Nonthermal transition of GaAs in ultra-intense laser radiation field

Using the technique of fight-binding electron-ion dynamics, we have calculated the response of crystalline GaAs when a femtosecond laser pulse excites 1-20% of the valence electrons. Above a threshold fluence, which corresponds to promotion of about 12% of the valence electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This result supports the conclusion that structural changes on a subpicosecond time scale observed in pump-probe experiments are of a nonthermal nature.