Laser-induced radiation leakage from microdroplets

Intensity distributions on the droplet surface and within the droplet are calculated and found helpful in explaining the observed laser-induced radiation leakage from various locations on or within the droplet. In an attempt to distinguish various possible mechanisms for laser-induced perturbation of micrometer-sized droplets, two types of experiment are conducted, and the measured intensity distribution is compared with that predicted by theoretical computation. The observed extra-light leakage of dye-laser radiation trapped in the droplet can be grouped into quasi-instantaneous and cumulative effects. One set of experiments uses a variable number of 100-ps laser pulses because of the laser’s low fluence at 0.1 GW/cm2. Quasi-instantaneous perturbations associated with an intensity-dependent index of refraction change are found to be important. Cumulative perturbations become evident as the number of 100-ps pulses increases. The second set of experiments uses one green laser beam and one near-IR laser beam, both with 7-ns duration, which because of the high fluence emphasize the electrostrictive- and temperature-related perturbations. Extra leakage from surface perturbations caused by the laser-induced temperature rise cannot be isolated but is found not to be dominant.