Optical cavity mode excitations in metal-coated microspheres

We present some recent results of our work on cavity mode excitations in metal-coated microspheres, which aims at the development of a novel type of bio-chemical sensor. In contrast to the well-known whispering gallery modes (WGM) of dielectric particles, metal-coated dielectric microspheres also allow for excitation of modes in radial direction, the so-called Fabry-Perot modes (FPM). One hurdle of such excitation is the proper adjustment of the reflectivity of the metallic coating, which either causes a low quality factor of the modes in case of insufficient thickness, or, otherwise, shields the inner cavity from outside excitation. The talk will present a novel concept on how such intricacies may be overcome by proper selection of excitation wavelength and materials choice, and will demonstrate that FPM modes may be excited in metal-coated microspheres with diameters down to I pm. First examples of utilization of such cavity modes for bio-chemical sensing will be given. Besides sensing, potential applications of metallic microcavities are related to the development of optical point sources, microscopic lasers, and to nonlinear nano-photonics.