Near Field Optics with High-Q Whispering-Gallery Modes

Abstract

High-Q whispering-gallery modes (WGM) are resonant electromagnetic modes which can be observed in dielectric microspheres with diameters larger than about ten times the wavelength [1]. They correspond to high angular momentum states of the electromagnetic field in which light propagates close to the sphere’s surface by repeated total internal reflection. The confinement of the light field in a small volume implies very high values of the electric field per photon. As a result, microdroplets have been shown to exhibit various non-linear optical effects induced at low power excitation. Cavity-enhanced stimulated emission [2], as well as laser action [3], has also been observed in dye-doped droplets. More recently, semiconductor microcavities have been investigated and microdisk lasers based on WGMs have been demonstrated [4]. Another remarkable feature can be achieved with silica microspheres. In this low-loss material, photon storage tinles in the range of one microsecond have been observed, which correspond to quality factors Q greater than 109 [5,6,7,8]. Such high Q values cOlnbined with small mode volumes are rather promising for the pursuit of Cavity Quantum Electrodynamics experiments in the strong coupling regime where a few atoms or ions are tightly coupled to a cavity mode which contains a small number of photons, or is even in the vacuum state [9]. These two properties have also allowed several groups to observe laser action in doped microspheres [10, 11] and we are currently investigating high-Q neodymium-doped microspheres showing very low-thresholds.