Abstract
Laser-induced ultrafast order-disorder transitions in silicon and gallium arsenide are studied by means of femtosecond time-resolved linear and nonlinear optical spectroscopy. Detailed measurements of the reflectivity and of the reflected second harmonic over a wide range of fluences reveal a complex picture of the phase transformation. We show that during the first 100 fs the changes of the optical constants and of the nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(2)}$ are determined by the various electronic excitation processes and only to a lesser extent by the process of disordering. On the other hand, time-resolved measurements of reflectivity spectra indicate that the development of a Drude-like metallic spectrum takes a few hundred femtoseconds. Our data show that the laser-induced structural changes develop slower than previously believed, occurring on a time scale of a few hundred femtoseconds.
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