertical directions according to the adjustment of the optical pumping method. The lasing thresholds and linewidths are in the range of 14–25 μJ cm−2 and 0.18–0.51 nm, respectively. This work may create possibilities for cavity design and fabrication of perovskite lasers in future potential applications.

X-ray reflectometry of whispering galleries (WGs), which propagate along meniscuses of deionized water or silica hydrosols (Ludox® SM) enriched by Cs+ ions, was analyzed for the first time. The measurements were performed using the diffractometer with a moving tube-detector system. The X-ray beam rotation angle reached a maximum value of 4° on a silica hydrosol sample at CuKα radiation, while for H2O it is only 1°. The intensities of WG modes propagating near the surface of a concave meniscus were found from the rigorous solution of the respective Helmholtz equation and applying the Monte Carlo technique for the random surface. For analysis of intensities of the X-ray scattering we used a hybrid model of the liquid surface with the upper transition layer having sinusoidal ripples in which the concentration of levitating Cs+ ions has a maximum near the surface, derived from the experiment, while the concentration maximum for SiO2 particles with sizes of ~6–15 nm is positioned at the depth of ~15 nm from the hydrosol’s surface. Our numerical results for reflectances are in a good agreement with the experimental data.

Using the transformation cavity, a gradient index cavity designed by transformation optics, we propose a hybrid resonator system to extract unidirectional narrow-beam emission from high-Q whispering gallery modes by embedding a transformation cavity inside a deformed uniform index cavity that exhibits unidirectional narrow-beam emission. For effective mode coupling between the transformation cavity and enclosing cavity, the embedded transformation cavity is designed to have bidirectional evanescent emission, which enables most of the emission from the transformation cavity to be laterally incident on the rim of the enclosing deformed cavity. Consequently, ultrahigh-Q resonances of this system can provide a sharp free-space light output, which is difficult to achieve by embedding a homogeneous disk cavity instead of the transformation cavity.

The high refractive index of ZnS (about 2.4 in the visible range) makes ZnS microstructures good candidates for the fabrication of light guides and resonant microcavities. Light guiding behaviour has been studied in several ZnS structures. In particular, in this work the behavior of rods and plates has been investigated and it is has been shown that the morphology of the studied structures plays a determinant role on the supported resonant modes. In ZnS:Ga and ZnS:In structures investigated in the present work Fabry-Perot as well as Whispering Gallery resonant modes are supported. A good agreement has been found between the experimental results and the theoretical model used to describe both types of resonances, which have been used to estimate the refractive index of the doped material. In ZnS:Ga wires with triangular cross-section a lasing effect has been observed in the blue-green range. To our knowledge, it is the first time that resonant modes in visible region have been studied in doped ZnS.

Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is, e.g., the linear electro-optic effect, the Pockels effect via externally applied electric fields. Due to the shape of the resonators a precise prediction of the electric field strength that affects the optical mode is non-trivial. Here, we study the average strength of the electric field in z-direction in the region of the optical mode for different configurations and geometries of lithium niobate whispering gallery resonators with the help of the finite element method. We find that in some configurations almost 100% is present in the cavity compared to the ideal case of a cylindrical resonator. Even in the case of a few-mode resonator with a very thin rim we find a strength of 90%. Our results give useful design considerations for future arrangements that may benefit from the strong electro-optic effect in bulk whispering gallery resonators made out of lithium niobate.

We realized a single-mode laser with an ultra-high quality factor in individual cesium lead bromide (CsPbBr3) perovskite micro-hemispheres fabricated by chemical vapor deposition. A series of lasing property analysis based on cavity size was reported under this material system. Due to good optical confinement capability of the whispering gallery resonant cavity and high optical gain of CsPbBr3 perovskite micro-hemispheres, single-mode lasing behavior was achieved with an ultra-high quality factor as large as 11,460 at room temperature. To study in detail the physical effects between lasing threshold and cavity, a set of cavity size dependence photoluminescence analyses were performed. We found that the lasing threshold increases while the cavity size decreases. Time-resolved PL analysis was conducted to confirm the relation between cavity size and lasing threshold. The larger cavity stands for longer PL lifetime and indicates easier-to-achieve carrier population inversion. Strong Purcell enhancement could be further investigated by the spontaneous emission coupling factor β and internal quantum efficiency as a function of cavity size. A high β-factor of 0.37 could be obtained from a 2.2 μm diameter hemisphere microcavity and a high Purcell factor of 14 in a 1.9 μm diameter hemisphere microcavity showing strong Purcell enhancement effect in our system.

Microresonator frequency combs have seen remarkable progress in the past decade. In this work, we investigate optical parametric oscillation in a mm-sized cadmium silicon phosphide (CdSiP2) whispering gallery resonator for X(2) frequency comb generation in the mid-infrared region. The comb formation starts with optical parametric oscillation (OPO) near degeneracy followed by a series of internally pumped second-order nonlinear processes. The first step of OPO has been achieved with thresholds below 100 μW, efficiencies of up to 17 % and wavelength tunability from 2.3 to 5 μm. The control of the wavelength tuning is hampered when the OPO is operated close to degeneracy. Nevertheless, we have observed sideband generation around the pump wavelength, indicating that frequency combs can be generated using this scheme. However, improved control of the wavelength tuning near degeneracy is needed to increase the number of sidebands, leading to broadband frequency combs in the mid-infrared.

Optical whispering-gallery mode (WGM) microresonators have been widely used as chemical sensors. In practice, most WGM sensing is based on either mode frequency shift (dispersive) or mode linewidth change (dissipative). However, WGM sensors based on the fractional change in throughput dip depth (dissipative) can offer better sensitivity under certain conditions. Recently it was demonstrated that for WGM dip-depth sensors multimode input can further improve the sensitivity as compared to single-mode input. This enhancement in sensitivity is achieved by using an asymmetric tapered fiber to couple light in and out of the microresonator. In this report, we develop a model which predicts the waist radius of the fabricated asymmetric tapered fiber. The radius predicted by the model is verified by measuring the beat length for the two fiber modes that are excited and couple into a WGM.

In many applications of whispering gallery (WG) resonators and optical nanofibers as well as their coupling systems, the freedom of polarization is not critical. Inspired by the results from near-field optics, we found the freedom of polarization may bring up interesting ideas into these systems. In this talk, I will introduce three experiments about the polarization-controlled light coupling in WG resonator-nanofiber coupling system. The first one is the realization of the full polarization control of a nanofiber; The second one is the cavity enhanced directional coupling; The third one is the polarization-controlled cavity input-output relations.

In this paper, we will present some of our recent work on whispering gallery microsphere and microbubble resonators and their applications in optical sensing, nonlinear optics and frequency comb generation. In particular, we have been working on direct blue frequency comb generation using such devices and our most recent results will be presented. It is hoped that this will show some of the advantages and limitations of WGM resonators coupled to tapered optical fibers for visible light comb generation.

Hybrid metallic nanoparticles encapsulated in oxide shells are currently intensely studied for plasmonic applications in sensing, medicine, catalysis, and photovoltaics. Here, we introduce a method for the synthesis of Au@Ag@SiO$_2$ cubes with a uniform silica shell of variable and adjustable thickness in the nanometer range; and we demonstrate their excellent, highly reproducible, and tunable optical response. Varying the silica shell thickness, we could tune the excitation energies of the single nanoparticle plasmon modes in a broad spectral range between 2.55 and 3.25\,eV. Most importantly, we reveal a strong coherent coupling of the surface plasmons at the silver-silica interface with the whispering gallery resonance at the silica-vacuum interface leading to a significant field enhancement at the encapsulated nanoparticle surface in the range of 100\,\% at shell thicknesses $t\,$$\simeq\,$20\,nm. Consequently, the synthesis method and the field enhancement open pathways to a widespread use of silver nanoparticles in plasmonic applications including photonic crystals and may be transferred to other non-precious metals.

International audience; In this article, we consider the radiation characteristics of a THz antenna made of a circular dielectric rod decorated with conformal graphene strip and illuminated by the field of a line magnetic current. The strip has arbitrary angular size and location and its surface impedance is characterized with Kubo theory. Our mathematically accurate analysis uses a dedicated hypersingular integral equation for the current induced on the strip. Discretization of this equation is carried out by the Nystrom-type method, which has a guaranteed convergence. We study the dependences of the powers radiated and absorbed in this configuration and also the directivity of antenna emission, in wide frequency range from 0 to 10 THz. They show very interesting interplay between the broadband inverse photonic-jet effect of lens-like dielectric rod and two types of resonances: On the moderate-Q plasmon modes of graphene strip and on the extremely high-Q whispering-gallery modes of the circular rod. © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.

We theoretically investigate a cavity quantum electrodynamics system in which a high-Q whispering-gallery-mode microcavity interacts simultaneously with a dipole quantum emitter and multiple Rayleigh scatterers. The scatterers are used to induce and to control mode coupling between counterclockwise and clockwise propagating light fields, which interact with the dipole quantum emitter. We study the effect of mode coupling on the linear and nonlinear output characteristics of coherent photon transport in a tapered fiber waveguide coupling the microcavity. It is found that enhanced optical Kerr nonlinearity with high linear transmission rate can be achieved efficiently when the system parameters are tuned properly. The present results illustrate the potential to utilize mode coupling for enhancing optical Kerr nonlinearities and controlling the transmission of light.

A new type of electro-optic crystal (PIN-PMN-PT) has been discovered. It has similar transmission wavelength range and transmittance with the traditional lithium niobate crystal. It has higher electro-optic coefficient and refractive index than lithium niobate crystal. We used this material to make a disk-shaped whispering gallery structure and studied it.

International audience; Pseudodifferential mode parabolic equations in curvilinear coordinates are derived from the horizontal refraction equations. An energy-conserving split-step Padé method for their numerical solution is developed. Several examples are considered including the propagation of sound in a perfect wedge, a whispering gallery formation near circular isobath, and the breaking of the whispering gallery waveguide in the vicinity of an inflection point of the isobath.

Exceptional points (EPs) in the optical whispering-gallery-mode (WGM) microcavity perturbed by Rayleigh scatterers have shown great potential in the areas of on-chip chiral photonics and enhanced sensors, creating a huge demand for deep theoretical insight into the unique physical properties of EPs. Here we propose an intuitive and first-principles-based model to clarify the formation of the EPs in the WGM microcavity perturbed by two nanoparticles. The model is constructed by considering a dynamical multiple-scattering process involving the clockwise and counterclockwise azimuthally propagating modes (APMs) at the nanoparticles. The complex resonance frequencies and resonant modes of the system can be comprehensively reproduced with the model. The conditions for the determination of EPs are derived, and the system parameters at the EP can be quantitatively predicted. The model unveils the relationship between the spatial chirality of the resonant modes near the EP and the scattering of APMs at the nanoparticles. The model quantitatively predicts the difference between the APM coefficients within different azimuthal regions divided by the two nanoparticles, which were considered to be identical in previous theoretical formalisms, and it exhibits the contributions of the APMs, other than those considered in the model, to forming the resonant modes. The proposition of the theoretical model will benefit the investigations of EPs in WGM microcavities and their applications.

We show how light can be controllably transported by light at microscale dimensions. We design a miniature device which consists of a short segment of an optical fiber coupled to transversely-oriented input-output microfibers. A whispering gallery soliton is launched from the first microfiber into the fiber segment and slowly propagates along its mm-scale length. The soliton loads and unloads optical pulses at designated input-output microfibers. The speed of the soliton and its propagation direction is controlled by the dramatically small, yet feasible to introduce permanently or all-optically, nanoscale variation of the effective fiber radius., Comment: 21 pages, 5 figures

To reduce the threshold and achieve unidirectional lasing emission in a whispering gallery mode microcavity, we propose and demonstrate a GaN-based eccentric microring with an inner hole located off the center. Compared to microdisk with the same outer diameter, the eccentric microring structure exhibits a remarkable reduction of lasing threshold by up to 53%. The introduction of the hole disturbs and eventually suppresses the field distribution of the higher order modes. Laser emission with high unidirectionality with a far-field divergence angle of about 40° has been achieved, meanwhile the Q factor of the whispering gallery modesis remains high as 6388. Finite-difference time-domain numerical simulation is carried out to prove that the far-field profile of the eccentric microring structure can be controlled by the position and the size of the hole. The properties of the whispering gallery mode microcavities are improved greatly through a simple structure and process, which has an important guiding significance to the research and development of the microcavity lasers.