Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t similar to 10(-13) s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms ("cold liquid") or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. (C) 1999 American Institute of Physics. [S1063-7834(99)02308-4].