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$°$, and reaches values of almost 45%. Assuming an exponential electron distribution, a temperature of 420 keV at $4\times {10}^{18}$ 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.