Whispering Gallery Mode Resonators Research Articles

The Impact of the Gain Medium Properties and the Resonator Morphology on the Whispering Gallery Mode Spectrum of Polystyrene Microspheres Coated with AgInS2/ZnS Quantum Dots

The modulation of fluorophore emission by whispering gallery modes (WGMs) in active spherical resonators holds great potential for advanced photonic devices. In the present work, we fabricate and systematically study a series of active WGM resonators based on polystyrene microspheres and heavy metal-free ternary AgInS2/ZnS quantum dots with broad and tunable photoluminescence bands coated on their surface as a gain medium. The variation in the mean size and emission wavelength of the quantum dots allowed us to establish a correlation between their characteristics and the WGM properties, including the resonance peaks intensity profile and the Q-factors. Other aspects such as coupling efficiency, polarization-dependent mode damping, and peak broadening are also discussed. Furthermore, we applied statistical analysis on multiple individual resonators within the batch to calculate the standard deviation of the resonator size and ellipticity. Our study highlights that both the cavity and the gain medium influence the WGM parameters in active spherical resonators.

Coupling package and WGM resonator filled with Terfenol-D for magnetic field sensing and tuning

In this work, we propose a whispering gallery mode (WGM) resonator filled with Terfenol-D (WGMRFT), which can be applied to magnetic field sensing and tuning. We used a hollow quartz rod to fabricate the resonator, and then filled it with Terfenol-D to create a magnetic field-sensitive device. The WGM of the resonator were excited using a tapered fiber evanescent field. As the external magnetic field increased, the WGM resonant spectrum of the resonator shifted toward the short wavelength direction. The sensitivity of the resonator is -3.05pm/mT. The structure has many advantages, such as simple structure, low cost and easy operation. In addition, we have demonstrated a resonator coupling and packaging device, using this device to adjust the coupling state of the resonator within a certain range. The package demonstrated good repeatability and reversibility in magnetic field measurement experiments. This lays the foundation for the application of millimeter-scale large WGM resonators.

Highly Sensitive Force Sensor Based on High-Q Asymmetric V-Shaped CaF2 Resonator.

Whispering gallery mode (WGM) resonators have high-quality factors and can be used in high-sensitivity sensors due to the narrow line width that allows for the detection of small external changes. In this paper, a force-sensing system based on a high-Q asymmetric V-shaped CaF2 resonator is proposed. Based on the dispersion coupling mechanism, the deformation of the resonator is achieved by loading force, and the resonant frequency is changed to determine the measurement. By adjusting the structural parameters of the asymmetric V-shaped resonator, the deformation of the resonator under force loading is improved. The experimental results show that the sensitivity of the V-shaped tip is 18.84 V/N, which determines the force-sensing resolution of 8.49 μN. This work provides a solution for force-sensing measurements based on a WGM resonator.

Carbon Dot-Decorated Polystyrene Microspheres for Whispering-Gallery Mode Biosensing

Whispering gallery mode (WGM) resonators doped with fluorescent materials find impressive applications in biological sensing. They do not require special conditions for the excitation of WGM inside that provide the basis for in vivo sensing. Currently, the problem of materials for in vivo WGM sensors are substantial since their fluorescence should have stable optical properties as well as they should be biocompatible. To address this we present WGM microresonators of 5–7 μm, where the dopant is made of carbon quantum dots (CDs). CDs are biocompatible since they are produced from carbon and demonstrate bright optical emission, which shows different bands depending on the excitation wavelength. The WGM sensors developed here were tested as label-free biosensors by detecting bovine serum albumin molecules. The results showed WGM frequency shifting, with the limit of detection down to 10−16 M level.

A Wearable Thin-Film Hydrogel Laser for Functional Sensing on Skin.

Flexible photonics offers the possibility of realizing wearable sensors by bridging the advantages of flexible materials and photonic sensing elements. Recently, optical resonators have emerged as a tool to improve their oversensitivity by integrating with flexible photonic sensors. However, direct monitoring of multiple psychological information on human skin remains challenging due to the subtle biological signals and complex tissue interface. To tackle the current challenges, here, we developed a functional thin film laser formed by encapsulating liquid crystal droplet lasers in a flexible hydrogel for monitoring metabolites in human sweat (lactate, glucose, and urea). The three-dimensional cross-linked hydrophilic polymer serves as the adhesive layer to allow small molecules to penetrate from human tissue to generate strong light--matter interactions on the interface of whispering gallery modes resonators. Both the hydrogel and cholesteric liquid crystal microdroplets were modified specifically to achieve high sensitivity and selectivity. As a proof of concept, wavelength-multiplexed sensing and a prototype were demonstrated on human skin to detect human metabolites from perspiration. These results present a significant advance in the fabrication and potential guidance for wearable and functional microlasers in healthcare.

Phosphate Glass Microspheres with Silver Nanoparticles for Whispering Gallery Mode Resonators.

Glass microspheres have gained significant attention over the years in the field of photonics due to their application in whispering gallery mode (WGM) microresonator platforms. However, the synthesis of glass spheres in the micro regime remains challenging, while it relies mostly on complicated synthetic methods or sol-gel chemistry. Herein, we demonstrate the controlled formation of phosphate glass microspheres by means of a simple, fast, low-temperature, post-glass melting thermal treatment of previously quenched glass. Moreover, we report on the simultaneous formation of silver nanoparticles (AgNPs) on the surface of glass spheres upon the same treatment. The formation of metal nanoparticles onto the glass spheres induces attractive optical and plasmonic properties, believed to be suitable for WGM resonator-based applications, as well as a wide range of optoelectronic, photonic, and sensing applications.

Packaging of WGM resonator coupled with tapered fiber for various application scenarios

In this study, we proposed a packaging that couples light from a tapered fiber to a whispering gallery mode resonator. The packaging is suitable for a wide range of resonators, such as quartz rods and crystal microdisks. The device uses a self-designed brass base with a special flexible structure to realize the three-dimensional precision adjustment of the relative position of the resonator and the tapered fiber. This structural is different from the packaging proposed in previous studies. It can adjust the coupling state of the resonator by adjusting the relative position between the resonator and the tapered fiber. We demonstrated the coupling process using a crystal resonator as an example and demonstrated three typical coupling states of the WGM system. The packaging also has the advantages of high stability, small size, low cost, and high portability. In addition, we tested the long-term robustness of the packaging and verified the application of the packaging for magnetic field sensing and temperature tuning.

Multiple Fano resonances by MIM waveguide coupling whispering gallery mode resonator and application in Refractive index sensing

In this study, a baffled metal–insulator–metal (MIM) waveguide coupled with a whispering gallery mode resonator (WGMR) is proposed. This structure could excite quadruple Fano resonances. The asymmetric Fano resonance transmittance spectrum and the electric field at the resonance peak were numerically simulated using the finite difference time domain method. The obtained data were fitted using the multimode interference coupled mode theory. The number of Fano resonance peaks was tuned by the outer radius of the WGMR. With other geometric parameters unchanged, the number of Fano resonance peaks increased with increasing outer radius of the WGMR. Thus, it achieved multiple Fano resonances by adjusting solely the radius of the WGMR. When the ring width was fixed, the structure could excite multiple Fano resonances within a certain outer radius range. This structure was used for refractive index sensing and its sensitivity and figure of merit (FOM) were 3004 nm/RIU and 3.14 × 104 in a gas environment, respectively. Therefore, the proposed structure can excite multiple Fano resonances and achieve high sensitivity and FOM, providing a theoretical basis for micro and nano applications.

Observing Variation in Whispering Gallery Mode Resonance of a Trapped and Levitated Dye Doped Microdrop.

Whispering gallery mode (WGM) resonance was created in a spherical micro drop. A gradual shift in the resonance were observed. For a 600 nm droplet radius, the blue shift were 1.5, 0.7, 3.7 nm. It was estimated that such a shift corresponds to a reduction in optical radius of the droplet by 1.3, 0.6, 3.3 nm respectively. The droplet was created from a solution of glycerol, methanol and rhodamine 6G dye, and was trapped and levitated in a modified Paul trap. The WGMs were created by optically exciting the dye material from an external 532 nm cw laser beam. A shift in the WGM was observed during a gradual increase in power of the excitation laser, and a reason for such a shift was thought to be thermal evaporation of the liquid. For a larger droplet an initial 0.1 nm thermal expansion was also estimated, preceding the volume contraction. Such an expansion was negligible for a smaller droplet. The rate of change of the blue shift depends upon initial radius of the droplet. For the smaller droplet the estimated rate of change of WGM with a change in optical radius, was 0.771. For larger droplet, this rate is lower.

Bioinspired Carbonized Polymer Microspheres for Full-Color Whispering Gallery Mode Emission for White Light Emission, Unclonable Anticounterfeiting, and Chemical Sensing Applications.

Light-element-based fluorescent materials, colloidal graphene quantum dots, and carbon dots (CDs) have sparked an immense amount of scientific interest in the past decade. However, a significant challenge in practical applications has emerged concerning the development of solid-state fluorescence (SSF) materials. This study addresses this knowledge gap by exploring the unexplored photonic facets of C-based solid-state microphotonic emitters. The proposed synthesis approach focuses on carbonized polymer microspheres (CPMs) instead of conventional nanodots. These microspheres exhibit remarkable SSF spanning the entire visible spectrum from blue to red. The highly spherical shape of CPMs imparts built-in photonic properties in addition to its intrinsic CD-based attributes. Leveraging their excitation-dependent photoluminescence property, these microspheres exhibit amplified spontaneous emission, assisted by the whispering gallery mode resonance across the visible spectral region. Remarkably, unlike conventional semiconductor quantum dots or dye-doped microresonators, this single microstructure showcases adaptable resonant emission without structural/chemical modifications. This distinctive attribute enables a plethora of applications, including microcavity-assisted energy transfer for white light emission, highly sensitive chemical sensing, and secure encrypted anticounterfeiting measures. This interdisciplinary approach, integrating photonics and chemistry, provides a robust solution for light-element-based SSF with inherent photonic functionality and wide-ranging applications.

In-Fiber Liquid Microsphere Whispering Gallery Mode Resonator Fabricated by Femtosecond Laser

In-Fiber Liquid Microsphere Whispering Gallery Mode Resonator Fabricated by Femtosecond Laser

Compact high-Q Ka-band sapphire distributed Bragg resonator

In a class of high quality (Q-) factor dielectric resonators with low radiative losses, including popular whispering-gallery mode (WGM) resonators with high azimuthal mode numbers, due to high confinement of modal field in dielectric, the Q-factor is limited by the value of inverse dielectric loss tangent of dielectric material. Metal enclosures necessary for device integration only marginally affect the Q-factor while eliminating the residual radiative loss and allowing the optimization of input and output coupling. While very high Q-factors ∼200000 are available in sapphire WGM resonators in X-band, at millimeter wave frequencies increasing dielectric loss limits the Q-factor to much smaller values, e.g. ∼50000 and ∼25000 for quasi-TE and quasi-TM modes, correspondingly, at 36 GHz. The use of distributed Bragg reflection (DBR) principle allows to push modal energy outside dielectric while also isolating it from Joule losses in metallic enclosure walls. Very high Q∼600000>tgδ has been demonstrated in X-band [C. A. Flory and R. C. Taber, IEEE Trans. Ultrason., Ferroelectr., Freq. Control 44, 486–495 (1997).] at the expense of impractically large dimensions. In this work, we report on the assembly and testing of a compact Ka-band sapphire distributed Bragg reflector cavity characterized with Q-factor seven times larger than one predicted by the material’s dielectric loss at the frequency of interest. An intrinsic Q-factor of ∼200000 is demonstrated at 36 GHz for the lowest order TM-mode of a sapphire DBR. The resonator has 50 cm3 volume, smaller than previously demonstrated DBRs.

Underwater Low-Frequency Acoustic Wave Detection Based on a High-Q CaF2 Resonator

Whispering gallery mode (WGM) resonators with an ultra-high quality (Q) factor provide a new idea for high-precision underwater acoustic sensing. However, acoustic energy loss due to watertight encapsulation has become an urgent problem for its underwater application. In order to solve this problem, this paper proposes a hollowed-out array structure. The finite element simulation shows that the acoustic wave transmission loss is improved by 30 dB compared with that of the flat plate encapsulation structure. Using a calcium fluoride (CaF2) resonator with a Q factor of 1.2 × 108 as an acoustic sensitive unit, the amplitude and frequency of the loaded acoustic wave are retrieved by means of the dispersion coupling response mechanism. The resonator’s underwater experimental test range is 100 Hz–1 kHz, its acoustic sensing sensitivity level reaches −176.3 dB re 1 V/µPa @ 300 Hz, and its minimum detectable pressure can be up to 0.87 mPa/Hz1/2, which corresponds to a noise-equivalent pressure (NEP) of up to 58 dB re 1 µPa/Hz1/2.

A WGM-FP hybrid microcavity for simultaneous measurement of NaCl concentration and temperature

Whispering gallery mode (WGM) resonator is sensitive to the external medium change due to its strong light-matter interaction. However, the temperature sensitivity of silica-based WGM resonators would cause crosstalk, which is a fatal shortcoming for those high-quality (Q) resonators. In this paper, a hybrid microcavity including WGM resonance and Fabry–Pérot (FP) interference is proposed for simultaneous measurement of NaCl concentration and temperature, using the same microcavity in the same system to obtain two types of optical signals. The experimental results indicate that the maximum measurement error of NaCl concentration and temperature without compensation is about ± 0.026 % and ± 1.1 °C, while they would be ± 0.012 % and ± 0.3 °C after compensation. Sharing one microcavity enables temperature compensation to achieve temperature synchronization, and sharing of one system ensures the simplicity and stability of the sensing platform.

On‐Chip Multimode WGM Microresonator with Cross‐Correlation Algorithm for Enhanced Sensing

AbstractOptical whispering‐gallery‐mode (WGM) microresonators emerge as a frontrunner for precision measurement due to their high sensitivity and compact footprint. They have demonstrated superior capabilities in detecting a wide range of objects, spanning from variations in temperature, humidity, and magnetic field, to local perturbations caused by viruses or nanoparticles. Traditional methods often rely on monitoring spectral changes in a single WGM resonance to extract the relevant information of sensing targets. However, in scenarios involving multiple sensing targets of interest (e.g., molecule detection in a temperature‐fluctuating environment), analyzing only one resonant mode may be insufficient, leading to inaccuracies or misinterpretations in deriving target information. To address this issue, a multimode‐based cross‐correlation algorithm is proposed to distinguish local perturbations from global environmental fluctuations. For validation, on‐chip silicon microresonators and pulley bus waveguides are integrated for robust multimode excitation. A proof‐of‐concept experiment demonstrates the effectiveness of this method in detecting nanoparticles in an environment with temperature changes. This approach showcases versatility and applicability in various optical sensors, introducing new prospects for achieving high‐sensitivity sensing through scalable integrated photonic platforms.

Microfluidic flowmeter based on a liquid crystal-filled nested capillary

Microfluidic flowmeters are a powerful and highly accurate tool, enabling precise monitoring and measurements of flows of gases and fluids in a range of applications. Here we proposed and experimentally demonstrated a whispering gallery modes flowmeter composed of a liquid crystal-filled nested capillary. Whispering gallery modes are excited by a tapered fiber coupled perpendicularly to the nested capillary. The air flowing through the capillary cools it down, which leads to a temperature-induced change of the refractive index of the nematic liquid crystals. This change in turn leads to a spectral shift of the whispering gallery modes resonances, which can be linked to the airflow rate in the capillary. The temperature change in the liquid crystals was simulated considering the heat transfer between the liquid crystals and airflow in the capillary, which indicated that the liquid crystals temperature decreases in a nonlinear manner with the increase of the airflow rate. A flowmeter with the maximum sensitivity of 0.3423 nm·min·mL−1 in the flowrate range from 0 to 2.52 nm·min·mL−1 and a resolution of 5.72 pm was demonstrated in our experiment. The proposed sensor provides a platform for whispering gallery modes flowmeters and offers the advantages of good stability, high sensitivity, and miniature size.

Perfect chirality constructed by position-dependent backscattering in a whispering gallery mode microresonator

Unidirectional propagation of photons originated from perfect chirality meets the critical requirement for building a high-performance quantum network. However, it not only requires that the circular dipole emitter is precisely located at points of circularly polarized electric fields, which leads to non-reciprocal interactions for photons with opposite propagation directions, but also the light-emitter interaction strength should be strong enough to guarantee a π phase shift. Unfortunately, these perfect chirality points are scarce and accessible points with elliptically polarized fields result in non-ideal photon-emitter chiral interactions and emitters radiating photons bidirectionally. Meanwhile, reflection properties, phase shifts, and non-reciprocal interactions are sensitive to frequency detunings and dissipations. Here, without engineering the dipole and optimizing the distribution of the field, a scatter such as a nanotip placed at the evanescent field of a whispering gallery mode resonator (WGMR) is adopted to control the transporting properties of single photons under non-ideal chiral interactions. By properly adjusting the relative position between the nanotip and the atom or the overlap between the nanotip and the mode volume of the WGMR, amplitudes of reflected photons in different pathways are changed. Consequently, complete destructive interference appears and thus no photons are reflected. The corresponding phase shifts of π and non-reciprocal interactions are guaranteed simultaneously. Significantly, the perfect chirality reconstructed here is robust against frequency detunings and dissipations. Therefore, the atom-WGMR-nanotip structure can be regarded as a compound chiral atom with radiating photons in only one direction.

Theoretical analysis of the influence of crystalline orientation on Brillouin gain of whispering gallery mode resonators

Brillouin scattering, arising from the interaction between photons and acoustic phonons, is well-known for its narrow gain profile, imposing stringent requirements for Brillouin lasers. However, in crystalline whispering gallery mode (WGM) optical resonators, the circular phonon path leads to intriguing gain profiles for Brillouin scattering due to the strong dependence of the phase velocity of acoustic waves on the crystalline axes’ orientation. Here, we present a theoretical investigation of Brillouin gain profiles in crystalline WGMRs considering different tangential directions and angles. Various host crystals ranging from tetragonal, cubic, and trigonal crystal groups, including magnesium fluoride, calcium fluoride, barium fluoride, lithium fluoride, strontium fluoride, sapphire, and lithium niobate, are studied. Our findings reveal significant variations in Brillouin shift depending on the crystal and its orientation, with the frequency shift broadening ranging from MHz to GHz.

Real-Time Tracking of Carbon Dioxide Concentration Using an Optical Microsphere Resonator Sensor.

Whispering gallery mode resonator sensors are nondisruptive optical sensors that can detect and monitor perturbations in a gaseous environment. Through its resonant properties of peak wavelength, amplitude, and quality factor (Q factor), changes in concentration can be quantified within seconds and monitored over days with great stability. In addition, the small footprint, low cost, and high sensitivity are ideal properties for a disposable sensor that can be utilized in extreme environments. The large Q factor of the resonant cavity enables long interaction lengths and amplifies the effect of small changes in the background refractive index, which is detectable in picometer shifts of the resonance wavelength. However, this measurement is susceptible to changes in other environmental factors such as temperature, pressure, and humidity, which manifest on the picometer wavelength scale, reinforcing the need to decouple the variables. In this work, we compare the spectral response of different diameter resonators to carbon dioxide, nitrogen, and its mixtures, observing the spectral shifting and broadening of the cavity resonance near 1550 nm. In addition, the effect of environmental temperature on spectral shifting due to the thermo-optic effect is characterized and quantified. Lastly, the gas concentrations are changed in real time to showcase the tracking and recovery capabilities of the resonator sensor.

Temperature Sensitivity Control of an Inkjet-Printed Optical Resonator on Pillar.

We report a whispering gallery mode resonator on a pillar using inkjet printing combined with traditional microfabrication techniques. This approach enables several different polymers on the same chip for sensing applications. However, polymers inherently exhibit sensitivity to multiple stimuli. To mitigate temperature sensitivity, careful selection of design parameters is crucial. By precisely tuning the undercut-to-radius ratio of the resonator, a linear dependence in temperature sensitivity ranging from -41.5 pm/°C to 23.4 pm/°C, with a zero-crossing point at 47.6% is achieved. Consequently, it is feasible to fabricate sensing devices based on undercut microdroplets with minimal temperature sensitivity. The lowest measured temperature sensitivity obtained was 5.9 pm/°C, for a resonator with an undercut-to-radius ratio of 53%.