Reflection and Transmission of High Intensity Femtosecond Laser Pulse Focused on Very Thin Plastic Foils

The 150 fs pulse of the LOA Ti:Sapphire laser has been focused at an intensity of 5x10(17) W/cm(2) on a 800 Angstrom plastic foil. The reflected and transmitted laser radiation resulted strongly affected by Self Phase Modulation effects. Even if the measurements were time integrated, the analysis of the spectra give information on laser plasma interaction at different times. A peculiar characteristic oflaser produced plasmas with femtosecond pulses is the negligible hydrodynamic expansion during the pulse. In fact, the scale length of the plasma density perpendicular to the target surface, given by L=c(s) Delta t, is much shorter than the vacuum wavelength lambda of the impinging laser radiation, that is : lambda much greater than L, where c(s) is the sound velocity, and Delta t the pulse duration. In these experimental conditions the interaction geometry is particularly well defined. Thus the laser radiation impinges at a given angle theta on a fairly flat plasma surface having a sharp density profile.