Coupling approaches and new geometries in whispering-gallery-mode resonators

In order to fully exploit the unique properties of micro-optical resonators with whispering gallery modes (WGMs), both for fundamental investigations as well as for practical applications, a critical point is an efficient, controllable, and robust coupling of the light to the cavity WGMs. We present the results of our studies on phase-matched evanescent field couplers, with particular reference to the coupling to high-index crystalline resonators like lithium niobate disks. We focus on couplers based on different types of waveguide configurations and include demonstration of optical coupling to high-Q lithium niobate resonators from integrated planar waveguides as well as from angle polished waveguides. These systems are all in guided optics architectures. We also briefly present our recent achievements in the development of microbubble resonators fabricated from silica capillaries. We show that, as high-Q hollow 3-D cavities with intrinsic microfluidics, these resonators represent a promising biochemical sensing platform.