Optical probing of a plasma created by a 100 femtosecond laser

Solid targets have been irradiated by 80 fs laser pulses at 10¹⁶ W/cm² laser irradiance. A time-resolved Schlieren imaging technique has been developed to measure the effect of the laser amplified spontaneous emission (ASE). This diagnostic shows that the critical density is located at less than 1 μm away from the target surface in our experimental condition. This is in agreement with the X-ray spectra from solid aluminum target in the 0.7-0.9 nm wavelength range, which are characteristic of plasma with near solid electron density (N_e∼10²³cm^-3).