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 produced by irradiating a transparent solid target with 100 fsec laser pulses at a peak intensity of 5[times]10[sup 14] W/cm[sup 2]. Time-resolved measurements of reflectivity, transmissivity, and frequency shift of probe light, incident on the rear of the plasma, showed rapid formation of an overdense plasma layer with a steep gradient that penetrates supersonically into the bulk target. Calculations using a simple nonlinear heat wave model, driven by electron thermal transport, show good agreement with the experimental results.