We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

/pdf/whispering-gallery-mode-sensors-3vzjirob2b.pdf

Whispering gallery mode sensors

Whispering-gallery modes (WGM) on a spherical surface were first described by Lord Rayleigh at the beginning of the last century, but only after the invention of laser did they start to have some scientific relevance and only during the last two decades there has been a substantial move towards real devices and practical applications. WGM resonators have peculiar properties, the most notable being the potential of having an ultrahigh quality factor Q, which makes them very appealing both as laser cavities and as extremely sensitive sensors. Among the different types of WGM resonators, the microspherical ones represent a very important category, due to their simplicity, easy fabrication, and very high quality. In this review we provide a description of their fundamental properties and we summarize recent works on their application as filters, sensors and lasers.

Spherical whispering‐gallery‐mode microresonators

Confinement of light into small volumes has become an essential requirement for photonic devices; examples of this trend are provided by optical fibers, integrated optical circuits, semiconductor lasers, and photonic crystals. Optical dielectric resonators supporting Whispering Gallery Modes (WGMs) represent another class of cavity devices with exceptional properties, like extremely small mode volume, very high power density, and very narrow spectral linewidth. WGMs are now known since more than 100 years, after the papers published by John William Strutt (Lord Rayleigh), but their importance as unique tools to study nonlinear optical phenomena or quantum electrodynamics, and for application to very low-threshold microlasers as well as very sensitive microsensors, has been recognized only in recent years. This paper presents a review of the field of WGM resonators, which exist in several geometrical structures like cylindrical optical fibers, microspheres, microfiber coils, microdisks, microtoroids, photonic crystal cavities, etc. up to the most exotic structures, such as bottle and bubble microresonators. For the sake of simplicity, the fundaments of WGM propagation and most of the applications will be described only with reference to the most common structure, i.e. microspherical resonators.

/pdf/whispering-gallery-mode-microresonators-fundamentals-and-6yxtews7tz.pdf

Whispering gallery mode microresonators: Fundamentals and applications

Glass microsphere resonators that support optical resonances known as whispering-gallery modes are unique tools for studying and exploiting optical effects under extremely well controlled conditions. In this paper, a review focusing mostly on glass microsphere resonators is presented. First, a brief historical background is given in which we see how the state-of-the-art has grown from novel optical resonators to the ultrahigh Q cavities used in cutting-edge experiments. After the basic properties of microsphere resonators are outlined we will discuss some of the recent experiments involving microsphere resonators, although some discussion involving polymeric microspheres is also included. The use of doped and undoped microspheres in optical signal processing, optical sensing and quantum optics is highlighted. Finally, there is a brief review of recent optomechanical experiments that use microspheres.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201000025

WGM microresonators: sensing, lasing and fundamental optics with microspheres

Optical Whispering Gallery Modes (WGMs) derive their name from a famous acoustic phenomenon of guiding a wave by a curved boundary observed nearly a century ago. This phenomenon has a rather general nature, equally applicable to sound and all other waves. It enables resonators of unique properties attractive both in science and engineering. Very high quality factors of optical WGM resonators persisting in a wide wavelength range spanning from radio frequencies to ultraviolet light, their small mode volume, and tunable in- and out- coupling make them exceptionally efficient for nonlinear optical applications. Nonlinear optics facilitates interaction of photons with each other and with other physical systems, and is of prime importance in quantum optics. In this paper we review numerous applications of WGM resonators in nonlinear and quantum optics. We outline the current areas of interest, summarize progress, highlight difficulties, and discuss possible future development trends in these areas.

https://iopscience.iop.org/article/10.1088/2040-8978/18/12/123002/pdf

Nonlinear and quantum optics with whispering gallery resonators

A micro-optical force sensor concept based on the morphology-dependent shifts of optical modes of dielectric microspheres is investigated. The optical resonances, commonly referred to as the whispering gallery modes (WGM), were excited by evanescently coupling light from a tunable diode laser using a tapered single-mode fiber. A compressive force applied to the sphere induces a change in both the shape and the index of refraction of the sphere leading to a shift in WGM. By tracking the shifts, the force magnitude is determined using solid silica as well as solid and hollow Polymethyl-methacrylate (PMMA) microsphere resonators. A measurement sensitivity as high as dlambda/dF=7.664 nm/N was demonstrated with a 960 mum hollow PMMA sphere.

Micro-optical force sensor concept based on whispering gallery mode resonators.

The effect of hydrostatic pressure on solid and hollow microsphere optical resonators was investigated. The primary goal was to explore the feasibility of a micro-optical pressure sensor based on whispering gallery modes (WGMs) and to quantify the deleterious effect of environmental pressure changes on other WGM-based sensors. Expressions were developed for WGM shifts due to changes in hydrostatic pressure of the environment surrounding the spherical resonators. These expressions were validated through experiments in which the pressure-induced WGM shifts of hollow polymethyl methacrylate microspheres were monitored. The effect of atmospheric pressure variations on silica resonators is negligible, but hydrostatic pressure may be effective in the optical tuning of hollow polymer spheres.

Pressure tuning of whispering gallery mode resonators

Performance characteristics of a force sensor concept based on the morphology dependent resonance (MDR) shifts of micro-optical resonators have been investigated. Previous experimental studies have indicated that microsphere sensors with diameters ranging between 30 and 950 μm may have force resolutions reaching 10−5 N [T. Ioppolo et al., Appl. Opt. 47, 3009 (2008)]. In the present, we carry out a systematic analysis and experiments to investigate the sensitivity, resolution, and bandwidth limits of MDR-based force sensors. Expressions for MDR shifts due to applied force in the polar direction are obtained for microspheres of various dielectric materials in the diameter range of 300–950 μm. The analyses are compared with experimental results for polymethylmethacrylate and polydimethylsyloxane (PDMS) microsphere sensors. The results show that the strain effect on MDR shifts is dominant over that of mechanical stress. It also indicates that force sensitivities of the order of a 1 pN are feasible using hollow PDMS spheres. The sensor bandwidths range between 1 kHz and 1 MHz, depending on the sphere material.

High-resolution force sensor based on morphology dependent optical resonances of polymeric spheres

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of a WGM-based microsensor for electric field measurement. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both strain and stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that detection of electric fields as small as ~500V/m may be possible using water filled, hollow solid polydimethylsiloxane (PDMS) microspheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid PDMS spheres agree well with the analytical prediction.

Tuning of whispering gallery modes of spherical resonators using an external electric field

A simple plane wave, ray-tracing approach was used to derive approximate equations for the dielectric microsphere whispering gallery mode (WGM) resonant wavenumber and quality factor, as dependent on the surrounding medium’s refractive index. These equations are then used to determine the feasibility of a micro-optical sensor for species concentration. Results indicate that the WGMs are not sensitive enough to refractive index changes in the case of gas media. However, they can be sufficiently sensitive for measurements in liquids. Experiments were carried out to validate the analysis and to provide an assessment of this sensor concept.

Tindaro Ioppolo

Papers

Micro-optical force sensor concept based on whispering gallery mode resonators.

Pressure tuning of whispering gallery mode resonators

High-resolution force sensor based on morphology dependent optical resonances of polymeric spheres

Tuning of whispering gallery modes of spherical resonators using an external electric field

Whispering gallery modes of microspheres in the presence of a changing surrounding medium: A new ray-tracing analysis and sensor experiment