High intensity 30 femtosecond laser pulse interaction with thin foils

An experimental investigation on the interaction of 30 femtosecond laser pulses with 0.1 and 1.0 mu m thick plastic foils has been performed at intensities from 5x10(16) to 5x10(18) W/cm(2). The interaction physics was found to be definitely different whether the nanosecond low intensity prepulses led to an early plasma formation or not. In the first case high reflectivity and very low transmittivity were observed, together with second and three-half harmonic generation. In absence of precursor plasma, with increasing intensity, reflectivity dropped to low values, while transmittivity increased up to an almost complete transparency. No harmonic generation was observed in this latter condition, while ultra-fast ionisation was inferred by the blue-shift of the transmitted pulse. Finally, intense hard X-ray emission was detected at the maximum laser intensity level. Current theories or numerical simulations cannot explain the observed transparency. A new model of magnetically induced optical transparency (MIOT) is briefly introduced.