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.

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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

Microlasers have experienced tremendous development in the past decade and become an essential part in laser evolution, as miniature lasers provide strong optical confinement and feature greatly enhanced light–matter interactions. Among all the configurations, whispering gallery mode (WGM) microcavities and microlasers exhibit outstanding optical performances with high quality factors and small mode volumes, thus ensuring low lasing thresholds. In addition, some unique properties inherent to WGM cavities, like bi-directional propagation and an evanescent field that spans several hundred nanometers across the boundary, can be exploited for novel applications. Therefore, designing and engineering innovative WGM microcavities and microlasers has attracted increasing research interest. The fundamentals and characteristics of WGM are introduced here, and then the developments and current status of WGM microcavities and microlasers are reviewed in terms of the evolution of fabrication techniques and built-up materials. In particular, the melting of glassy materials in early studies, top-down and bottom-up approaches with semiconductors, coating structures, as well as flexible, soft microresonators in recent years are presented. Finally, the application prospects of microlasers including the wavelength manipulation, sensing and microresonator coupling, are discussed.

Advances and prospects for whispering gallery mode microcavities

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

Evanescent coupling of light from glass channel waveguides into the whispering gallery modes of glass microspheres of radius 15µm and 100µm is studied experimentally at wavelengths near 1550nm. Fitting the positions, widths and heights of resonances in the experimental spectra to the characteristic equation for microsphere modes and to universal coupled microresonator theory, we establish sphere radius and index, identify mode numbers, and determine losses. The results provide detailed information for the design of optical devices incorporating microsphere resonators in planar lightwave circuits.

Whispering gallery mode spectra of channel waveguide coupled microspheres

A neodymium-doped BK7 glass microsphere laser integrated with a planar ion-exchanged waveguide pumped at 0.8µm has been demonstrated. The pump radiation was launched by evanescent coupling from the waveguide, and the signal radiation was coupled out through the same waveguide, offering the potential for robustly assembled fully integrated active optical circuits. The dependence of the lasing spectra on pump power and wavelength were studied in detail to clarify the whispering-gallery-mode behavior at the pump and lasing wavelengths.

Integrated Nd-doped borosilicate glass microsphere laser.

Glass microsphere resonators have the potential to add significant functionality to planar lightwave circuits when coupled to waveguides where they can provide wavelength filtering, delay and low-power switching, and laser functions. Design of such photonic circuits requires precise coupling between spheres and waveguides to allow control of Q-factor and hence of stored energy and resonator bandwidth. In this paper an erbium-doped silicate glass microsphere is coupled to an ion-exchanged glass waveguide, and excitation spectra for the sphere whispering-gallery modes are determined as a function of spatial separation. Modal assignment allows extraction of the physical parameters of the microsphere and the dependence of Q-factor with separation is compared with theory. All practical microspheres exhibit a small degree of ellipticity and the effects of this upon whispering-gallery mode excitation and wavelength splitting are explored. It has been shown that appropriate displacement and orientation of slightly deformed microspheres with respect to the waveguide can be used to control the effective Q-factor and optimize the spectral shape of the optical devices. This can result in either single high-Q peaks or substantially broadened and spectrally flattened resonances.

Position-dependent coupling between a channel waveguide and a distorted microsphere resonator

Topographical structures to position microsphere resonators accurately upon planar optical waveguides have been designed and fabricated. The methods being employed to assemble the microspheres on the patterned planar waveguides are discussed.

Micropositioning of microsphere resonators on planar optical waveguides

Multicomponent glass microspheres self-assembled on optical waveguides combine tailored optical properties with strong light/material interaction potentially leading to compact low-power photonic devices. Progress and prospects for microsphere/waveguide integration will be described.

Yuwapat Panitchob

Papers

Whispering gallery mode spectra of channel waveguide coupled microspheres

Integrated Nd-doped borosilicate glass microsphere laser.

Position-dependent coupling between a channel waveguide and a distorted microsphere resonator

Micropositioning of microsphere resonators on planar optical waveguides

Integrated microsphere planar lightwave circuits