Whispering Gallery Mode Spectra Research Articles

Direct response of liquid crystal to pH changes for wide range pH sensing via the whispering gallery mode.

pH sensing is essential in various fields including healthcare, environmental monitoring, food safety, and agriculture. In this Letter, we propose a method for a wide-range pH measurement based on the direct response of liquid crystal (LC) to pH variations via the whispering gallery mode (WGM). The liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) is selected for its excellent optoelectronic properties and the birefringence of 5CB microdroplets serving as spherical resonators. This sensor achieves wide-range pH sensing without any intermediaries. Polarized optical microscopy (POM) and WGM provide a qualitative morphological analysis and accurate quantitative spectral measurements. Under alkaline conditions (pH 7.6 to 13.3), POM images of 5CB microdroplets show a complete transition from bipolar to radial structures, with an average sensitivity of 1.78 nm/pH of redshift in WGM spectra. In acidic conditions (pH 6.65 to 1.5), structural changes observed by POM are minor, with a sensitivity of 1.06 nm/pH of blueshift in WGM spectra. This difference is attributed to the varying refractive indices of HCl and NaOH solutions. This method offers a reference for theoretical studies on pH and LC interactions and holds promise for the development of LC-based pH sensors.

Optical whispering gallery mode resonators: analysing thermo-optic tuning in a silicon sphere

ABSTRACT In this work, we discuss and experimentally investigate the whispering gallery modes (WGMs) in a 500 μm radius silicon sphere. We begin by reviewing the basics of WGM resonators, followed by simulations and experimental results obtained with the aforementioned silicon spherical WGM resonator. The recorded WGM signatures in the transmission and scattering spectra excited, in the near-infrared (near-IR) region, agree well with the simulations. Thermo-optic tuning of these WGMs is achieved by introducing a pump laser in the violet-blue region. Red-shifts in wavelength are observed, which increase with the increase in the pump power. A shift of 0.63 nm is observed at a pump power of 6.32 mW. We also study switching by analysing the transient response of the WGM spectra. The measured WGMs exhibit high quality factors ( Q ∼ 10 5 ), which along with the aforesaid tunability, demonstrates the potential of spherical silicon resonator as a platform for photonic applications, e.g. sensing and communication. In summary, this work contributes towards the understanding of the fundamental physics of WGMs and provides insights into silicon WGM resonators.

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.

Highly sensitive and label-free detection of biotin using a liquid crystal-based optofluidic biosensor.

A liquid crystal (LC)-based optofluidic whispering gallery mode (WGM) resonator has been applied as a biosensor to detect biotin. Immobilized streptavidin (SA) act as protein molecules and specifically bind to biotin through strong non-covalent interaction, which can interfere with the orientation of LCs by decreasing the vertical anchoring force of the alignment layer in which the WGM spectral wavelength shift is monitored as a sensing parameter. Due to the double magnification of the LC molecular orientation transition and the resonance of the WGM, the detection limit for SA can reach 1.25 fM (4.7 × 10-13 g/ml). The measurable concentration of biotin and the wavelength shift of the WGM spectrum have an excellent linearity in the range of 0 to 0.1 pg/ml, which can achieve ultra-low detection limit (0.4 fM), i.e., seven orders of magnitude improvement over conventional polarized optical microscope (POM) method. The proposed optofluidic biosensor is highly reproducible and can be used as an ultrasensitive real-time monitoring biosensor, which will open the door for applications to other receptor and ligand models.

Laser structure based on an erbium-doped fiber ring and a whispering gallery mode microbottle resonator.

A novel fiber laser structure, to the best of our knowledge, based on an erbium-doped fiber ring (EDFR) and a silica microbottle resonator (MBR) is proposed and investigated experimentally. Two fiber laser samples based on MBRs with different geometries and diameters of 200 and 150µm are fabricated, and their performance is studied experimentally. Periodic whispering gallery mode spectra of the MBRs are dependent on the position of the fiber taper used for coupling of light into the MBR, and this dependence is explored to achieve lasing at different wavelengths by moving the light coupling point along the axis of the microbottle incorporated into the proposed EDFR-MBR system. The influences of the pump laser power and light polarization on the system performance and laser stability are also investigated. Single-mode lasing with a maximum optical signal-to-noise ratio of 32dB is demonstrated.

Whispering gallery mode resonant microspheres as labels for biological imaging.

Optical whispering gallery modes (WGMs) arise from total internal reflection of light from an optical cavity, such as microspheres. Whispering gallery mode phenomena produce resonant frequencies at the reflected wavelengths that appear as peaks in spectrographic data. In microspheres, whispering gallery mode spectra vary by the size and material used. These differences allow for distinct labeling of many targets using only a single fluorescent input and output. In this work we use resonant microspheres that, conjugated to antibodies, label bacterial and mammalian cell populations in vitro. We use 5 µm resonant microspheres of polystyrene and polystyrene-divinylbenzene conjugated with antibodies against biological targets of interest including Epidermal Growth Factor Receptor (EGFR) and Erb-B2 Receptor Tyrosine Kinase 2 (ErbB2) to label human cells and lipopolysaccharide to detect Gram-negative bacterial infection. Beads are distinguished using their whispering gallery mode spectra. This work presents an early step towards labeling and biodetection of many biological targets using a single fluorescent input and output.

Indirect bandgap MoSe2 resonators for light-emitting nanophotonics.

Transition metal dichalcogenides (TMDs) are promising for new generation nanophotonics due to their unique optical properties. However, in contrast to direct bandgap TMD monolayers, bulk samples have an indirect bandgap that restricts their application as light emitters. On the other hand, the high refractive index of these materials allows for effective light trapping and the creation of high-Q resonators. In this work, a method for the nanofabrication of microcavities from indirect TMD multilayer flakes, which makes it possible to achieve pronounced resonant photoluminescence enhancement due to the cavity modes, is proposed. Whispering gallery mode (WGM) resonators are fabricated from bulk indirect MoSe2 using resistless scanning probe lithography. A micro-photoluminescence (μ-PL) investigation revealed the WGM spectra of the resonators with an enhancement factor up to 100. The characteristic features of WGMs are clearly seen from the scattering experiments which are in agreement with the results of numerical simulations. It is shown that the PL spectra in the fabricated microcavities are contributed by two mechanisms demonstrating different temperature dependences. The indirect PL, which is quenched with the temperature decrease, and the direct PL which almost does not depend on the temperature. The results of the work show that the suggested approach has great prospects in nanophotonics.

Prefab Hollow Glass Microsphere-Based Immunosensor with Liquid Crystal Sensitization for Acute Myocardial Infarction Biomarker Detection.

Quantitative detection of cardiac troponin biomarkers in blood is an important method for clinical diagnosis of acute myocardial infarction (AMI). In this work, a whispering gallery mode (WGM) microcavity immunosensor based on a prefab hollow glass microsphere (HGMS) with liquid crystal (LC) sensitization was proposed and experimentally demonstrated for label-free cardiac troponin I-C (cTnI-C) complex detection. The proposed fiber-optic immunosensor has a simple structure; the tiny modified HGMS serves as the key sensing element and the microsample reservoir simultaneously. A sensitive LC layer with cTnI-C recognition ability was deposited on the inner wall of the HGMS microcavity. The arrangement of LC molecules is affected by the cTnI-C antigen—antibody binding in the HGMS, and the small change of the surface refractive index caused by the binding can be amplified owing to the birefringence property of LC. Using the annular waveguide of the HGMS, the WGMs were easily excited by the coupling scanning laser with a microfiber, and an all-fiber cTnI-C immunosensor can be achieved by measuring the resonant wavelength shift of the WGM spectrum. Moreover, the dynamic processes of the cTnI-C antigen—antibody binding and unbinding was revealed by monitoring the wavelength shift continuously. The proposed immunosensor with a spherical microcavity can be a cost-effective tool for AMI diagnosis.

Magnetic-Based Polydimethylsiloxane Microspheres for Magnetic Field Measurement

Magnetic-based polydimethylsiloxane microspheres (MPMs) are fabricated and its applications for measurement of magnetic field are investigated. The MPMs are prepared by first incorporating the uniformly sized Mn₃O₄ nanocrystals into polydimethylsiloxane (PDMS) matrix, and then using a self-assembling method to obtain a chain of microspheres on the fiber taper. The MPMs are regarded as optical resonators, demonstrating whispering gallery modes (WGM) with a high Q factor in the 1310 nm band. Experimental results show that the resonance wavelengths of the WGM spectra depend on the strength of the magnetic field since the change in the applied filed cause the mechanical deformation of the MPMs, leading to a shift in the resonant wavelength of the resonator. Using as magnetic field sensors, the performances of the MPMs in terms of sensitivities and measurement ranges have been optimized. Thus, the sensitivity of 101.9 pm/mT and the measurement range up to 8 mT are achieved with a MPM (~270 μm in diameter). Besides, the performance of the MPM after two-week storage is analyzed, showing the excellent long-term stability.

Whispering gallery mode resonators in continuous flow: spectral assignments and sensing with monodisperse microspheres.

Whispering gallery mode resonator (WGMR) microspheres yield highly structured optical spectra that are extremely sensitive to their environment and are of intense interest for use in a variety of sensing applications. Many efforts to leverage the unique sensitivities of WGMRs have relied on stringent experimental requirements to correlate specific spectral shifts/changes to an analyte/stimulus such as (1) precise positional knowledge, (2) reference spectra for each microsphere, and (3) high mechanical stability. Consequently, these factors can hinder adequate mixing or incorporation of analytes and can create challenges for remote sensing. This work describes a continuous flow technique for measuring whispering gallery mode (WGM) spectra of dye-doped microspheres suspended in solution and an accompanying analysis scheme that can extract the local refractive index without a priori knowledge of the microsphere size and position and without a reference spectrum. This measurement technique and analysis scheme was shown to accurately measure the refractive index of a range of alcohol and saline solutions down to a few thousandths of a refractive index unit (RIU). Additionally, a spectral clustering algorithm was applied to the fit results of two batches of microspheres suspended in water and was able to accurately assign spectra back to either batch of microspheres.

Liquid crystal-amplified optofluidic biosensor for ultra-highly sensitive and stable protein assay

Protein assays show great importance in medical research and disease diagnoses. Liquid crystals (LCs), as a branch of sensitive materials, offer promising applicability in the field of biosensing. Herein, we developed an ultrasensitive biosensor for the detection of low-concentration protein molecules, employing LC-amplified optofluidic resonators. In this design, the orientation of LCs was disturbed by immobilized protein molecules through the reduction of the vertical anchoring force from the alignment layer. A biosensing platform based on the whispering-gallery mode (WGM) from the LC-amplified optofluidic resonator was developed and explored, in which the spectral wavelength shift was monitored as the sensing parameter. The microbubble structure provided a stable and reliable WGM resonator with a high Q factor for LCs. It is demonstrated that the wall thickness of the microbubble played a key role in enhancing the sensitivity of the LC-amplified WGM microcavity. It is also found that protein molecules coated on the internal surface of microbubble led to their interactions with laser beams and the orientation transition of LCs. Both effects amplified the target information and triggered a sensitive wavelength shift in WGM spectra. A detection limit of 1 fM for bovine serum albumin (BSA) was achieved to demonstrate the high-sensitivity of our sensing platform in protein assays. Compared to the detection using a conventional polarized optical microscope (POM), the sensitivity was improved by seven orders of magnitude. Furthermore, multiple types of proteins and specific biosensing were also investigated to verify the potential of LC-amplified optofluidic resonators in the biomolecular detection. Our studies indicate that LC-amplified optofluidic resonators offer a new solution for the ultrasensitive real-time biosensing and the characterization of biomolecular interactions.

Optical whispering-gallery mode barcodes for high-precision and wide-range temperature measurements

Temperature is one of the most fundamental physical properties to characterize various physical, chemical, and biological processes. Even a slight change in temperature could have an impact on the status or dynamics of a system. Thus, there is a great need for high-precision and large-dynamic-range temperature measurements. Conventional temperature sensors encounter difficulties in high-precision thermal sensing on the submicron scale. Recently, optical whispering-gallery mode (WGM) sensors have shown promise for many sensing applications, such as thermal sensing, magnetic detection, and biosensing. However, despite their superior sensitivity, the conventional sensing method for WGM resonators relies on tracking the changes in a single mode, which limits the dynamic range constrained by the laser source that has to be fine-tuned in a timely manner to follow the selected mode during the measurement. Moreover, we cannot derive the actual temperature from the spectrum directly but rather derive a relative temperature change. Here, we demonstrate an optical WGM barcode technique involving simultaneous monitoring of the patterns of multiple modes that can provide a direct temperature readout from the spectrum. The measurement relies on the patterns of multiple modes in the WGM spectrum instead of the changes of a particular mode. It can provide us with more information than the single-mode spectrum, such as the precise measurement of actual temperatures. Leveraging the high sensitivity of WGMs and eliminating the need to monitor particular modes, this work lays the foundation for developing a high-performance temperature sensor with not only superior sensitivity but also a broad dynamic range.

Photonic Microresonators Created by Slow Optical Cooking

Silica and water are known as exceptionally inert chemical materials whose interaction is not completely understood. Here we show that the effect of this interaction can be significantly enhanced by optical whispering gallery modes (WGMs) propagating in a silica microcapillary filled with water. Our experiments demonstrate that WGMs, which evanescently heat liquid water over several hours, induce permanent alterations in silica material characterized by the subnanometer variation of the WGM spectrum. We use the discovered effect to fabricate optical WGM microresonators having potential applications in optical signal processing and microfluidic sensing. Our results pave the way for the ultraprecise fabrication of resonant optical microdevices and the ultra-accurate characterization of physical and chemical processes at solid–liquid interfaces.

Highly stable in-fiber integrated silica microresonator

We demonstrate a compact in-fiber integrated microresonator based on a silica microsphere and microstructure fiber. Whispering-gallery-modes (WGMs) are effectively excited through evanescent field coupling between the silica microsphere and one of the input tube waveguides of negative curvature fiber. The WGM spectrum with the Q-factor of ∼2.56 × 103 and the extinction ratio of ∼15.2 dB is achieved. The transmission spectra evolution for 5 h reveals the long-term stability of WGM excitation. The experimental results indicate that the in-fiber integrated resonator has the advantages of high stability, high integration, robustness, alignment-free assembly structure, and low cost, motivating potential applications in the lab-in-fiber platform.

Chirality driven effects in multiphoton excited whispering gallery mode microresonators prepared by a self-assembly technique

Organic microresonators are highly attractive for numerous applications including biosensing, optical switching and metrology. Adding chiral materials to resonators can enhance their sensing properties due to the appearance of chiroptical effects. Here we study experimentally resonant multiphoton processes in spherical microresonators with the diameter of 2–12 m made of 1,1’-bi-2-naphthol derivatives by a self-assembly technique. Using nonlinear single-particle microscopy, we demonstrate that these microspheres provide excitation of whispering gallery modes (WGMs) in two- and three-photon excited luminescence with a Q-factor of up to 300. We also demonstrate that the nonlinear circular dichroism is enhanced due to the WGM excitation and reaches 7.7% and 11% in the two- and three-photon absorption, respectively. Pronounced modulation of the WGMs spectrum due to the mode’s splitting is observed in coupled binol-based microresonators, which opens a path for the control over their resonant response and makes them suitable for nonlinear optical sensing applications.

Fast Detection of Myricetin With the Use of Dedicated Microdroplets

A 4-cyano-4’-pentylbiphenyl(5CB) microdroplet-based sensing method for myricetin (MY) detection, which is produced by a syringe pump connected tapered capillary microtube and functionalized by dodecyltrimethylammonium bromide (DTAB) and deoxyribonucleic acid (DNA) at the aqueous/liquid crystal (LC) interface is proposed and demonstrated. As the concentration of MY increases, the 5CB microdroplet exhibits a structural transition from a bipolar configuration to a radial configuration. We also exploit the 5CB microdroplet to act as an optical microcavity of the whispering gallery mode (WGM) and obtain typical WGM lasing spectra under a pulsed laser pump. The WGM spectrum, which is related to the orientation of LC molecules at the aqueous/LC interface, presents a spectral blue shift with the increase of the MY addition. The sensitivity within the detection range is 0.04 nm/ $\mu \text{M}$ . The experimental results demonstrate the feasibility of this simple and small size system for MY detection.

Strain-induced tunable dual-bottle-shaped optical microresonator.

We present a novel dual-bottle microresonator with a high Q-factor exceeding 107. Such a resonator consists of two lateral resonators and one central resonator. The maximum diameter of the lateral resonator is ∼180µm, and the central diameter is 124.91 µm. Characteristics of the whispering gallery mode spectra are investigated and explained by the microbottle resonance, mode interference, and scattering effects. Strain-tuning sensitivity varies from 0.126 to 1.7 pm/µε when a tapered fiber is placed at the resonator's different positions. With a coupled area at the middle of the central resonator, the highest, to the best of our knowledge, strain sensitivity of 1.7 pm/µε, which is higher than previously reported solid microresonator strain sensors, is achieved.

Thermal and electrical tuning of whispering gallery modes in liquid crystal-filled hollow glass microsphere

This paper demonstrates a thermally and electrically tunable hollow glass microsphere resonant cavity filled with dye-doped nematic liquid crystal. We controlled the orientation of in-sphere liquid crystal and encapsulated it with polydimethylsiloxane film. The typical whispering gallery modes (WGMs) lasing spectra were obtained under a pulsed laser pump and the effects of the temperature and electric field on the peaks wavelengths were studied. The WGM spectra present a blueshift significantly with increasing temperature and electric field due to the decrease in the effective refractive index.

Full-wave theory for WGM lasing of fully anisotropic nanoparticles

A full-wave theory for the prediction of the complex whispering-gallery-mode (WGM) resonances of fully anisotropic nano/microresonators is presented. The theory is based on the volume integral equation (VIE) method, supported by appropriate perturbation theory which enables analytical closed-form expressions valid for extremely large indices of the involved spherical wavefunctions. These closed-form expressions allow for the instant computation of the eigenbasis required for the solution of the VIE. The theory is applied to the calculation of WGM spectra of uniaxial semiconductor crystals, to gyroelectric magnetic garnets, and to geometrical birefringence enhancement due to large Verdet constants. Combined with a state-of-the-art open-source algorithm for complex roots computation, this theory constitutes a rigorous tool for the interpretation of experimental data obtained from photonic and optomagnonic applications, including WGM lasing of fully anisotropic nanoparticles and magneto-optical coupling.

Mode-Splitting for Refractive Index Sensing in Fluorescent Whispering Gallery Mode Microspheres with Broken Symmetry.

Whispering gallery mode (WGM) resonators have become increasingly diverse in terms of both architecture and applications, especially as refractometric sensors, allowing for unprecedented levels of sensitivity. However, like every refractometric sensor, a single WGM resonator cannot distinguish temperature variations from changes in the refractive index of the surrounding environment. Here, we investigate how breaking the symmetry of an otherwise perfect fluorescent microsphere, by covering half of the resonator with a high-refractive-index (RI) glue, might enable discrimination of changes in temperature from variations in the surrounding refractive index. This novel approach takes advantage of the difference of optical pathway experienced by WGMs circulating in different equatorial planes of a single microsphere resonator, which induces mode-splitting. We investigated the influence of the surrounding RI of the microsphere on mode-splitting through an evaluation of the sphere’s WGM spectrum and quality factor (Q-factor). Our results reveal that the magnitude of the mode-splitting increases as the refractive index contrast between the high-refractive-index (RI) glue and the surrounding environment increases, and that when they are equal no mode-splitting can be seen. Investigating the refractive index sensitivity of the individual sub modes resulting from the mode-splitting unveils a new methodology for RI sensing, and enables discrimination between surrounding refractive index changes and temperature changes, although it comes at the cost of an overall reduced refractive index sensitivity.