NONLINEAR-INTERACTION OF KRF LASER-RADIATION WITH SMALL WATER DROPLETS

Results have been obtained for the interaction of KrF excimer laser radiation (lambda = 248 nm, t(pulse) = 17 ns) with 60-mu-m diam distilled water droplets for irradiance values ranging from 3 to 230 GW/cm2. Laser images of the droplet breakup during the time intervals from 0 to 100 ns indicate that the dynamic breakup processes are highly nonlinear. At low irradiance (3 GW/cm2) material is ejected from both the illuminated and shadow hemispheres of the droplet in qualitative agreement with the location of the electric field peaks predicted by plane wave Lorenz-Mie theory calculations. As the irradiance is increased, the interaction on the shadow hemisphere becomes stronger while the interaction on the illuminated hemisphere decreases. This nonlinear behavior is attributed to rapid electrical breakdown of the droplet near the shadow surface. The breakdown region destroys the ability of the droplet to redirect energy toward the front hemisphere of the droplet. Without this mechanism, the localized electric field maxima near the illuminated surface of the droplet cannot arise. Measurements were also taken of the average material velocities during the time intervals from 0 to 50 ns after arrival of the high energy pulse. Velocities ranged from 1000 m/s at 3 GW/cm2 to 6000 m/s at 230 GW/cm2.