Cavity-enhanced emission in microdroplets

Abstract

Micrometer-sized transparent droplets act as natural high-Q cavities to visible light, and this paper shows them to exhibit strongly enhanced dye emission owing to cavity quantum-electrodynamic effects. A vibrating-orifice aerosol generator1 was used to produce a linear stream of virtually identical (less than 2 parts in 105 diameter variation) falling droplets 14 μm diameter, 10-5 M solution of Rhodamine 6G in ethanol; the droplets were excited by a cw argon-ion laser. Droplet emission was examined with a double 1 m Spex spectrograph and a photomultiplier. Recent development of elastic-scattering-size spectroscopy1,2 allowed identification of the various cavity modes contributing to the emission. This was necessary because of the different Q values and spatial properties of the resonant modes. Q values were calculated from Lorenz-Mie theory and were found to be consistent with observed fluorescence linewidths for the experimentally resolvable fourth-order modes. It was experimentally determined that Q's of only 103 were sufficient to observe sizable enhancements in the A coefficient. Lasing was observed to occur at reduced thresholds, e.g., 400 W/cm2 for fourth order modes having Q's of 2 × 103. This corresponds to a cavity induced enhancement in the B coefficient of 103.