Chemical lasing in pendant droplets: lasing-spectra, emission-pattern, and cavity-lifetime measurements

Abstract

We demonstrate lasing in liquid pendant droplets through a chemiluminescence process, which uniquely provides spatially uniform pumping throughout the sample. Pendant droplets of 2-mm equatorial radius are formed at the tip of a capillary tube through which the chemiluminescence material is injected. The chemiluminescence spectra along the highlighted rim of the droplet show redshifted intensity enhancement in the wavelength region where the absorption is low. The lasing threshold is found by addition of different amounts of absorbers. The observed nonuniform laser-emission intensity distribution along the droplet rim is caused by a spatially varying rate of diffractive-light leakage related to the droplet surface curvature. Using WKB approximation, we express the diffractive-light leakage rate on a curved surface as an exponentially decreasing function of angle of incidence. The standard laser rate equation with distributed leakage loss is employed to express the laser-emission output intensity from the pendant droplet. The light leakage from the surface was further investigated by localized perturbations formed by poking of the surface with a sharply tipped fiber. The Q of cavity modes as high as 3.5×108 in the pendant droplet was determined from the cavity-lifetime measurement.