TIME-RESOLVED PROBING OF ELECTRON THERMAL TRANSPORT IN PLASMA PRODUCED BY FEMTOSECOND LASER-PULSES

We present the first direct evidence for a supersonically moving steep ionization front in a solid density hot plasma. The plasma was produc ed by irradiating a transparent solid target with 100 fsec laser pulse s at a peak intensity of 5 x 10(14) W/cm2. Time-resolved measurements of reflectivity, transmissivity, and frequency shift of a probe light, incident on the rear of the plasma, showed rapid formation of an over -dense plasma layer with a steep gradient that penetrates supersonical ly into the bulk target. Calculations using a simple nonlinear heat wa ve model, driven by electron thermal transport, show good agreement wi th the experimental results.