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.

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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

Active control of high Reynolds number and high-speed jets has been hampered due to the lack of suitable actuators. Some of the attributes that would make an actuator suitable for such flows are: high amplitude and bandwidth; small size for distribution around the jet; phase-locking ability for jet azimuthal mode forcing; and sufficient ruggedness for hot jets. We have been developing a class of actuators termed ‘localized arc filament plasma actuators,’ which possess such characteristics. In this paper, we present the development and characterization of these actuators as well as preliminary results on their applications in high Reynolds number Mach 0.9 and ideally expanded Mach 1.3 jets.

Development and characterization of plasma actuators for high-speed jet control

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.

A shear stress sensor for measuring the shear force of a fluid flowing along a wall. A floating member, flush with the wall, senses a shear force of the flowing fluid. The floating member is mounted by support means to a base element that is placed in the wall, so that the floating member is flush with the wall and a shear force, sensed by the floating member, is translated via the support means to a Fiber Bragg Grating. The force acting on the Fiber Bragg Grating changes the shape and the refractive index of the Fiber Bragg Grating, thereby changing the resonant frequency of the Fiber Bragg Grating and causing a shift in the spectrum of wavelengths of light that is introduced to the Fiber Bragg Grating. This shift in the spectrum of wavelengths is representative of the shear force of the flowing fluid.

Shear stress measurement apparatus

The density of electrons formed in binary collisions of 2p53s neon atoms was measured in the afterglow of a low pressure glow discharge by observing the high energy tail of the electron energy distribution function (EEDF) using a Langmuir probe. In the afterglow, the bulk plasma electrons thermalize but the density of 2p53s neon atoms remains significant. In an ionizing collision, a pair of these atoms releases high energy (∼11.6 eV) electrons which form a characteristic peak in the EEDF. Simultaneously with the chemi-ionization electrons, the densities of 1s2, 1s3, 1s4, and 1s5 neon atoms were independently measured using diode laser absorption spectroscopy. It was found that the data obtained are described well by a single chemi-ionization reaction when the 2p53s configuration is considered a single state. The corresponding rate coefficient, found to be (3.2±0.4)×10−10 cm3 s−1 at a temperature of 310 K, is recommended for use in discharge modeling.

Chemi-ionization in neon plasma

Real-time assessment of granule densification in high shear wet granulation and application to scale-up of a placebo and a brivanib alaninate formulation.

A unique method of blocking sound waves using a glow discharge plasma sheet was demonstrated experimentally The main thrust of the investigation was the determination of the effectiveness of using glow discharge plasma as a sound barrier in aerospace applications Experiments were conducted in an anechoic chamber where the attenuation of single-frequency (125-kHz) sound propagating through a plasma sheet formed between two electrodes was studied The sound attenuation was measured at different locations inside the chamber with the discharge plane oriented normal to the direction of sound-wave propagation Some measurements were also carried out with the plasma plane oriented 45 deg to the sound-wave propagation direction The measurements clearly demonstrated that the plasma attenuates sound The highest attenuation of about 23 dB was obtained when the plasma was oriented 45 deg to the sound propagation direction In this configuration, sound reflection from the glow discharge region was also observed The present results suggest that the dominant mechanism responsible for the sound attenuation is the change in the index of refraction caused by gas temperature gradient at the plasma boundaries

Valery Sheverev

Papers

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

Shear stress measurement apparatus

Chemi-ionization in neon plasma

Real-time assessment of granule densification in high shear wet granulation and application to scale-up of a placebo and a brivanib alaninate formulation.

Sound Attenuation by Glow Discharge Plasma