Coupled-mode induced transparency in a bottle whispering-gallery-mode resonator

All-optically controlled mode-coupling induced transparency with tunable efficiency in a microsphere resonator

The optical analogs of electromagnetically induced transparency (EIT) have attracted vast attention recently. The generation and manipulation of EIT in microcavities have sparked research in both fundamental physics and photonic applications, including light storage, slow light propagation, and optical communication. In this Letter, the generation and tuning of an all-optically controlled mode-coupling induced transparency (MCIT) are proposed, experimentally demonstrated, and theoretically analyzed. The MCIT effect originated from the intermodal coupling between the plethora of modes generated in our fabricated optical microcavity, and the tuning of the transparency mode utilized the cavity's thermal bistability nature. Furthermore, based on our method, a novel, to the best of our knowledge, controlling of the mode shifting efficiency is also achieved with an increase up to two times and more. The proposed scheme paves a unique, simple, and efficient way to manipulate the induced transparency mode, which can be useful for applications like cavity lasing and thermal sensing.

Laser frequency stabilization based on Fano resonance in a microcylinder cavity

We investigate the application of Fano resonance in microcylinder cavities for laser frequency stabilization. By combining Fano resonance and the differential subtraction method, we successfully reproduce the error signal of the traditional Pound-Drever-Hall (PDH) technique. The frequency noise of the laser, when locked to both microsphere and microcylinder cavities, approaches the thermal noise limit. The microcylinder cavity, with a high Q factor of ∼108, benefiting from its large mode volume, exhibits a significant reduction in frequency noise by one order of magnitude compared with a microsphere in the frequency range of 0.1 to 10 kHz, achieving a minimum noise of ∼2.25 Hz2/Hz at 10 kHz. As this approach eliminates the need for additional electronic circuits typically used in the PDH technique, it holds promise as a cost-effective and reliable solution for laser frequency stabilization.

Intrinsic mode coupling in mirror-symmetric whispering gallery resonators

Rotationally symmetric micro-cavities with disk, ring or toroidal shape displaying whispering gallery modes (WGMs) play an essential role in modern-day photonics. Due to the reduced symmetry of such resonators compared to spheres, an exact analytical model yielding WGMs as solutions does not exist. The established WGM classification scheme based on approximated analytical solutions is generally useful but neglects a possible interaction between the different modes. In this paper, we assess the limitation of the validity of this established classification based on extensive finite element method (FEM) simulations. We investigate respective mode couplings as well as underlying selection rules based on avoided crossings of the modes' resonance wavelengths. We propose conserved mode properties solely based on true symmetries of the underlying refractive-index distribution and deduce a novel WGM classification scheme.

Packaged optofluidic microbottle resonator for high-sensitivity bidirectional magnetic field sensing

We demonstrate a high-sensitivity bidirectional magnetic field sensor based on a packaged optofluidic microbottle resonator (OFMBR) filled with magnetic fluid (MF). The relationship between sensitivity and different wall thicknesses and radial modes of OFMBR is theoretically analyzed. Then the thin-wall OFMBR is fabricated by etching a capillary with the fusion discharge process. The OFMBR and tapered fiber is packaged with a portable and robust coupling configuration. By applying perpendicular or parallel magnetic field directions to the OFMBR, opposite refractive index responses of the MF can be obtained, with resonant wavelengths redshifted or blueshifted as the magnetic field intensity is increased. A magnetic field sensitivity of 98.23 pm/mT can be obtained by using the second-order radial mode when the magnetic field is perpendicular to the packaged OFMBR. When the magnetic field is parallel to the packaged OFMBR, the sensitivity is -304.80 pm/mT by using the third-order radial mode and the detection limit reaches 0.0656 mT. The proposed sensor has the advantages of easy fabrication, high sensitivity, and reliability, showing a great potential in bidirectional magnetic field application.

Electromagnetically induced transparency with a single optomechanical microring resonator

An all-optical realization scheme of electromagnetically induced transparency (EIT) in a single silicon optomechanical microring resonator is proposed and demonstrated. Due to the strong mechanical Kerr effect and well-designed microring resonator, two modes with a resonant frequency separation of 292 GHz (2.35 nm) can be tuned into resonance when the control power is about 4.3 µW, and the EIT spectrum is achieved. Our work provides a constructive solution for realizing EIT in a single microcavity with a low mode density. Furthermore, this device is fully integrated on-chip and compatible with current complementary metal-oxide semiconductor (CMOS) processing and has great potential in applications such as light storage, optical sensing, and quantum optics.

Simulation and Optimization of SNAP-Taper Coupling System in Displacement Sensing

Sensing applications based on whispering gallery mode (WGM) microcavities have attracted extensive attention recently, especially in displacement sensing applications. However, the traditional displacement sensing scheme based on shift in a single resonance wavelength, has a lot of drawbacks. Herein, a novel displacement sensing scheme based on the surface nanoscale axial photonics (SNAP) is proposed to achieve a wide range and high-resolution displacement sensor through analyzing the transmittance of multiple axial modes. By analyzing the surface plot of the resonance spectrum with different coupling positions, the ideal coupling parameters and ERV for displacement sensing are obtained. In the following, displacement sensing with high sensitivity and a wide range is theoretically realized through adjusting the sensitivity threshold and the number of modes. Finally, we present our views on the current challenges and the future development of the displacement sensing based on an SNAP resonator. We believe that a comprehensive understanding on this sensing scheme would significantly contribute to the advancement of the SNAP resonator for a broad range of applications.

Coupled-mode induced transparency via Ohmic heating in a single polydimethylsiloxane-coated microbubble resonator

We demonstrate an approach for the realization of coupled-mode induced transparency (CMIT) in a hybrid polydimethylsiloxane (PDMS)-coated silica microbubble resonator, with an Au microwire inserted in the hollow channel. Owing to the large negative thermo-optics coefficient of PDMS, different radial order modes with opposite thermal sensitivities can coexist in this hybrid microcavity. By applying a current through the Au microwire, which acts as a microheater, the generated Ohmic heating could thermally tune the resonance frequencies and the frequency detuning of the coupled mode to achieve controllable CMIT. This platform offers an efficient and convenient way to obtain controllable CMIT for applications, such as label-free biosensing and quantum information processing.

Autler-Townes splitting and induced transparency windows in a multimode microfiber knot

In this Letter, Autler-Townes splitting and induced transparency windows are observed in a multimode microfiber knot. The microfiber knot is fabricated using tapered single-mode fiber, with the knot position located at the transition area of the tapered fiber. The spectrum, in analogy to Autler-Townes splitting, derives from the mode splitting of two high-order excited modes, which is theoretically explained by the multimode transfer matrix method. Moreover, without adding resonators, two induced transparency windows are realized with the tunable coupling coefficients and phase difference of excited knot modes. The tunable, easily fabricated, compact, and robust microfiber knot has potential applications in optical sensing, filters, slow light, and optical switching.

Manipulation of optomechanically induced transparency and absorption by indirectly coupling to an auxiliary cavity mode

We theoretically study the optomechanically induced transparency (OMIT) and absorption (OMIA) phenomena in a single microcavity optomechanical system, assisted by an indirectly coupled auxiliary cavity mode. We show that the interference effect between the two optical modes plays an important role and can be used to control the multiple-pathway induced destructive or constructive interference effect. The three-pathway interference could induce an absorption dip within the transparent window in the red sideband driving regime, while we can switch back and forth between OMIT and OMIA with the four-pathway interference. The conversion between the transparency peak and absorption dip can be achieved by tuning the relative amplitude and phase of the multiple light paths interference. Our system proposes a new platform to realize multiple pathways induced transparency and absorption in a single microcavity and a feasible way for realizing all-optical information processing.

Magnetic-field tuning whispering gallery mode based on hollow microbubble resonator with Terfenol-D-fixed

We propose a method of magnetic-field tuning whispering gallery modes (WGMs) based on a hollow microbubble resonator (HMBR) with Terfenol-D-fixed. WGMs are excited by the evanescent field from a tapered fiber coupling with an HMBR. Both ends of the HMBR are fixed with Terfenol-D and vary with different lengths of the Terfenol-D. The length of the Terfenol-D varies with the external magnetic field for the high magnetostriction coefficient of Terfenol-D. The magnetic field sensitivity of 0.081 pm/mT in the magnetic field range of 0.14 mT-21.8 mT is achieved. The $Q$Q-factor of the HMBR can be regulated up to ${2.07} \times {{10}^4}$2.07×10⁴ with physical stretching HMBR. This work provides a novel tuning whispering gallery mode scheme and a broad application prospect in the fields of optical measurement and precise optical clocks in the future.

A general model for taper coupling of multiple modes of whispering gallery resonators and application to analysis of coupling-induced Fano interference in a single cavity

In this paper, we give a general model for analysis of multimode Whispering Gallery Mode (WGM) resonators coupled to multimode tapered fibers based on the coupled-mode theory. Such formulation takes into account the asymmetry of the taper-resonator coupling. Simulations for a microsphere show that the tapered fiber coupling mechanism induces cross-coupling between coherent orthonormal WGMs. We show that the degree of such cross-coupling depends basically on the fiber diameter, air-gap between the taper and resonator, intrinsic losses and eccentricity. The WGM cross-coupling affects the total transmission and spectral line-shape of the internal powers resulting in a controllable transformation of the line-shape to non-Lorentzian spectra. This analysis can be utilized to precisely determine the output and intra-cavity intensity of multimode microresonators, which is important in lasers, nonlinear optical signal generation and realization of optical delays.

Effects of end surface and angle coupling on mode splitting and suppression in a cylindrical microcavity

A cylindrical microcavity conventionally has the characteristics of simple fabrication and high Q factor, where the rich physics of mode splitting and suppression caused by mode excitation, coupling, and interference have been realized for highly sensitive sensing and cavity quantum electrodynamics. In this paper, we show experimentally and theoretically a simple method to tailor these two mechanisms via near-end surface and angle coupling in a single-mode fiber cylindrical microcavity. Mode splitting can be enhanced due to the interference between localized and axial modes as the effect of near-end surface coupling, validated by the coupled-mode model. Besides, we also demonstrate that the coupling angle between the fiber taper and cylindrical microcavity can efficiently affect the mode suppression in the transmission spectrum. Such a device has a simple structure, simple fabrication process, and simple mechanism to tailor the mode splitting and suppression for applications in cavity quantum electrodynamics, sensitive sensing, and other topics of photonics.

Electrospun polymer bottle microresonators for stretchable single-mode lasing devices

We report a simple electrospinning method to fabricate polymer bottle microresonators, which are doped with a lasing gain material and supported by electrospun polymer micro/nanofibers on a flexible grooved polymer substrate. The fabricated bottle microresonators have smooth outer surfaces and high quality. By using an interference light pump approach, single whispering gallery mode lasing is obtained, with a side-mode suppression factor over 20 dB. By mechanically stretching the grooved substrate, tunability of the lasing peaks is demonstrated. Our method has the advantages of saving time and being low in cost and may have promising applications in stretchable lasing and sensing devices.

Whispering gallery mode resonator sensor for in situ measurements of hydrogel gelation

Whispering gallery mode (WGM) resonators are compact and ultrasensitive devices, which enable label-free sensing at the single-molecule level. Despite their high sensitivity, WGM resonators have not been thoroughly investigated for use in dynamic biochemical processes including molecular diffusion and polymerization. In this work, the first report of using WGM sensors to continuously monitor a chemical reaction (i.e. gelation) in situ in a hydrogel is described. Specifically, we monitor and quantify the gelation dynamics of polyacrylamide hydrogels using WGM resonators and compare the results to an established measurement method based on rheology. Rheology measures changes in viscoelasticity, while WGM resonators measure changes in refractive index. Different gelation conditions were studied by varying the total monomer concentration and crosslinker concentration of the hydrogel precursor solution, and the resulting similarities and differences in the signal from the WGM resonator and rheology are elucidated. This work demonstrates that WGM alone or in combination with rheology can be used to investigate the gelation dynamics of hydrogels to provide insights into their gelation mechanisms.

Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering

Single-mode lasing in whispering-gallery mode (WGM) microresonators is challenging to achieve. In bottle microresonators, the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability. In this work, by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. The pump intensity is engineered into an interference distribution on the bottle microresonator surface. By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection. Attractive advantages of the system, including high side-mode suppression factors >20 dB, large spectral tunability >8 nm, low-lasing threshold and reversible control, are presented. Our demonstrated approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics.

Enhanced Fano resonance in a non-adiabatic tapered fiber coupled with a microresonator

We achieved enhanced Fano resonance by coupling a bottle resonator with a special non-adiabatic tapered fiber, where there is a high intensity distribution ratio between high-order and fundamental modes in the tapered region, as well as single mode propagation in the waist region. The resonance line shape is theoretically proved to be related to the intensity distribution ratio of the two fiber modes and their phase shift. An enhanced Fano line shape with an extinction ratio over 15 dB is experimentally reached by improving the intensity distribution ratio and tuning the phase shift. The results can remarkably improve the sensitivity of whispering-gallery mode microresonators in the field of optical sensing.

Electromagnetically induced transparency and absorption in a compact silicon ring-bus-ring-bus system

We have theoretically and experimentally demonstrated electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) phenomena in a compact silicon ring-bus-ring-bus (RBRB) system. The two ring resonators in our RBRB system are both directly coupled and indirectly coupled through an asymmetric tricoupler. The coherent interference between a radiant mode and a subradiant mode in the two rings results in EIT and EIA effects at the through and drop ports, respectively. A theoretical model is established to analyze the proposed system based on temporal coupled mode theory. Finite-difference time-domain method is also employed to simulate the characteristics of this system. Consequently, RBRB structures were fabricated on a silicon-on-insulator platform and EIT and EIA transmissions have been observed simultaneously in the two outputs. The experimental results agree with our theoretical modeling and numerical simulations.

Stable controlling of electromagnetically induced transparency-like in a single quasi-cylindrical microresonator

We experimentally and theoretically demonstrate electromagnetically induced transparency (EIT)-like and Fano resonance in a single quasi-cylindrical microresonator (QCMR). Stable controlling of the EIT and Fano resonance lineshapes can be achieved by vertically moving the resonator along its axis while in touch with the tapered fiber. Moreover, by horizontally scanning the coupling point along the tapered fiber, asymmetric Fano resonances of the transmission spectra are observed and can be engineered to vary periodically. Interestingly, the two different kinds of mechanisms that induce the Fano or EIT resonances can work on the same mode simultaneously. Our device offers a stable platform for controlling the EIT and Fano resonances and holds unique potential in all-optical switching, quantum information processing and sensitivity-enhanced sensing applications.

Tunable polarization beam splitter based on optofluidic ring resonator

An efficient polarization beam splitter (PBS) based on an optofluidic ring resonator (OFRR) is proposed and experimentally demonstrated. The PBS relies on the large effective refractive index difference between transverse-electric (TE) and transverse-magnetic (TM) polarization states, since the silica-microcapillary-based OFRR possesses a slab-like geometry configuration in the cross section through which the circulating light travels. To the best of our knowledge, this is the first OFRR-based PBS. In our work, the maximum polarization splitting ratio of up to 30 dB is achieved. Besides, water and ethanol are pumped into the core of the silica microcapillary respectively, and the maximum wavelength tuning range of 7.02 nm is realized when ethanol flows through the core, verifing the tuning principle of the PBS effectively. With such a good performance and simple scheme, this OFRR-based PBS is promising for applications such as tunable optical filters, demultiplexers, and routers.