Optical Whispering Gallery Mode Research Articles

High-sensitivity whispering-gallery mode sensor for simultaneous measurement of humidity and temperature.

A novel, to the best of our knowledge, optical fiber whispering-gallery mode (WGM) sensor for simultaneously measuring humidity and temperature is proposed and investigated. The proposed sensor is realized by a polyvinyl alcohol (PVA)-coated capillary tube coupling with an optical single-mode fiber (SMF), which is integrated with a fiber Bragg grating (FBG). The as-fabricated sensor can be used not only for relative humidity (RH) sensing but also for temperature detection. The achieved humidity and temperature sensitivities are -181.2 pm/%RH and 10.5 pm/°C, respectively. The easy fabrication and very high sensitivity of the proposed sensor are attractive features for high-precision and wide-range RH sensing applications.

Highly-selective and temperature-compensated toluene sensor based on MOF-actuated optical WGM microresonator

This paper reports an optical fiber toluene sensor based on whispering gallery mode (WGM) microbottle resonator. The sensor includes a single-mode fiber (SMF)- no-core fiber (NCF)- SMF offset structure, which can be used as not only a coupler to couple light into the microbottle but also a multimode interferometer. The microbottle is formed by self-assembly of polydimethylsiloxane (PDMS) polymer doped with metal-organic framework (MOF) under the action of surface tension and gravity. The high specific surface area, suitable pore size, nitrogen-containing functional groups of MOF-Fe-PD and nonpolar groups on PDMS impart the high sensitivity and specific response of the sensor to toluene. When toluene is captured by the MOF and PDMS, the refractive index and volume of the cavity change, which affects the mode shift of WGM. According to test results, WGM of the sensor shows a linear sensitivity of 12.44 pm/ppm with toluene concentration between 0 ppm and 337 ppm at room temperature, but it is affected by temperature crosstalk. While multimode interference dip is only sensitive to temperature, which can compensate for the mode shift of WGM caused by temperature change. Repeatability experiments show that the response and recovery time of the sensor are less than 70 s. The method provides a toluene sensor with high selectivity, stable performance and simple preparation, which has potential environmental benefits and good industrial application prospects.

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.

Enhanced chiral sensing by optical whispering gallery mode microresonator.

Chiral sensing is essential in biochemistry, biomedicine, and food industries. Traditional chiral sensing mainly focuses on enhancing the chiral near fields, where the quality factor of the resonator has largely been ignored. Here, we propose a whispering gallery mode (WGM) optical microresonator to enhance the chiral signal by exploiting its high quality factor. The enhancement factor in the WGM resonator is linearly dependent on the quality factor of the microresonator and exponentially increases with the decreasing thickness of the chiral analyte. Based on this, we achieve a chiral enhancement factor of more than two orders of magnitude, which features a background-free chiroptical signal. Our results open a route to attain highly sensitive chiral sensing based on the WGM optical resonator, which promotes the realization of chiral applications in integrated optics.

High-Q WGM microcavity-based optofluidic sensor technologies for biological analysis.

High-quality-factor (Q) optical microcavities have attracted extensive interest due to their unique ability to confine light for resonant circulation at the micrometer scale. Particular attention has been paid to optical whispering-gallery mode (WGM) microcavities to harness their strong light-matter interactions for biological applications. Remarkably, the combination of high-Q optical WGM microcavities with microfluidic technologies can achieve a synergistic effect in the development of high-sensitivity optofluidic sensors for many emerging biological analysis applications, such as the detection of proteins, nucleic acids, viruses, and exosomes. They can also be utilized to investigate the behavior of living cells in human organisms, which may provide new technical solutions for studies in cell biology and biophysics. In this paper, we briefly review recent progress in high-Q microcavity-based optofluidic sensor technologies and their applications in biological analysis.

Design and performance simulation of a silica microdisk cavity optical pressure sensor

The opto-mechanical system of optical whispering-gallery mode (WGM) microcavities confines resonant photons in micro-scale resonators for a long time, which can strongly enhance the interaction between light and matter, making it an ideal platform for various sensors. To measure the slim optical pressure in the interaction between the laser and matter, a silica microdisk cavity sensor with metal film is designed in this paper. In this study, the finite element method was employed to investigate the opto-mechanical coupling mechanism in a microdisk cavity. From the aspects of optics and mechanics, the structural parameters of the sensor were optimized and the performance was simulated. The simulation results show that at 1550 nm, the sensor’s optical quality factor (Q) can reach ∼104, the free spectral range is ∼5.3nm, the sensing sensitivity is 5.32mPa/Hz1/2, and the optical force resolution is 6.61×10−12N, which is better than the thin-film interferometry and optical lever method.

Engineering Dispersive-Dissipative Modal Coupling in Hybrid Optical Microresonators.

Hybrid microresonators have served an intriguing platform for fundamental research and applied photonics. Here, we study the plasmonics-engineered coupling between degenerate optical whispering gallery modes, which can be tuned in a complex space featuring the dissipative strong, dispersive strong, and weak coupling regimes. Experimentally, the engineering of a single plasmonic resonance to a cavity mode family is examined in a waveguide-integrated high-Q microdisk, from which the complex coupling coefficients are extracted and agree well with theoretical predictions. The coupling strength over 10GHz is achieved for both dissipative and dispersive interactions, showing a remarkable enhancement compared to that induced by a dielectric scatterer. Furthermore, the far fields of hybridized cavity modes are measured, revealing the coherent interference between the radiative channels. Our results shed light on the engineering of whispering gallery modes through plasmonic resonances, and provide fundamental guidance to practical microcavity devices.

Whispering-Gallery Quantum-Well Exciton Polaritons in an Indium Gallium Arsenide Microdisk Cavity.

Despite appealing high-symmetry properties that enable strong spatial confinement and ultrahigh-Q, optical whispering-gallery modes of spherical and circular resonators have been absent from the field of quantum-well exciton polaritons. Here we observe whispering-gallery exciton polaritons in a gallium arsenide microdisk cavity filled with indium gallium arsenide quantum wells, the test bed materials of polaritonics. Strong coupling is evidenced in photoluminescence and resonant spectroscopy accessed through concomitant confocal microscopy and near-field optical techniques. Excitonic and optical resonances are tuned by varying temperature and disk radius, revealing Rabi splittings between 5 and 10meV. A dedicated analytical quantum model for such circular whispering-gallery polaritons is developed, which reproduces the measured values. At high power, lasing is observed and accompanied by a blueshift of the emission consistent with the regime of polariton lasing. With experimental methods and theory now established, whispering-gallery-mode polaritons in round dielectric resonators appear as a new viable platform toward low loss polaritonics.

Anomalous optical whispering-gallery mode induced by rotational symmetry breaking

Anomalous optical whispering-gallery mode induced by rotational symmetry breaking

Hybrid quantum-classical polarizability model for single molecule biosensing.

Optical whispering gallery mode biosensors are able to detect single molecules through effects of their polarizability. We address the factors that affect the polarizability of amino acids, which are the building blocks of life, via electronic structure theory. Amino acids are detected in aqueous environments, where their polarizability is different compared to the gasphase due to solvent effects. Solvent effects include structural changes, protonation and the local field enhancement through the solvent (water). We analyse the impact of these effects and find that all contribute to an increased effective polarizability in the solvent. We also address the excess polarizability relative to the displaced water cavity and develop a hybrid quantum-classical model that is in good agreement with self-consistent calculations. We apply our model to calculate the excess polarizability of 20 proteinogenic amino acids and determine the minimum resolution required to distinguish the different molecules and their ionised conformers based on their polarizability.

Direct observation of split-mode exciton-polaritons in a single MoS2 nanotube.

A single nanotube synthesized from a transition metal dichalcogenide (TMDC) exhibits strong exciton resonances and, in addition, can support optical whispering gallery modes. This combination is promising for observing exciton-polaritons without an external cavity. However, traditional energy-momentum-resolved detection methods are unsuitable for this tiny object. Instead, we propose to use split optical modes in a twisted nanotube with the flattened cross-section, where a gradually decreasing gap between the opposite walls leads to a change in mode energy, similar to the effect of the barrier width on the eigenenergies in the double-well potential. Using micro-reflectance spectroscopy, we investigated the rich pattern of polariton branches in single MoS2 tubes with both variable and constant gaps. Observed Rabi splitting in the 40-60 meV range is comparable to that for a MoS2 monolayer in a microcavity. Our results, based on the polariton dispersion measurements and polariton dynamics analysis, present a single TMDC nanotube as a perfect polaritonic structure for nanophotonics.

Packaged magnetic sensor based on a saddle-shape Whispering Gallery Mode microresonator

Incorporating optical Whispering Gallery Mode (WGM) microresonators in real life sensing applications is of great interest due to their tiny volumes and very high quality factors. Taper-fiber coupled microresonators, in particular, suggest high simplicity, low cost, and wide range of achievable coupling regimes. Here we apply a unique packaging method based on a tapered fiber-coupled saddle-shape microresonator (SSM), providing high mechanical stability while preserving the quality (Q) factor of the microresonator. The ability to tune the SSM's WGMs mechanically is harnessed here to detect the elongation of a magnetostrictive Terfenol-D (Ter-D) rod, and therefore to measure variations in the ambient magnetic field. As we demonstrate here, this packaging technique enables the production of tightly packaged WGM-based devices. The simplicity in manufacturing SSMs, their high Q factors, the ability to tune their WGMs mechanically/magnetically, and the high mechanical stability they provide, make them great candidates as a platform for practical miniature WGM-devices.

Achieving chirality and unidirectional emission in optical microcavity via external perturbations

Optical whispering-gallery mode (WGM) microcavities, featured with ultrahigh quality (Q) factors and small mode volumes, are widely investigated in linear optics, nonlinear optics and quantum optics. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders their potential in integrated photonic circuits (PCs). Despite the rapid development of microcavity physics, it is essential to explore simple and flexible method to tailor the outputs from the microcavity. In this work, we numerically prove that the regular resonances confined within the stable islands can be affected by purposely manufacturing the energy fluctuation in chaotic sea. By evanescently coupling a waveguide with deformed microcavity, the waveguide induced asymmetric scattering disturbs the balance between the counter-clockwise (CCW) and clockwise (CW) components of the chaotic modes, which sequently brings such inequivalence into stable modes through chaotic-to-regular tunneling. A theoretical model of 4 × 4 non-Hermitian Hamiltonian matrix is employed to illustrate the working principle of the proposed mechanism. Moreover, for the regular modes with angular momentum above critical line, the energy refracting out of the cavity is suppressed, highly improving the collection ratio of waveguide. In this sense, the waveguide simultaneously plays the role of constructing chirality and collecting energy from microcavity. The presented design offers alternative but robust approach to manipulate the chirality of the microcavity. We believe that this research is of great significance to understand the basics and expand the potentials of microcavity.

Waveguide-integrated chip-scale optomechanical magnetometer.

Optomechanical magnetometers enable highly sensitive magnetic field sensing. However, all such magnetometers to date have been optically excited and read-out either via free space or a tapered optical fiber. This limits their scalability and integrability, and ultimately their range of applications. Here, we present an optomechanical magnetometer that is excited and read-out via a suspended optical waveguide fabricated on the same silicon chip as the magnetometer. Moreover, we demonstrate that thermomechanical noise limited sensitivity is possible using portable electronics and laser. The magnetometer employs a silica microdisk resonator selectively sputtered with a magnetostrictive film of galfenol (FeGa) which induces a resonant frequency shift in response to an external magnetic field. Experimental results reveal the retention of high quality-factor optical whispering gallery mode resonances whilst also demonstrating high sensitivity and dynamic range in ambient conditions. The use of off-the-shelf portable electronics without compromising sensor performance demonstrates promise for applications.

Fabrication of optical WGM microcavities using high-resistance wire

This paper describes a method for fabricating whispering gallery mode (WGM) optical microcavities based on optical fiber using a nickel-chromium wire. The above method makes it possible to manufacture optical WGM microcavities with high reproducibility and low cost compared to other methods.

Photo-elasticity of silk fibroin harnessing whispering gallery modes

Silk fibroin is an important biomaterial for photonic devices in wearable systems. The functionality of such devices is inherently influenced by the stimulation from elastic deformations, which are mutually coupled through photo-elasticity. Here, we investigate the photo-elasticity of silk fibroin employing optical whispering gallery mode resonation of light at the wavelength of 1550 nm. The fabricated amorphous (Silk I) and thermally-annealed semi-crystalline structure (Silk II) silk fibroin thin film cavities display typical Q-factors of about 1.6 × 104. Photo-elastic experiments are performed tracing the TE and TM shifts of the whispering gallery mode resonances upon application of an axial strain. The strain optical coefficient K’ for Silk I fibroin is found to be 0.059 ± 0.004, with the corresponding value for Silk II being 0.129 ± 0.004. Remarkably, the elastic Young’s modulus, measured by Brillouin light spectroscopy, is only about 4% higher in the Silk II phase. However, differences between the two structures are pronounced regarding the photo-elastic properties due to the onset of β-sheets that dominates the Silk II structure.

Superfluid Helium Drops Levitated in High Vacuum.

We demonstrate the trapping of millimeter-scale superfluid helium drops in high vacuum. The drops are sufficiently isolated that they remain trapped indefinitely, cool by evaporation to 330mK, and exhibit mechanical damping that is limited by internal processes. The drops are also shown to host optical whispering gallery modes. The approach described here combines the advantages of multiple techniques, and should offer access to new experimental regimes of cold chemistry, superfluid physics, and optomechanics.

Simultaneous determination of AC and DC magnetic field by a whispering gallery mode optical resonator

Alternating current (AC) and direct current (DC) magnetic fields are monitored by an optical whispering gallery mode (WGM) microcapillary resonator filled with magnetostrictive material. The DC magnetic field is demodulated with the signal-to-noise ratio of an AC-modulated magnetic field at a particular frequency and a sensitivity of 2.1 dB/mT is obtained across a detection range of 0.9 mT. An AC field sensitivity of 2.08 μT/ H z at 346 kHz is obtained with the same resonator. Introducing a biased DC field of 90Gs provides a four-fold improvement in AC field sensitivity. The AC and DC fields may be used to improve the corresponding DC and AC sensing. Therefore, this scheme may play a role monitoring magnetic fields in defence, biomedical analysis, and consumer electronics.

Saddle-Shape Whispering Gallery Mode Microresonators

Optical Whispering Gallery Mode (WGM) resonators are an ideal platform for highly sensitive sensors and nonlinear interactions. They exhibit extremely high quality (Q) factors, providing a sensing platform with low detection limit to environmental changes and optical confinement for low threshold nonlinear stimulation. Here we demonstrate a new type of WGM resonator we coin as a Saddle-Shape Microresonator (SSM). The unique modal properties of the SSM's WGMs significantly differs them from spheres, disks, toroids, and other well-known microresonator types. We show that the SSM offers, in addition to the many attractive attributes that are found in "fiber-tip" sphere microresonators, the added characteristic of structural stability of a taper coupled resonator. The combination of these traits make it ideal for strain-based sensing, cavity quantum electrodynamics, and for real life applications in which mechanical tunability of the microresonator's WGMs is essential. The distinctive structure of SSMs – allowing exceptional high mechanical stability as well as mechanical tunability – opens a route to manufacture miniature packaged fiber-coupled WGM microresonators-based devices.

Revealing molecular diffusion dynamics in polymer microspheres by optical resonances.

Understanding the diffusion of small molecules in polymer microsystems is of great interest in diverse fundamental and industrial research. Despite the rapidly advancing optical imaging and spectroscopic techniques, entities under investigation are usually limited to flat films or bulky samples. We demonstrate a route to in situ detection of diffusion dynamics in polymer micro-objects by means of optical whispering-gallery mode resonances. Through mode tracking, interactions between solvent molecules and polymer microspheres, including sorption, diffusion, and swelling can be quantitatively analyzed. A turning point of mode response is observed, while the diffusion exceeds the sub-wavelength-thick outermost layer as the radial extent of resonances and starts penetrating the inner core. The estimated solubility in the glassy polymer is consistent with the predicted value using Flory-Huggins theory. Besides, the non-Fickian contribution is analyzed in such a glassy polymer-penetrant system. Our work represents a high-precision and label-free approach to describing characteristics in diffusion dynamics.