Plasma Formation Dynamics Within A Water Microdroplet On Femtosecond Time Scales

We demonstrate that a highly confined plasma inclusion is formed inside a micrometric water droplet that is excited by a femtosecond laser. An 800-nm laser pulse generates a plasma inclusion by laser-induced breakdown that is subsequently probed by the elastic scattering of a second (400-nm) time-delayed ultrashort pulse. For a 25-mum-radius droplet and an incident intensity of 7 X 1012 W/cm(2) the radius of the highly localized plasma is 1.9 +/- 0.4 mum. We probed the plasma formation dynamics on femtosecond time scales by varying the delay between the pump and the probe pulses. Good agreement with numerical simulations of the process was found. (C) 2003 Optical Society of America.