ABSTRACT
Optical frequency conversion is widely used for wavelength extension of laser sources. Whispering-gallery-mode resonators (WGMRs) play an extremely important role in modern optics, being fundamental not only in many laser and sensing devices, but also as tools for very accurate frequency measurements and for nonlinear optics experiments, such as frequency conversion. This chapter illustrates several nonlinear effects in silica microspheres. It deals with tunable optical harmonic generation with extremely narrow linewidth. The selection rules for silica microspherical WGMRs have been obtained systematically by taking into account the spatial, radial, and angular mode overlaps and the material and resonator dispersion profiles. Third-order sum-frequency generation (TSFG) has been studied in whispering-gallery mode structures, in particular in liquid droplets, starting from 1989. A model for TSFG in spherical dielectric microresonators can be based on the work of H. Chew et al. for emission from a polarization source within a sphere.
