Femtosecond laser diagnostics of ultrafast laser-induced melting of the GaAs surface

We report here the experimental results on the SHG and linear reflectivity studies of the dynamics of laser-induced melting of a GaAs surface layer under subpicosecond pulsed laser excitation. We find that the SH intensity drops in a time less than 100 fs after excitation, while the linear reflectivity reaches a value characteristic of molten GaAs on a time scale of about 1 ps. Thus, the experimental results unambiguously indicate that ultra-fast phase transition to a new solid phase with structural properties different both from that of initial material and that of molten GaAs takes place in the surface layer under strong femtosecond laser excitation. This transition occurs on a time scale of 100 fs which is about an order of magnitude less than the time required for the electron relaxation to the bottom of the conduction band.