Abstract

We present a detailed study of structural phase transitions of crystalline silicon and GaAs induced by femtosecond laser pulses. Using fs pump-probe techniques we have performed time-resolved measurements of the p- and s-polarized optical reflectiviy and measurements of the reflected second harmonic (SH) over a wide range of times and laser fluences. Our results provide strong evidence that two fundamentally different types of phase transitions can be produced. There is a distinct range of laser fluences where a relatively slow transition to a liquid state is observed which takes typically 100 ps to develop. For higher fluences, however, we observe a very dramatic change of the time evolution suggesting a different type of transition which develops on a subpicosecond time scale.^[1-4]

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