Microbubble Resonator Research Articles

A mechanically tuned Kerr comb in a dispersion-engineered silica microbubble resonator

This paper discusses the developement and investigation of a silica microbubble resonator (MBR) that is optimized to cancel mode dispersion with material dispersion, at a wavelength of approximately $1550\mathrm{\,nm}$and maintain a quality factor of an optical mode as large as $5.4\times10^{7}$.Benefitting from the near-zero dispersion and high quality factor,a primary optical comb is generated in the MBR using cascaded four-wavemixing processes, which span over $300\,\mathrm{nm}$ with severaltens of teeth. Furthermore, the frequency comb could be graduallytuned by mechanically stretching the MBR. This tunable Kerr comb has multiple potential applications in precision measurements and sensing applications, suchas molecular spectroscopy and ranging.

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.

Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer.

Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light–matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator’s interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

Resonant Microbubble as a Microfluidic Stage for All-Optical Photoacoustic Sensing

All-optical photoacoustic sensing is emerging as a sensitive technique for inspecting small volumes of fluids, for applications such as liquid biopsies, drug discovery, development of contrast agents for photoacoustic imaging, and analysis of gases. The authors implement a whispering-gallery-mode microbubble resonator as a multifunctional component for all-optical photoacoustic sensing. A laser pulse triggers photoacoustic activation of a sample in the cavity of the resonator, and the transient deformation of the cavity wall as a resonant oscillation is monitored. This configuration promises high sensitivity to optical absorbance, and removes the need for impedance-matched media.

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.

Optomechanics in anisotropic liquid crystal -filled micro-bubble resonators.

In this paper, we report on the realization of optomechanics and its broad tuning in liquid crystal-filled micro-bubble resonators (LC-MBR). Acoustic anisotropy of LC has enriched optomechanical resonant modes in such whispering-gallery modes (WGM) cavities. Meanwhile anisotropic dependence of sound speed of LC on temperature strongly enhances the tuning ability of the resonances. By applying magnetic field to control re-orientation of LC in MBR, optomechanical resonant frequency changes significantly after magnetic field goes beyond Fréedericksz transition threshold. By designing such an optomachanical system, optomechanical oscillator with broad tuning range can be realized.

Numerical Simulation of Mid-Infrared Optical Frequency Comb Generation in Chalcogenide As2S3 Microbubble Resonators

Mid-infrared optical frequency comb generation in whispering gallery mode microresonators attracts significant interest. Chalcogenide glass microresonators are good candidates for operating in the mid-infrared range. We present the first theoretical analysis of optical frequency comb generation in As2S3 microbubble resonators in the 3–4 μm range. The regime of dissipative soliton plus dispersive wave generation is simulated numerically in the frame of the Lugiato–Lefever equation. Using microbubble geometry allows controlling of the zero-dispersion wavelength and the obtaining of anomalous dispersion needed for soliton generation at the pump wavelength of 3.5 μm, whereas the zero-dispersion wavelength of the analyzed As2S3 glass is ~4.8 μm. It is shown that, for the optimized characteristics of microbubble resonators, optical frequency combs with a spectral width of more than 700 nm (at the level of −30 dB) can be obtained with the low pump power of 10 mW.

Ultra-sensitive biomolecular detection by external referencing optofluidic microbubble resonators

We propose an effective method for biomolecular detection based on an external referencing optofluidic microbubble resonator system (EROMBRS), which possesses good long-term stability and low noise. In this study, EROMBRSs were used for nonspecific detection of bovine serum albumin (BSA) molecules and specific detection of D-biotin molecules. Ultra-low practical detection limits of 1 fg/mL for nonspecific and specific biomolecular detection were achieved.

Guanidine derivative polymer coated microbubble resonator for high sensitivity detection of CO2 gas concentration.

A carbon dioxide (CO2) gas sensor based on a polyhexamethylene biguanide (PHMB)-coated whispering gallery mode (WGM) microbubble resonator is proposed and verified experimentally in this work. Microbubbles were fabricated using two reverse arc discharges focused on microcapillaries. The inner wall of the microbubble was coated with a layer of PHMB using a filling and sintering process. A significant WGM resonance was observed by coupling with a tapered fiber. The experimental results show that as the concentration of carbon dioxide increases, a blue shift appears in the spectrum. Addition, a high sensitivity (0.46 pm /ppm) and a good linear relationship were obtained in the measurement range of 200-700 ppm with a detection limit of 50 ppm. The sensor features include high sensitivity, simple structure, easy manufacture, and low cost.

Highly Sensitive Label-Free Detection of Small Molecules with an Optofluidic Microbubble Resonator.

The detection of small molecules has increasingly attracted the attention of researchers because of its important physiological function. In this manuscript, we propose a novel optical sensor which uses an optofluidic microbubble resonator (OFMBR) for the highly sensitive detection of small molecules. This paper demonstrates the binding of the small molecule biotin to surface-immobilized streptavidin with a detection limit reduced to 0.41 pM. Furthermore, binding specificity of four additional small molecules to surface-immobilized streptavidin is shown. A label-free OFMBR-based optical sensor has great potential in small molecule detection and drug screening because of its high sensitivity, low detection limit, and minimal sample consumption.

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.

Stabilizing and Tuning the Laser Frequencies in Er-Doped Fiber Ring Lasers Based on Microbubble Resonators

The laser frequency of an Er-doped fiber ring cavity is stabilized using the microbubble resonator (MBR) as the feedback element. The linewidth of the laser is nearly 86 MHz at ∼1560 nm measured by the Fabry–Perot interferometer. Since the thermooptic effect changes the reflectivity of the modes excited inside the MBR, the transition from the dual-wavelength operation to a stable single-wavelength output is found as increasing the pump power. The outstanding advantage of such laser is the microfluidic channel of the MBR which provides an excellent platform for the light to interact with the liquids. Thus, the laser wavelengths could be tuned by injecting different refractive index (RI) liquids into the microbubble. A coupled-cavities system is setup through inserting the MBR into the fiber cavity directly, of which the laser spectrum is very sensitive to the light polarization. Moreover, the laser wavelength could be tuned in a wide wavelength range if the thin-wall MBR is adopted. It provides a way to realize different lasing statues by controlling the light polarization or the RI of liquids inside the MBR.

Determining the geometric parameters of microbubble resonators from their spectra

A method for determining the diameters and shell thicknesses of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diameter, and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extracting the diameter and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in experiments. The model is tested for a small-diameter silica glass microbubble, with the free spectral range being simulated for a range of diameters and shell thicknesses. The numerically simulated spectra are then compared to the experimentally measured spectrum. The ability to determine the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diameters and thin shell thicknesses.

Determining the geometric parameters of microbubble resonators from their spectra

A method for determining the diameters and shell thicknesses of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diameter, and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extracting the diameter and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in experiments. The model is tested for a small-diameter silica glass microbubble, with the free spectral range being simulated for a range of diameters and shell thicknesses. The numerically simulated spectra are then compared to the experimentally measured spectrum. The ability to determine the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diameters and thin shell thicknesses.

Optical Microbubble Resonators with High Refractive Index Inner Coating for Bio-Sensing Applications: An Analytical Approach.

The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an analytical method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse electric (TE) and transverse magnetic (TM) polarization. The obtained results do provide physical insights for the development of OMBR-based biosensor.

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 1.4μm, the zero dispersion wavelength shifts to values shorter than 764nm, making phase matching possible around 765nm. With the optical Q-factor of the MBR modes being greater than 107, four-wave mixing is observed at 765nm for a pump power of 3mW. By increasing the pump power, parametric oscillation is achieved, and a frequency comb with 14 comb lines is generated at visible wavelengths.

Fluorescence biosensing in selectively photo–activated microbubble resonators

A procedure for a spatially selective immobilization of antibodies, via photochemical activation, on the inner surface of optical microbubble resonators (OMBRs) is reported. The method is based on a suitable UV mask process. As proof of concept of this technique, immunoassay in photo-activated OMBR was implemented and fluorescence investigation was conducted. Moreover, the possible application of this procedure towards the development of a biochemical multiplexed detection system based on OMBR is presented. Finally, the high Q factor values measured after the immunoassay event (>105) make these photo–activated OMBRs suitable devices for future label-free biosensor implementation.

Extinction ratio and resonant wavelength tuning using three dimensions of silica microresonators

In this paper, a multidimensional tuning method of the silica microcapillary resonator (MCR) is proposed and demonstrated whereby the extinction ratio (ER) as well as the resonant wavelength can be individually controlled. An ER tuning range of up to 17 dB and a maximum tuning sensitivity of 0.3 dB/μm are realized due to the tapered profile of the silica optical microfiber (MF) when the MF is adjusted along its axial direction. Compared to direct tuning of the coupling gap, this method could lower the requirement for the resolution of displacement stage to micrometers. When the MF is adjusted along the axial direction of the silica microcapillary, a resonance shift of 3.06 nm and maximum tuning sensitivity of 0.01 nm/μm are achieved. This method avoids the use of an applied external field to control the silica microresonators. Moreover, when air is replaced by ethanol and water in the core of the silica microcapillary, a maximum resonance shift of 5.22 nm is also achieved to further enlarge the resonance tuning range. Finally, a microbubble resonator with a higher Q factor is also fabricated to achieve an ER tuning range of 8.5 dB. Our method fully takes advantage of the unique structure of the MCR to separately and easily tune its key parameters, and may broaden its applications in optical signal processing and sensing.

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.