Deep‐learning powered whispering gallery mode sensor based on multiplexed imaging at fixed frequency

During the last decades the whispering gallery mode based sensors have become a prominent solution for label-free sensing of various physical and chemical parameters.At the same time,the widespread utilization of the approach is hindered by the restricted applicability of the known configurations for ambient variations quantification outside the laboratory conditions and their low affordability,where necessity on the spectrally-resolved data collection is among the main limiting factors.In this paper we demonstrate the first realization of an affordable whispering gallery mode sensor powered by deep learning and multi-resonator imaging at a fixed frequency.It has been shown that the approach enables refractive index unit(RIU)prediction with an absolute error at 3×10^(-6) level for dynamic range of the RIU variations from 0 to 2×10^(-3) with temporal resolution of several milliseconds and instrument-driven detection limit of 3×10−5.High sensing accuracy together with instrumental affordability and production simplicity places the reported detector among the most cost-effective realizations of the whispering gallery mode approach.The proposed solution is expected to have a great impact on the shift of the whole sensing paradigm away from the model-based and to the flexible self-learning solutions.

High resolution angular-displacement sensor based on whispering gallery mode resonance in bent optical fibers

A simple fiber sensor to measure angular displacement with high resolution, which is based on whispering gallery mode （WGM） resonance in bent optical fibers,is proposed. The sensor is composed of a single loop formed by loosely tying a knot using single mode fiber. To measure the transmission spectra, a tunable laser and an optic power meter are connected to the two ends of fi- ber loop, respectively. Significant WGM resonances occur over the investigated wavelength range for all the sensors with different bend radius. The angular-displacement sensitivity is studied in the range from -0. 1°to 0. 1°. The detection limit of 1.49 × 10 ^-7 rad can be achieved for the detecting system with the resolution of lpm. The simple loop-structure fiber sensor has potential application prospect in the field of architecture or bridge building with low detection limit and low cost.

Periodically poled lithium niobate whispering gallery mode microcavities on a chip

In this study, we investigate the fabrication of periodically poled lithium niobate(PPLN) microdisk cavities on a chip. These resonators are fabricated from a PPLN film with a 16 μm poling period on insulator using conventional microfabrication techniques.The quality factor of the PPLN microdisk resonators with a 40-μm radius and a 700-nm thickness is 6.7×10~5. Second harmonic generation(SHG) with an efficiency of 2.2×10^(-6) mW(-1) is demonstrated in the fabricated PPLN microdisks. The nonlinear conversion efficiency could be considerably enhanced by optimizing the period and pattern of the poled structure and by improving the cavity quality factors.

Surface-enhanced Raman scattering on dielectric microspheres with whispering gallery mode resonance

Conventionally, metallic nanostructures are used for surface-enhanced Raman spectroscopy(SERS), but recently there has been increasing interest in the enhancement of Raman scattering from dielectric substrates due to their improved stability and biocompatibility compared with metallic substrates. Here, we report the observation of enhanced Raman scattering from rhodamine 6 G molecules coated on silica microspheres. We excite the whispering gallery modes(WGMs) supported in the microspheres with a tapered fiber coupler for efficient WGM excitation, and the Raman enhancement can be attributed to the WGM mechanism. Strong resonance enhancement in pump laser intensity and modified Raman emission from the Purcell effect in the microsphere resonator are observed from the experiment and compared with theoretical results. A total Raman enhancement factor of 1.4 × 10~4 is observed, with contribution mostly from the enhancement in pump laser intensity. Our results show that, with an efficient pumping scheme, dielectric microspheres are a viable alternative to metallic SERS substrates.

UV-curable adhesive microsphere whispering gallery mode resonators

We report the fabrication and optical characterization of spherical whispering gallery mode(WGM) resonators made from ultraviolet(UV)-curable adhesive. The fabricated microspheres have good sphericity and surface smoothness, and can directly adhere to the tip of half-tapered fibers for easy manipulation. WGMs are e?ciently excited in the microsphere using an evanescent field of the tapered silica optical fibers. Resonances with quality factors of 1.3×10 5 are observed. The dependence of wavelength shifts of WGM resonances on the input light powers shows that the resonant wavelength of the proposed microsphere resonators can be tuned thermo-optically.

Applied whispering gallery modes on ZnO nanorods coated glass for humidity sensing application

In this letter,a humidity sensor is demonstrated by applying a whispering gallery mode(WGM)from a microsphere resonator onto the ZnO nanorods coated glass surface.The diameter of the microsphere was 234μm and the glass surface was coated with ZnO nanorods using the hydrothermal method at growth duration of 12 h.A significant response to humidity level ranging from 35%RH to 85%RH has been observed with the sensitivity of 0.0142 nm/%RH.The proposed humidity sensor has successfully employed to enhance interaction between the whispering gallery mode evanescent and surrounds analyte with the assistance of ZnO nanorods coated glass.

Optomechanically induced non-reciprocity in the whispering gallery mode microresonator

With the support by the National Natural Science Foundation of China,Chinese Academy of Sciences,and Ministry of Science and Technology of China,the research group led by Dr.Dong Chunhua(董春华)at the CAS Key Lab of Quantum Information,University of Science and Technology of China。

Photonic lenses with whispering gallery waves at Janus particles

We show that electric field on the plane surface of truncated sphere or cylinders(so called Janus particles)have sharp resonances versus the depth of removed segment of a sphere or cylinder.These resonances are related to the excited whispering gallery waves caused by truncation.It is a new mechanism of the field localization.Optimization of this effect for cylinders permits to reach a super resolution in the line thickness,which can be used for contact optical lithography.

Improving resolution of superlens based on solid immersion mechanism

Super-resolution imaging with superlens has been one of the fundamental research topics. Unfortunately, the resolution of superlens is inevitably restrained by material loss. To address the problem, we introduce the solid immersion mechanism into the slab superlens and the cylindrical superlens. The proposed solid immersion slab superlens(SISSL) and the solid immersion cylindrical superlens(SICSL) can improve the resolution by converting evanescent wave to propagating wave using high refractive index materials. From the perspective of applications, the cylindrical superlens with finite cross section and the ability of magnification or demagnification has more advantages than the slab superlens. Therefore,we focus on demonstrating analytically the super-resolution imaging of SICSL. Due to the impedance mismatching caused by solid immersion mechanism, the whispering gallery modes(WGMs) are excited between SICSL and the air interface.We clarify the excitation conditions of WGMs and analyze their influence on the imaging quality of SICSL. The SISSL and SICSL may pave a way to apply in lithography technique and real-time biomolecular imaging in future.

Double-triangular whispering-gallery mode lasing from a hexagonal GaN microdisk grown on graphene

The III-nitride semiconductor microdisk laser exhibits high quality factor and low excitation threshold with broad development potential in the field of nanophotonics such as nanoscale light-emitting devices and on-chip optical integration.So far,many works have been focused on process synthesis and nanostructural design of microdisk lasers.Nevertheless,there are still some limitations in the existing optical resonance mode which is an important internal influence factor for lasing characteristics.In this work,a new double-triangular whispering-gallery mode(D3-WGM)lasing from a hexagonal GaN microdisk with a high quality(Q)factor of 3049 and an excitation threshold of around 11.5μW has been experimentally demonstrated.In addition,the optical properties of hexagonal-WGM(6-WGM),triangular-WGM(3-WGM)and D3-WGM lasing from microdisk are explored by numerical calculation and Comsol simulation.These results confirm that the D3-WGM lasing has its own significant performance advantages,namely high Q factor and easy light emission.Based on this novel laser mode characteristic,microcavity lasers are expected to be further developed and applied in the field of nanophotonics.

Impact of nanoparticle-induced scattering of an azimuthally propagating mode on the resonance of whispering gallery microcavities

Optical whispering gallery microcavities with high-quality factors have shown great potential toward achieveing ultrahigh-sensitivity sensing up to a single molecule or nanoparticle, which raises a huge demand on a deep theoretical insight into the crucial phenomena such as the mode shift, mode splitting, and mode broadening in sensing experiments. Here we propose an intuitive model to analyze these phenomena from the viewpoint of the nanoparticle-induced multiple scattering of the azimuthally propagating mode(APM). The model unveils explicit relations between these phenomena and the phase change and energy loss of the APM when scattered at the nanoparticle; the model also explains the observed polarization-dependent preservation of one resonance and the particle-dependent redshift or blueshift. The model indicates that the particle-induced coupling between the pair of unperturbed degenerate whispering gallery modes(WGMs) and the coupling between the WGMs and the free-space radiation modes, which are widely adopted in current theoretical formalisms, are realized via the reflection and scattering-induced free-space radiation of the APM, respectively, and additionally exhibits the contribution of cross coupling between the unperturbed WGMs and other different WGMs to forming the splittingresonant modes, especially for large particles.

Raman lasing and other nonlinear effects based on ultrahigh-Q CaF_(2)optical resonator

The calcium fluoride(CaF_(2))whispering gallery mode crystalline resonator is an excellent platform for nonlinear optical applications because of the decreasing in threshold caused by ultrahigh quality(Q)factor.In this paper,we achieved the observation of Raman lasing,first-order Raman comb,and second-order Raman lasing in a CaF_(2)disk resonator with a diameter of 4.96 mm and an ultrahigh-Q of 8.43×10^(8)at 1550-nm wavelength.We also observed thermal effects in CaF_(2)disk resonator,and the threshold of thermo-optical oscillation is approximately coincident with Raman lasing,since the intracavity power increases rapidly when the power reaches the threshold,and higher input pump power results in longer thermal drift and higher Raman emission power.With a further increase in pump power,the optical frequency combs range is from 1520 nm to 1650 nm,with a wavelength interval of 4×FSR.It is a promising candidate for optical communication,biological environment monitoring,spectral analysis,and microwave signal sources.

Printable microlaser arrays with programmable modes for information encryption

Lasing emissions with multiple and tunable modes are promising in coding field as a novel cryptographic primitive.With the advantages of simple fabrication,full-color and high-quality-factor whispering gallery mode lasing inside a circular cross-section,polymer microfibers are attractive for photonic devices.However,polymer lasing microfibers for information encryption have never been reported.Herein,we propose a design of printable lasing microfiber encryption chip by in-situ tuning the effective refractive index of the microresonator arrays via a facile approach.Through inkjet printing high-refractive-index nanoparticles on the designated position of lasing microfiber arrays,the effective refractive index of the microcavities is regulated,and the ratio of wavenumber spacing between transverse electric and transverse magnetic mode to the free spectral range can be modulated,particularly with neglectable influence by the size factor.Thus,the programmable region selective encoding process can be conducted simply by a printing program within several minutes.Besides,the encoded microfiber arrays are encapsulated into polydimethylsiloxane to reduce the scattering loss and environmental interference,and a printable encryption chip is realized.This work is expected to provide a platform for the printable encrypted devices.

Low threshold fiber taper coupled rare earth ion-doped chalcogenide microsphere laser

We report the applications of a low-cost and environmentally friendly chalcogenide glass, 75GeS2-15Ga2S3-10CsI, in building active microsphere laser oscillators. A silica fiber taper is used as the coupling mechanism. With an 808-nm laser diode as a pump source, we show that a high-Q （-, 6× 10^4） laser mode could be obtained from a 75-μm diameter microsphere that is coupled with a 1.77-μm waist-diameter fiber taper. The threshold of the incident pump power is 1.39 mW, which is considerably lower than those of previously reported free-space coupled chalcogenide microsphere lasers. We also note an apparent enhancement in laser power generated from this chalcogenide microsphere laser.

High-efficiency photon-electron coupling resonant emission in GaN-based microdisks on Si

Resonance effects caused by the photon-electron interaction are a focus of attention in semiconductor optoelectronics,as they are able to increase the efficiency of emission.GaN-on-silicon microdisks can provide a perfect cavity structure for such resonance to occur.Here we report GaN-based microdisks with different diameters,based on a standard blue LED wafer on a Si substrate.A confocal photoluminescence spectroscopy is performed to analyze the properties of all microdisks.Then,we systematically study the effects of radial modes and axial modes of these microdisks on photon-electron coupling efficiency by using three-dimensional finite-difference time-domain simulations.For thick microdisks,photon-electron coupling efficiency is found to greatly depend on the distributions of both the radial modes and the axial modes,and the inclined sidewalls make significant influences on the axial mode distributions.These results are important for realization of high-efficiency resonant emission in GaN-based microcavity devices.

Fabrication and characterization of Ge-Ga-Sb-S glass microsphere lasers operating at ～1.9 μm

We report the fabrication and characterization of germanium gallium antimony sulfide（Ge–Ga–Sb–S or 2 S2 G, doped with Tm^（3＋）ions） microsphere lasers operating at ～1.9-μm spectral band. Compared to the chalcogenide glasses that are used in previous microsphere lasers, this 2 S2 G glass has a lower transition temperature and a higher characteristic temperature. This implies that 2 S2 G microspheres can be fabricated at lower temperatures and the crystallization problem in the sphere-forming process can be alleviated. We show that hundreds of high-quality microspheres（quality factors higher than 105） of various diameters can be produced simultaneously via a droplet sphere-forming method. Microspheres are coupled with silica fiber tapers for optical characterizations. We demonstrate that Whispering Gallery mode（WGM）patterns in the 1.7–2.0 μm band can be conveniently obtained and that once the pump power exceeds a threshold, single-and multi-mode microsphere lasers can be generated. For a typical microsphere whose diameter is 258.64 μm, we demonstrate its laser threshold is 0.383 mW, the laser wavelength is 1907.38 nm, and the thermal sensitivity of the microsphere laser is29.56 pm/？C.

Temperature sensing property of microdisk resonator

The transmission equation of microdisk resonator is obtained by the transfer matrix method.The physical model is built and the electric field distribution,output spectrum and phase of the microdisk resonator are simulated by three-dimensional finite element software.The influence of the structural parameters on transmission characteristics and the temperature sensing property of the microdisk resonator are studied deeply.The results show that the output spectrum will change significantly with the distance between the microdisk and the straight waveguide within a certain range but there is no apparent change in the phase of the output port.The extinction ratio and maxima sensitivity of the device will reach 30 dB and 45 pm/℃,respectively.Microdisk has higher integration,higher quality factor and wider free spectral range compared with common microring resonator.

A Cogwheel WGM Resonator Based on Spoof Surface Plasmon Polaritons

The optical WGM resonator plays an important role in modern physics due to its ultra-high quality factor and small volume mode. In optics, SPPs modes can effectively confine electromagnetic waves at the interface between metal and dielectric, providing extremely high sensitivity. New interesting WGM phenomena will emerge when the WGM is combined with the SPPs. In this paper, a cogwheel resonator based on spoof SPPs was designed, which can generate multi-order WGM modes. The transmission coefficients, dispersion relations and resonance modes of the WGM resonator were analyzed. The proposed resonator extends the WGM mode from optical band to microwave band, providing a new perspective for the applications of WGM mode at microwave band.

All-silicon low-loss THz temporal differentiator based on microring waveguide resonator platform

Microring resonators have been widely used in passive optical devices such as wavelength division multiplexers,differentiators,and integrators.Research on terahertz(THz)components has been accelerated by these photonics technologies.Compact and integrated time-domain differentiators that enable low-loss,high-speed THz signal processing are necessary for THz applications.In this study,an on-chip THz temporal differentiator based on all-silicon photonic technology was developed.This device primarily consisted of a microring waveguide resonator and was packaged with standard waveguide compatibility.It performed time-domain differentiation on input signals at a frequency of 405.45 GHz with an insertion loss of 2.5 dB and a working bandwidth of 0.36 GHz.Various periodic waveforms could be handled by this differentiator.This device could work as an edge detector,which detected step-like edges in high-speed input signals through differential effects.This development holds significant promise for future THz data processing technologies and THz communication systems.

Analysis of mode characteristics for microcircular resonators confined by different metallic materials

Mode characteristics of metallically confined microcircular resonators are theoretically studied by solv- ing eigenvalue equations for two-dimensional multilayer structures. The influences of conventional metals includ- ing Au, Ag, Cu, A1, and Ti, on the mode wavelengths and Q factors of whispering gallery modes （WGMs） are an- alyzed and compared. The results show silver has the best optical confinement among these metals, and aluminum presents similar behavior to Au. However, Ti, which is usually applied to enhance the adhesion of p-electrode to semiconductors, results in a great dissipation for confined modes. Furthermore, circular microlasers with A1 as both p-electrode and optical confinement medium are fabricated, and continuous-wave operations are realized at room temperature for the microlasers with a radius of 15μm.