Motivated by recent experiments, we have performed simulations which show in detail how the electrons and ions in GaAs respond to fast intense laser pulses ͑with durations of order 100 fs and intensities of order 1Ϫ10 TW/cm 2). The method of tight-binding electron-ion dynamics is used, in which an arbitrarily strong radiation field is included through a time-dependent Peierls substitution. The population of excited electrons, the atomic displacements, the atomic pair-correlation function, the band structure, and the imaginary part of the dielectric function are all calculated as functions of time, during and after application of each pulse. Above a threshold intensity, which results in promotion of about 10% of the electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This is most clearly revealed in the dielectric function ⑀(), which exhibits metallic behavior and loses its structural features after 100-200 fs. ͓S0163-1829͑98͒01843-8͔
Using a new formalism that modifies a tight-binding Hamiltonian to include interaction with a timedependent electromagnetic field, we have obtained an analytical expression for the second-order susceptibility. This expression has been used to calculate the energy dependence of (2) () for GaAs. The results are in agreement with previous calculations and with available experimental data.
A method is introduced for simulations of the coupled dynamics of electrons and ions in a molecule or material. It is applicable to general nonadiabatic processes, including interactions with an arbitrarily intense radiation field. The field is included in the electronic Hamiltonian through a time-dependent Peierls substitution. The time-dependent Schrödinger equation is solved with an algorithm that preserves orthogonality, and the atomic forces are obtained from a generalized Ehrenfest theorem. Calculations for GaAs and Si demonstrate that the method is reliable and quantitative.
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