Femtosecond-scale response of semiconductors to laser pulses

We report simulations of the response of InSb, GaAs, and Si to 70-femtosecond laser pulses of various intensities. In agreement with the experiments of Mazur and coworkers, and other groups, there is a nonthermal phase transition for each of these semiconductors above a threshold intensity. Our simulations employ tight-binding electron-ion dynamics (TED), a technique which is briefly described in the text. In the experimental pump-probe observations, the dielectric function ε (ω) and the second-order susceptibility χ⁽²⁾ can be measured. These same quantities can be calculated during a TED simulation, and there is good agreement in the behavior with respect to both time and frequency. The simulations provide much additional microscopic information which is experimentally inaccessible: for example, the time-dependence of the atomic pair-correlation function, electronic energy bands, occupancies of excited states, kinetic energy of the atoms, and excursions of atoms from their initial positions.