Onset of diffuse reflectivity and fast electron flux inhibition in 528-nm-laser solid interactions at ultrahigh intensity

Using a high-power femtosecond frequency-doubled Nd:glass laser system with a contrast ratio of 10(12), the interaction between light and matter up to intensities of 10(19) W cm(-2) has been investigated. The absorption of the laser light in solid aluminum is almost independent of the polarization, peaks at about 25 degrees, and reaches values of almost 45%. Assuming an exponential electron distribution, a temperature of 420 keV at 4 x 10(18) W cm(-2) was measured. These experiments and the detection of the hard-x-ray radiation (60 keV-1 MeV) implied a conversion efficiency of 10(-4)-10(-3) into suprathermal electrons. A second low-energy electron distribution either with trajectories mainly parallel to the target surface or with a reduced penetration depth due to flux inhibition was also inferred from K alpha line radiation measurements.