Kerr parametric oscillations and frequency comb generation from dispersion compensated silica micro-bubble resonators

Machine learning-assisted high-accuracy and large dynamic range thermometer in high-Q microbubble resonators

Whispering gallery mode (WGM) resonators provide an important platform for fine measurement thanks to their small size, high sensitivity, and fast response time. Nevertheless, traditional methods focus on tracking single-mode changes for measurement, and a great deal of information from other resonances is ignored and wasted. Here, we demonstrate that the proposed multimode sensing contains more Fisher information than single mode tracking and has great potential to achieve better performance. Based on a microbubble resonator, a temperature detection system has been built to systematically investigate the proposed multimode sensing method. After the multimode spectral signals are collected by the automated experimental setup, a machine learning algorithm is used to predict the unknown temperature by taking full advantage of multiple resonances. The results show the average error of 3.8 × 10^-3°C within the range from 25.00°C to 40.00°C by employing a generalized regression neural network (GRNN). In addition, we have also discussed the influence of the consumed data resource on its predicted performance, such as the amount of training data and the case of different temperate ranges between the training and test data. With high accuracy and large dynamic range, this work paves the way for WGM resonator-based intelligent optical sensing.

Optical Whispering-Gallery-Mode Microbubble Sensors

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

Tunable optofluidic microbubble lens

Optofluidic microlenses are one of the crucial components in many miniature lab-on-chip systems. However, many optofluidic microlenses are fabricated through complex micromachining and tuned by high-precision actuators. We propose a kind of tunable optofluidic microbubble lens that is made by the fuse-and-blow method with a fiber fusion splicer. The optical focusing properties of the microlens can be tuned by changing the refractive index of the liquid inside. The focal spot size is 2.8 µm and the focal length is 13.7 µm, which are better than those of other tunable optofluidic microlenses. The imaging capability of the optofluidic microbubble lens is demonstrated under a resolution test target and the imaging resolution can reach 1 µm. The results indicate that the optofluidic microbubble lens possesses good focusing properties and imaging capability for many applications, such as cell counting, optical trapping, spatial light coupling, beam shaping and imaging.

Directed Self-Assembly Driven Mesoscale Lithography Using Laser-Induced and Manipulated Microbubbles: Complex Architectures and Diverse Applications

A microbubble nucleated due to the absorption of a tightly focused laser at the interface of a liquid-solid substrate enables directed and irreversible self-assembly of mesoscopic particles dispersed in the liquid at the bubble base. This phenomenon has facilitated a new microlithography technique which has grown rapidly over the past decade and can now reliably pattern a vast range of soft materials and colloids, ranging from polymers to metals to proteins. In this review, we discuss the science behind this technology and the present state-of-the-art. Thus, we describe the physics of the self-assembly driven by the bubble, the techniques for generating complex mesoarchitectures, both discrete and continuous, and their properties, and the various applications demonstrated in plastic electronics, site-specific catalysis, and biosensing. Finally, we describe a roadmap for the technique to achieve its potential of successfully patterning "everything" mesoscopic and the challenges that lie therein.

Wall-thickness-controlled microbubble fabrication for WGM-based application

We present a wall-thickness-controlled microbubble fabrication model for whispering-gallery-mode (WGM)-based application. The process of fabricating the model is divided into three sequenced steps: geometry size change of the microcapillary during drawing, expanding the process under internal injection air pressure, and microcapillary waist swell into a microbubble. Experiments were carried out to verify the effectiveness of the model. Experiment results show that wall thickness can reach 1.28 µm-1.46 µm at different injection pressure ranges of 50 kPa. The expected wall thickness of the microbubble can be achieved by changing injection pressure while keeping the diameter, which helps to prepare the required microbubble for practical application.

Optofluidic microbubble Fabry-Pérot cavity

An optofluidic microbubble Fabry-Pérot (OMBFP) cavity was investigated. In contrast to plane-plane FP (PPFP) cavities, the optical mode confinement and stability in an OMBFP were significantly enhanced. The optical properties of the OMBFP cavity, including the quality (Q) factor, effective mode area, mode distribution as a function of the core refractive index, microbubble position, and mirror tilt angle, were investigated systematically using the finite element method. In optofluidic lasing experiments, a low lasing threshold of 1.25 µJ/mm², which was one order magnitude lower than that of the PPFP, was achieved owing to improved modal lateral confinement. Since the microbubble acts as a micro-lens and microfluidic channel in the parallel FP cavity, mode selection and cell-dye laser were easily realized in the OMBFP cavity.

Nonlinear Optics in Microspherical Resonators

Nonlinear frequency generation requires high intensity density which is usually achieved with pulsed laser sources, anomalous dispersion, high nonlinear coefficients or long interaction lengths. Whispering gallery mode microresonators (WGMRs) are photonic devices that enhance nonlinear interactions and can be exploited for continuous wave (CW) nonlinear frequency conversion, due to their capability of confine light for long time periods in a very small volume, even though in the normal dispersion regime. All signals must be resonant with the cavity. Here, we present a review of nonlinear optical processes in glass microspherical cavities, hollow and solid.

Packaged microbubble resonator optofluidic flow rate sensor based on Bernoulli Effect

A novel flow sensor based on dynamic fluid pressure changing in a packaged microbubble resonator without additional modification on its structure has been proposed and experimentally demonstrated. The results of sensing performance under both tunable laser source and broadband light source are presented. The flow rate sensitivity can reach up to 0.0196 pm / (µL/min). The fluid pressure variation caused by Bernoulli Effect is also analyzed theoretically.

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.

Orthogonal Demodulation Pound-Drever-Hall Technique for Ultra-Low Detection Limit Pressure Sensing

We report on a novel optical microcavity sensing scheme by using the orthogonal demodulation Pound-Drever-Hall (PDH) technique. We found that larger sensitivity in a broad range of cavity quality factor (Q) could be obtained. Taking microbubble resonator (MBR) pressure sensing as an example, a lower detection limit than the conventional wavelength shift detection method was achieved. When the MBR cavity Q is about 105-106, the technique can decrease the detection limit by one or two orders of magnitude. The pressure-frequency sensitivity is 11.6 GHz/bar at wavelength of 850 nm, and its detection limit can approach 0.0515 mbar. This technique can also be applied to other kinds of microcavity sensors to improve sensing performance.

A Single-Ended Ultra-Thin Spherical Microbubble Based on the Improved Critical-State Pressure-Assisted Arc Discharge Method

Hollow core microbubble structures are good candidates for the construction of high performance whispering gallery microresonator and Fabry-Perot (FP) interference devices. In the previous reports, most of interest was just focused on the dual-ended microbubble, but not single-ended microbubble, which could be used for tip sensing or other special areas. The thickness, symmetry and uniformity of the single-ended microbubble in previous reports were far from idealization. Thus, a new ultra-thin single-ended spherical microbubble based on the improved critical-state pressure-assisted arc discharge method was proposed and fabricated firstly in this paper, which was fabricated simply by using a commercial fusion splicer. The improvement to former paper was using weak discharge and releasing pressure gradually during the discharging process. Thus, the negative influence of gravity towards bubble deformation was decreased, and the fabricated microbubble structure had a thin, smooth and uniform surface. By changing the arc discharge parameters and the fiber position, the wall thicknesses of the fabricated microbubble could reach the level of 2 μm or less. The fiber Fabry-Perot (FP) interference technique was also used to analyze the deformation characteristic of microbubble under difference filling pressures. Finding the ends of the microbubbles had a trend of elongation with axial compression when the filling pressure was increasing. Its sensitivity to the inner pressure of microbubble samples was about ~556 nm/MPa, the bubble wall thickness was only of about 2 μm. Besides, a high whispering gallery mode (WGM) quality factor that up to 107 was realized by using this microbubble-based resonator. To explain the upper phenomenon, the microbubble was modeled and simulated with the ANSYS software. Results of this study could be useful for developing new single-ended whispering gallery mode micro-cavity structure, pressure sensors, etc.

Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity

The large normal dispersion of the fundamental mode (TE_n=1 mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TE_n=2 mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO₂ or HfO₂) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TE_n=2 mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.

Packaged Droplet Microresonator for Thermal Sensing with High Sensitivity

Liquid droplet and quasi-droplet whispering gallery mode (WGM) microcavities have been widely studied recently for the enhanced spatial overlap between the liquid and WGM field, especially in sensing applications. However, the fragile cavity structure and the evaporation of liquid limit its practical applications. Here, stable, packaged, quasi-droplet and droplet microcavities are proposed and fabricated for thermal sensing with high sensitivity. The sensitivity and electromagnetic field intensity distribution are analyzed by Mie theory, and a quantified definition of the quasi-droplet is presented for the first time to the best of our knowledge. By doping dye material directly into the liquid, lasing packaged droplet and quasi-droplet microcavity sensors with a high thermal sensitivity of up to 205.3 pm/°C are experimentally demonstrated. The high sensitivity, facile fabrication, and mechanically robust properties of the optofluidic, packaged droplet microresonator make it a promising candidate for future integrated photonic devices.

Optical Spring Effect in Micro-Bubble Resonators and Its Application for the Effective Mass Measurement of Optomechanical Resonant Mode

In this work, we present a novel approach for obtaining the effective mass of mechanical vibration mode in micro-bubble resonators (MBRs). To be specific, the effective mass is deduced from the measurement of optical spring effect (OSE) in MBRs. This approach is demonstrated and applied to analyze the effective mass of hollow MBRs and liquid-filled MBRs, respectively. It is found that the liquid-filled MBRs has significantly stronger OSE and a less effective mass than hollow MBRs, both of the extraordinary behaviors can be beneficial for applications such as mass sensing. Larger OSE from higher order harmonics of the mechanical modes is also observed. Our work paves a way towards the developing of OSE-based high sensitive mass sensor in MBRs.

Unified theory of whispering gallery multilayer microspheres with single dipole or active layer sources

The development of a fast and reliable whispering gallery mode (WGM) simulator capable of generating spectra that are comparable with experiment is an important step forward for designing microresonators. We present a new model for generating WGM spectra for multilayer microspheres, which allows for an arbitrary number of concentric dielectric layers, and any number of embedded dipole sources or uniform distributions of dipole sources to be modeled. The mode excitation methods model embedded nanoparticles, or fluorescent dye coatings, from which normalized power spectra with accurate representation of the mode coupling efficiencies can be derived. In each case, the emitted power is expressed conveniently as a function of wavelength, with minimal computational load. The model makes use of the transfer-matrix approach, incorporating improvements to its stability, resulting in a reliable, general set of formulae for calculating whispering gallery mode spectra. In the specific cases of the dielectric microsphere and the single-layer coated microsphere, our model simplifies to confirmed formulae in the literature.

Tunable erbium-doped microbubble laser fabricated by sol-gel coating

In this work, we show that the application of a sol-gel coating renders a microbubble whispering gallery resonator into an active device. During the fabrication of the resonator, a thin layer of erbium-doped sol-gel is applied to a tapered microcapillary, then a microbubble with a wall thickness of 1.3 μm is formed with the rare earth ions diffused into its wall. The doped microbubble is pumped at 980 nm and lases in the emission band of the Er³⁺ ions at 1535 nm. The laser wavelength can be shifted by aerostatic pressure tuning of the whispering gallery modes of the microbubble. Up to 240 pm tuning is observed with 2 bar of applied pressure. We also show that the doped microbubble could be used as a compact, tunable laser source.

Four-wave mixing parametric oscillation and frequency comb generation at visible wavelengths in a silica microbubble resonator

Frequency comb generation in microresonators at visible wavelengths has found applications in a variety of areas such as metrology, sensing, and imaging. To achieve Kerr combs based on four-wave mixing in a microresonator, dispersion must be in the anomalous regime. In this Letter, we demonstrate dispersion engineering in a microbubble resonator (MBR) fabricated by a two-CO₂ laser beam technique. By decreasing the wall thickness of the MBR to 1.4 μm, the zero dispersion wavelength shifts to values shorter than 764 nm, making phase matching possible around 765 nm. With the optical Q-factor of the MBR modes being greater than 10⁷, four-wave mixing is observed at 765 nm for a pump power of 3 mW. By increasing the pump power, parametric oscillation is achieved, and a frequency comb with 14 comb lines is generated at visible wavelengths.

Precise measurement of micro bubble resonator thickness by internal aerostatic pressure sensing

We develop a new, simple and non-destructive method to precisely measure the thickness of thin wall micro bubble resonators (MBRs) by using internal aerostatic pressure sensing. Measurement error of 1% at a bubble wall thickness of 2 μm is achieved. This method is applicable to both thin wall and thick wall MBR with high measurement accuracy.

Resonance Frequency of Optical Microbubble Resonators: Direct Measurements and Mitigation of Fluctuations

This work shows the improvements in the sensing capabilities and precision of an Optical Microbubble Resonator due to the introduction of an encaging poly(methyl methacrylate) (PMMA) box. A frequency fluctuation parameter σ was defined as a score of resonance stability and was evaluated in the presence and absence of the encaging system and in the case of air- or water-filling of the cavity. Furthermore, the noise interference introduced by the peristaltic and the syringe pumping system was studied. The measurements showed a reduction of σ in the presence of the encaging PMMA box and when the syringe pump was used as flowing system.

Dispersion analysis of whispering gallery mode microbubble resonators

This paper examines the opportunities existing for engineering dispersion in non-silica whispering gallery mode microbubble resonators, for applications such as optical frequency comb generation. More specifically, the zero dispersion wavelength is analyzed as a function of microbubble diameter and wall thickness for several different material groups such as highly-nonlinear soft glasses, polymers and crystalline materials. The zero dispersion wavelength is shown to be highly-tunable by changing the thickness of the shell. Using certain materials it is shown that dispersion equalization can be realized at interesting wavelengths such as deep within the visible or mid-infrared, opening up new possibilities for optical frequency comb generation. This study represents the first extensive analysis of the prospects of using non-silica microbubbles for nonlinear optics.

Stimulated Brillouin laser and frequency comb generation in high-Q microbubble resonators

We report on the stimulated Brillouin laser (SBL) and over-dense frequency comb generation in high-Q microbubble resonators (MBRs). Both first-order and cascaded SBL are achieved due to the rich high-order axial modes in the MBRs, although the free spectral range (FSR) of azimuthal mode of the MBR is severely mismatched with the Brillouin shift. The SBL is also generated by varying the internal pressure of MBR at fixed initially non-resonant pump light wavelength. In addition, over-dense frequency combs are realized with comb spacings that are one and two FSRs of aixal mode.

Dispersion in silica microbubble resonators

We explore the scope for engineering dispersion in whispering gallery mode silica microbubbles for nonlinear applications, such as optical frequency comb generation. In particular, the zero dispersion wavelength is shown to be highly tunable by changing the thickness of the shell. Using a small diameter and small wall thickness, dispersion equalization within the visible is predicted. This opens up the possibility of realizing visible frequency combs for a range of different applications.

Degenerate four-wave mixing in a silica hollow bottle-like microresonator

A hollow, bottle-like microresonator (BLMR) was fabricated from a microcapillary with a nearly parabolic profile. From simulations at 1.55 μm the fundamental bottle mode is shown to be in the anomalous dispersion regime, while the conventional whispering gallery mode, confined to the center of the BLMR, is in the normal dispersion regime. Therefore, we have experimentally shown that, for a BLMR with a diameter of 102 um, degenerate four-wave mixing can only be observed by judicious selection of the tapered fiber coupling position. Dispersion tuning in such a system is also briefly discussed theoretically. BLMRs are promising devices for the implementation of sparsely distributed, widely spanned frequency combs at the telecommunications C-band.

Packaged optofluidic microbubble resonators for optical sensing

A microbubble resonator (MBR) coupled with a fiber taper is packaged with low-index polymer. The cladding polymer serves as a protective matrix for the coupling system to avoid environmental disturbance. The packaged structure is portable and provides good performance to maintain high Q factors for a long working period. The hollow structure of the MBR makes the packaged system useful for practical chemical and biomedical sensing applications. To evaluate the performance of the packaged MBRs-based sensor, we carry out bulk refractive index and surface-sensing measurements with achieved sensitivities of 18.8 nm/RIU and 31.29 pm/nm, respectively.

Strong coupling of hybrid and plasmonic resonances in liquid core plasmonic micro-bubble cavities

A thin-wall plasmonic micro-bubble resonator, which is a high-Q optofluidic silica bubble cavity with a thin Ag film on the inside wall of the bubble, is proposed and fabricated to manipulate coupling among various types of resonant modes by changing its wall thickness and refractive index of the liquid in the core. Coupling of high-Q whispering gallery mode/plasmonic resonant mode forms hybrid mode; the hybrid mode can again strongly couple with another interior plasmonic resonant mode in the bubble cavity to achieve tunable high-Q plasmonic resonance that can be feasibly accessed by standard tapered fiber coupling. Therefore, the novel cavity structure provides a unique, yet general, platform to study plasmonic/photonic, hybrid/plasmonic, and plasmonic/plasmonic coupling.

Generation of hyper-parametric oscillations in silica microbubbles

Cavity resonant enhanced stimulated Raman scattering (SRS), four-wave mixing, and broadband hyper-parametric oscillation in silica microbubble whispering gallery mode resonators (WGMR) in forward and backward directions are reported in this Letter. We show that microbubbles can operate not only in a highly ideal two-photon emission regime, but also generate combs, both natively and multi-mode spaced. The nonlinear process is phase matched because of the interaction of different mode families of the resonator.

2-D optical/opto-mechanical microfluidic sensing with micro-bubble resonators

In this paper a new sensing scheme by simultaneously measuring optical refractive index change and sound speed change in an optofluidic thin wall micro-bubble resonator is reported. Sensitivity of sound speed is 4.2-6.8 MHz/ (km/s) for 3 types of mechanical modes. A 2-D optical/opto-mechanical sensing map is plotted by detecting both the whispering gallery mode resonance shift and the optomechanical resonance shift. This novel scheme provides a supplementary support to optical sensing when analytes do not respond to refractive index (RI) change.

Confocal reflectance microscopy for determination of microbubble resonator thickness

Optical Micro Bubble Resonators (OMBR) are emerging as new type of sensors characterized by high Q-factor and embedded micro-fluidic. Sensitivity is related to cavity field penetration and, therefore, to the resonator thickness. At the state of the art, methods for OMBR's wall thickness evaluation rely only on a theoretical approach. The purpose of this study is to create a non-destructive method for measuring the shell thickness of a microbubble using reflectance confocal microscopy. The method was validated through measurements on etched capillaries with different thickness and finally it was applied on microbubble resonators.

Material candidates for optical frequency comb generation in microspheres

This paper evaluates the opportunities for using materials other than silica for optical frequency comb generation in whispering gallery mode microsphere resonators. Different materials are shown to satisfy the requirement of dispersion compensation in interesting spectral regions such as the visible or mid-infrared and for smaller microspheres. This paper also analyses the prospects of comb generation in microspheres within aqueous solution for potential use in applications such as biosensing. It is predicted that to achieve comb generation with microspheres in aqueous solution the visible low-loss wavelength window of water needs to be exploited. This is because efficient comb generation necessitates ultra-high Q-factors, which are only possible for cavities with low absorption of the evanescent field outside the cavity. This paper explores the figure of merit for nonlinear interaction efficiency and the potential for dispersion compensation at unique wavelengths for a host of microsphere materials and dimensions and in different surroundings.

Quasi-droplet microbubbles for high resolution sensing applications

Optical properties and sensing capabilities of fused silica microbubbles were studied numerically using a finite element method. Mode characteristics, such as quality factor (Q) and effective refractive index, were determined for different bubble diameters and shell thicknesses. For sensing applications with whispering gallery modes (WGMs), thinner shells yield improved sensitivity. However, the Q-factor decreases with reduced thickness and this limits the final resolution. Three types of sensing applications with microbubbles, based on their optimized geometrical parameters, were studied. Herein the so-called quasi-droplet regime is defined and discussed. It is shown that best resolution can be achieved when microbubbles act as quasi-droplets, even for water-filled cavities at the telecommunications C-band.