Transient gratings and second-harmonic probing of the phase transformation of a GaAs surface under femtosecond laser irradiation

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Abstract
Using a transient-grating diffraction technique, we have compared the ultrafast dynamics of the linear reflectivity from a GaAs surface at the fundamental and second-harmonic frequencies with the dynamics of the second-harmonic generation (SHG) in reflection at an excitation level exceeding the melting threshold. It is shown that the ultrafast (within 100 fs) drop in the SHG efficiency cannot be accounted for by changes in the linear dielectric susceptibility that take place on a longer time scale. This fact indicates a fast change of the long-range crystalline symmetry of GaAs within a semiconductorlike phase, preceding a transition to the metallic phase as the crystal is melted.