ABSTRACT
Coating a whispering-gallery mode (WGM) microspherical optical cavity by a thin film, either passive or active, represents a powerful tool to tailor the properties of a resonator. This chapter discusses those effects which a thin coating has on characteristics of the optical modes of spherical WGM resonators and different coating methods that can be used to produce a uniform coating on top of a microresonator. It outlines how such a coating can greatly increase the applicability of the microspheres for different photonic applications such as lasing, sensing, and nonlinear optics. An optically active resonator, however, can also be produced by coating the sphere with a thin fluorescent glass or quantum dot film. When coating a microsphere with a film having a higher refractive index, the electric field of the whispering-gallery modes is present both in the coating and in the core of the microsphere, the evanescent field of the microsphere extending in the surrounding medium.
