Whispering Gallery Mode Microresonators Research Articles

Fluorescent and lasing whispering gallery mode microresonators for sensing applications

AbstractWhispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM‐modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence‐based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.image

Femtosecond laser fabrication of high‐Q whispering gallery mode microresonators via two‐photon polymerization

ABSTRACTWhispering gallery mode microresonators have been triggering considerable advances in science due to their ability to confine light within small dielectric volumes, which makes them suitable for a wide range of applications. Lithographic approaches have been the dominant technique for fabricating microresonators; however, they restrict the choice of materials due to their multistep processing nature. As an alternative, they report the direct laser fabrication of acrylic based hollow microcylinder resonators, via two‐photon polymerization, with good structural integrity and sidewall roughness of 1.5 nm, which make them promising candidates for photonic applications in the near‐infrared. Such polymeric microresonators exhibit finesse close to 103 and a quality factor of 1 105, a performance achieved without any additional processing step, which would restrict the choices of materials to be incorporated into the polymeric resonator. This advantage thereby broadens the widespread use of the polymeric microresonators, making them an excellent platform for lasing and nonlinear optics studies in the near‐infrared. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 569–574

Optical multistability and Fano line-shape control via mode coupling in whispering-gallery-mode microresonator optomechanics

We study a three-mode (i.e., a clockwise mode, a counterclockwise mode, and a mechanical mode) coherent coupling regime of the optical whispering-gallery-mode (WGM) microresonator optomechanical system by considering a pair of counterpropagating modes in a general case. The WGM microresonator is coherently driven by a strong control laser field and a relatively weak probe laser field via a tapered fiber. The system parameters utilized to explore this process correspond to experimentally demonstrated values in the WGM microresonator optomechanical systems. By properly adjusting the coupling rate of these two counterpropagating modes in the WGM microresonator, the steady-state displacement behaviors of the mechanical oscillation and the normalized power transmission and reflection spectra of the output fields are analyzed in detail. It is found that the mode coupling plays a crucial role in rich line-shape structures. Some interesting phenomena of the system, including optical multistability and sharp asymmetric Fano-shape optomechanically induced transparency (OMIT), can be generated with a large degree of control and tunability. Our obtained results in this study can be used for designing efficient all-optical switching and high-sensitivity sensor.

Microresonator stabilized 2 μm distributed-feedback GaSb-based diode laser.

We report on the stabilization of a high-power distributed feedback (DFB) semiconductor laser operating at 2.05μm wavelength, using a crystalline whispering gallery mode microresonator. The laser's frequency noise is measured to be below 100 Hz/Hz1/2 at Fourier frequencies ranging from 10Hz to 1MHz. The instantaneous linewidth of the laser is improved by four orders of magnitude compared with the free-running DFB laser, and is measured to be 15Hz at 0.1ms measurement time. The integral linewidth approaches 100Hz. The stabilized DFB laser is integrated with a polarization maintaining output fiber and an integrated optical isolator.

Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials.

The unavailability of highly transparent materials in the mid-infrared has been the main limitation in the development of ultra-sensitive molecular sensors or cavity-based spectroscopy applications. Whispering gallery mode microresonators have attained ultra-high-quality (Q) factor resonances in the near-infrared and visible. Here we report ultra-high Q factors in the mid-infrared using polished alkaline earth metal fluoride crystals. Using an uncoated chalcogenide tapered fibre as a high-ideality coupler in the mid-infrared, we study via cavity ringdown technique the losses of BaF2, CaF2, MgF2 and SrF2 microresonators. We show that MgF2 is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF2 and BaF2 the lower multiphonon absorption leads to ultra-high Q factors at 4.5 μm. These values correspond to an optical finesse of , the highest value achieved for any type of mid-infrared resonator to date.

High-Q silk fibroin whispering gallery microresonator.

We have experimentally demonstrated an on-chip all-silk fibroin whispering gallery mode microresonator by using a simple molding and solution-casting technique. The quality factors of the fabricated silk protein microresonators are on the order of 105. A high-sensitivity thermal sensor was realized in this silk fibroin microtoroid with a sensitivity of -1.17 nm/K, that is 8 times higher than previous WGM resonator-based thermal sensors. This opens the way to fabricate biodegradable and biocompatible protein based microresonators on a flexible chip for biophotonics applications.

Biosensing by WGM Microspherical Resonators.

Whispering gallery mode (WGM) microresonators, thanks to their unique properties, have allowed researchers to achieve important results in both fundamental research and engineering applications. Among the various geometries, microspheres are the simplest 3D WGM resonators; the total optical loss in such resonators can be extremely low, and the resulting extraordinarily high Q values of 108–109 lead to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. They can also be coated in order to better control their properties or to increase their functionality. Their very high sensitivity to changes in the surrounding medium has been exploited for several sensing applications: protein adsorption, trace gas detection, impurity detection in liquids, structural health monitoring of composite materials, detection of electric fields, pressure sensing, and so on. In the present paper, after a general introduction to WGM resonators, attention is focused on spherical microresonators, either in bulk or in bubble format, to their fabrication, characterization and functionalization. The state of the art in the area of biosensing is presented, and the perspectives of further developments are discussed.

Integrated microsphere whispering gallery mode probe for highly sensitive refractive index measurement

We report an integrated whispering gallery mode microresonator–based sensor probe for refractive index sensing. The probe was made by sealing a borosilicate glass microsphere into a thin-wall glass capillary pigtailed with a multimode optical fiber. The intensities of the resonant peaks were found decreasing exponentially (linearly in a log scale) with the increasing refractive index of the medium surrounding the capillary. The sensing capability of the integrated probe was tested using sucrose solutions of different concentrations and the resolution was estimated to be about 2.5×10− 5 in the index range of 1.3458 to 1.3847. The integrated sensor probe may prove useful in many chemical and biological sensing applications where highly sensitive refractive index monitoring is needed.

Generation of optical frequency combs in a fiber-ring/microresonator laser system.

We propose and experimentally demonstrate a simple scheme for generating optical frequency combs (OFCs) in a fiber-ring/microresonator laser system. The ultrahigh Q whispering gallery mode microresonator is employed both as a mode reflection mirror to generate erbium lasing and as a Kerr-nonlinearity initiator that introduces optical parametric oscillation signals to form OFCs. By controlling the coupling position between the fiber taper and microresonator, optimizing the fiber polarization, as well as the pump power from a 974 nm laser diode (LD), versatile OFCs can be tuned out from single-wavelength states. The OFCs have single, multiple, or combined free spectral ranges. In addition, a Raman-gain-assisted OFC is also observed with a bandwidth of ∼230 nm. This LD-pumped and multifunctional laser system could find applications in precision spectroscopy, biochemical sensing, and optical fiber communication systems.

Millisecond Photon Lifetime in a Slow-Light Microcavity.

Optical microcavities with ultralong photon storage times are of central importance for integrated nanophotonics. To date, record quality (Q) factors up to 10^{11} have been measured in millimetric-size single-crystal whispering-gallery-mode (WGM) resonators, and 10^{10} in silica or glass microresonators. We show that, by introducing slow-light effects in an active WGM microresonator, it is possible to enhance the photon lifetime by several orders of magnitude, thus circumventing both fabrication imperfections and residual absorption. The slow-light effect is obtained from coherent population oscillations in an erbium-doped fluoride glass microsphere, producing strong dispersion of the WGM (group index n_{g}∼10^{6}). As a result, a photon lifetime up to 2.5ms at room temperature has been measured, corresponding to a Q factor of 3×10^{12} at 1530nm. This system could yield a new type of optical memory microarray with ultralong storage times.

Ringing phenomenon based whispering-gallery-mode sensing.

Highly sensitive sensing is one of the most important applications of whispering-gallery-mode (WGM) microresonators, which is usually accomplished through a tunable continuous-wave laser sweeping over a whispering-gallery mode with the help of a fiber taper in a relative slow speed. It is known that if a tunable continuous-wave laser sweeps over a high quality whispering-gallery mode in a fast speed, a ringing phenomenon will be observed. The ringing phenomenon in WGM microresonators is mainly used to measure the Q factors and mode-coupling strengths. Here we experimentally demonstrate that the WGM sensing can be achieved based on the ringing phenomenon. This kind of sensing is accomplished in a much shorter time and is immune to the noise caused by the laser wavelength drift.

Enhancement of a whispering gallery mode microtoroid resonator by plasmonic triangular gold nanoprism for label-free biosensor applications

Whispering gallery mode microresonators coupled to plasmonic nanoparticles have shown great promise for ultra-sensitive and label-free biological sensing. We analyze a whispering gallery mode microtoroid biosensor with a triangular gold nanoprism bound to its surface. We choose triangular nanoprism because of its capability of considerable enhancement of electromagnetic field at the tips and because its localized surface plasmon resonance band position is readily tunable. The local electric field enhancement at different points of plasmonic triangular gold nanoprism is calculated and the effect of rounded tip is investigated. This field enhancement permits the detection and characterization of some large protein molecules. We have studied the detection of single bovine serum albumin protein using our hybrid microcavity.

Two photon versus one photon fluorescence excitation in whispering gallery mode microresonators

We investigate the feasibility of both one photon and two photon fluorescence excitation using whispering gallery mode microresonators. We report the linear and non linear fluorescence real-time detection of labeled IgG covalently bonded to the surface of a silica whispering gallery mode resonator (WGMR). The immunoreagents have been immobilized onto the surface of the WGMR sensor after being activated with an epoxy silane and an orienting layer. The developed immunosensor presents great potential as a robust sensing device for fast and early detection of immunoreactions. We also investigate the potential of microbubbles as nonlinear enhancement platform. The dyes used in these studies are dylight800, tetramethyl rhodamine isothiocyanate, rhodamine 6G and fluorescein. All measurements were performed in a modified confocal microscope.

Tunable continuous wave emission via phase-matched second harmonic generation in a ZnSe microcylindrical resonator.

Whispering gallery mode microresonators made from crystalline materials are of great interest for studies of low threshold nonlinear phenomena. Compared to amorphous materials, crystalline structures often exhibit desirable properties such as high indices of refraction, high nonlinearities, and large windows of transparency, making them ideal for use in frequency comb generation, microlasing and all-optical processing. In particular, crystalline materials can also possess a non-centrosymmetric structure which gives rise to the second order nonlinearity, necessary for three photon processes such as frequency doubling and parametric down-conversion. Here we report a novel route to fabricating crystalline zinc selenide microcylindrical resonators from our semiconductor fibre platform and demonstrate their use for tunable, low power continuous wave second harmonic generation. Visible red light is observed when pumped with a telecommunications band source by a process that is phase-matched between different higher order radial modes, possible due to the good spatial overlap between the pump and signal in the small volume resonator. By exploiting the geometrical flexibility offered by the fibre platform together with the ultra-wide 500–22000 nm transmission window of the ZnSe material, we expect these resonators to find use in applications ranging from spectroscopy to quantum information systems.

All-electronic line width reduction in a semiconductor diode laser using a crystalline microresonator

This study examines the capability to significantly suppress the frequency noise of a semiconductor distributed feedback diode laser using a universally applicable approach: a combination of a high-Q crystalline whispering gallery mode microresonator reference and the Pound–Drever–Hall locking scheme using an all-electronic servo loop. An out-of-loop delayed self-heterodyne measurement system demonstrates the ability of this approach to reduce a test laser’s absolute line width by nearly a factor of 100. In addition, in-loop characterization of the laser stabilized using this method demonstrates a 1-kHz residual line width with reference to the resonator frequency. Based on these results, we propose that utilization of an all-electronic loop combined with the use of the wide transparency window of crystalline materials enable this approach to be readily applicable to diode lasers emitting in other regions of the electromagnetic spectrum, especially in the UV and mid-IR .

Perovskite Microdisk Microlasers Self-Assembled from Solution.

Single-crystalline square microdisks of CH3NH3PbBr3 are prepared by using a one-step solution self-assembly method. Single-mode lasing at 557 nm is achieved based on a built-in whispering gallery mode microresonator at room temperature. By partial replacement of Br with Cl, lasing wavelengths are continuously tuned from 525 to 557 nm.

Thermal characterisation of (bio)polymers with a temperature-stabilised whispering gallery mode microsensor

In this work, we theoretically and experimentally investigate the thermal response of whispering gallery mode microresonators operating in an aqueous glycerol medium. Thermal stabilisation of the resonance wavelength is realised by appropriate choice of the resonator radius and glycerol concentration, with a 60 fold reduction in thermal sensitivity demonstrated. Finally, we employ our stabilised system to determine the thermal dependence of the molecular polarisability of adsorbed bovine serum albumin molecules and the refractive index of dextran and poly(diallyldimethylammonium chloride) coatings.

Cavity ring-up spectroscopy for ultrafast sensing with optical microresonators.

Spectroscopy of whispering-gallery mode microresonators has become a powerful scientific tool, enabling the detection of single viruses, nanoparticles and even single molecules. Yet the demonstrated timescale of these schemes has been limited so far to milliseconds or more. Here we introduce a scheme that is orders of magnitude faster, capable of capturing complete spectral snapshots at nanosecond timescales—cavity ring-up spectroscopy. Based on sharply rising detuned probe pulses, cavity ring-up spectroscopy combines the sensitivity of heterodyne measurements with the highest-possible, transform-limited acquisition rate. As a demonstration, we capture spectra of microtoroid resonators at time intervals as short as 16 ns, directly monitoring submicrosecond dynamics of their optomechanical vibrations, thermorefractive response and Kerr nonlinearity. Cavity ring-up spectroscopy holds promise for the study of fast biological processes such as enzyme kinetics, protein folding and light harvesting, with applications in other fields such as cavity quantum electrodynamics and pulsed optomechanics.

High-Q Factor Optical Whispering-Gallery Mode Microresonators and Their Use in Precision Measurements

Basic data concerning the use of optical dielectric whispering-gallery mode microresonators are presented. Their properties, fabrication technologies, and ranges of application as well as the most important recent advances are described. Such microresonators may be used in highly stable sources of microwave signals and optical frequency comb oscillators, making it possible to link together optical and radio frequency standards.

Nano-partical sensing based on Raman laser in the whispering gallery mode microresonators

In this review, the recent development of nano-particle detection using Raman lasers in the whispering gallery mode microcavities is presented. The fabrication of the microcavity, the working principles are given and the recent experimental progress is reviewed. Recent years, the demand for nano-particle sensing techniques was increased, since more and more nano-particles of sizes between 1 nm and 100 nm are employed in areas such as biomedical science and homeland security. In these applications, label-free, rapid and real-time sensing requirements are necessary. Whispering gallery mode (WGM) micro-resonators have high-quality factors and small mode volumes, and have achieved significant progress in the nano-particle sensing field. There are various measurement mechanisms for nano-particle sensing using WGM cavities, including resonance mode broadening, resonance frequency shift, and mode splitting changes. The key point to improve sensing limit is to narrow the resonance mode linewidth, which means reducing the optical cavity losses, or equivalently to enhance quality factor. An important approach to narrowing the mode linewidth is to fabricate active resonators that provide gain and produce laser by doping rare earth irons. According to Schawlow-Townes formula, the linewidth of corresponding laser will be narrower than that of the original optical cavity mode. Active resonators have outstanding performances in particle detection. However, doping process requires complex fabrication steps, and rare earth irons laser demands a certain pumping wavelength band. A new approach is to use low threshold Raman laser in an optical micro-resonator. The binding of nano-particles on WGM micro-resonator induces resonance mode splitting. Raman lasers of the two splitting modes irradiate the same photon detector and generate a beat note signal. By monitoring the jumps of the two split mode differential signals, one can easily recognize the nano-particle binding events, thus achieving real time nanoparticle detection. Using Raman laser in WGM cavities in nano-particle sensing is no longer limited by the stringent requirement of a suitable pump light source, which greatly expands the applicability of this method in different environments. It does not need additional fabrication process as compared with the rare earth doping method. It has also better biological compatibility, which makes it a promising technique in biomedical applications. Recently, two groups, i.e., Li et al. (Proc. Natl. Acad. Sci. 111 14657) from Peking University, and zdemir et al. from University of Washington and Tsinghua University, have successfully completed the demonstration experiments. zdemir et al. (Proc. Natl. Acad. Sci. 111 E3836) have achieved a nano-particle sensing limit down to 10 nm without labelling, and Li et al. (Proc. Natl. Acad. Sci. 111 14657) realized real-time detection of single nano-particles with WGM cavity Raman laser in an aqueous environment. Both experiments have shown the great potential of the new approach. The new technique can also be used in other photonic systems, such as the photonic crystal or metal materials.