Abstract

Picosecond laser pulse experiments on semiconductors show that a thermal melting model can explain the behavior of these materials when irradiated with such pulses.¹ This model assumes that energy is quickly transferred from the electrons excited by the laser pulse to the lattice through phonon emission. The lattice then heats up to the melting temperature and undergoes a phase transition to the liquid phase. However, the thermal model breaks down if the energy of the laser pulse is deposited into the electronic system on a time scale which is short compared to the electron-lattice energy relaxation time, as in the case of femtosecond laser pulses exciting semiconductors with relaxation times of a few picoseconds.² In this case, the bond breaking which occurs through excitation of valence electrons to the conduction band can lead directly to ionic disordering while the lattice is still cold.³ Experiments performed on silicon with femtosecond laser pulses indeed suggest disordering of a cold lattice.⁴ We have performed a series of measurements on GaAs using 160-fs pulses which show that above a threshold pump fluence of 0.1 J/cm², the material undergoes an ultrafast transition to a disordered, metallic phase in less than 0.5 ps.

© 1992 The Author(s)