Micrometer-size droplets as optical cavities: lasing and other nonlinear effects

Just within the liquid-air interface of micrometer-size droplets, the internal field at the incident wavelength is enhanced and efficient optical feedback is provided for the internally generated inelastic radiation. At specific discrete wavelengths corresponding to morphology-dependent resonances of a droplet (e.g., a sphere or spheroid), the droplet can be envisioned as a spherical Fabry-Perot for two internally generated waves counterpropagating around the circumference and experiencing gain when these waves pass through the localized regions of the incident pump wave. Examples are presented of laser emission from dye-containing ethanol and ethylene glycol droplets and of stimulated Raman emission from CCl4 and water droplets (with or without anions such as NO 3 − ; and SO 4 2 − ). Examples of asymmetrically broadened spectra of the stimulated emission caused by intensity-dependent index of refraction modulation are given. At higher pumping levels, laser-induced plume ejection and avalanche breakdown giving rise to continuum and discrete atomic emission have been observed. Photographs of individual droplets which have achieved lasing, stimulated Raman oscillation, and avalanche breakdown thresh-olds are shown.