Abstract

Femtosecond laser-pulse induced phase transition on the surface of Silicon and GaAs are of great current interest.^[1-2] Unusual behavior of structural properties of semiconductor under pulsed femtosecond irradiation might be caused by the fact that the exiting pulse duration is much shorter than characteristic times of energy relaxation in semiconductors (electron-phonon, phonon-phonon etc). Measurements of ultrafast dynamics of optical constants and especially comparison of linear and nonlinear susceptibilities provide a valuable structural information since the latter are strongly structure-dependent. In this way several research groups observed ultrafast (less than 1 ps) phase transition in Si and GaAs^[2] which can not be accounted for within simple thermal model. However, many questions remain in regard to competing processes capable in principle of ultrafast changing nonlinear properties of GaAs (Si).

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