Dynamical theory of the laser-induced lattice instability of silicon

By extending our previous theory we analyze the observed time-resolved instability of the diamond lattice of silicon, which is induced by a dense electron-hole plasma and which may result from an intense laser pulse. The transverse acoustic phonons become instable if more than 8% of the valence electrons are excited into the conduction band. Furthermore, if 15--20 % of the valence electrons are excited, the average atomic displacement increases to more than 1 {} within less than 200 fsec after the laser pulse. This very rapidly destroys the symmetries of the lattice and leads to a strong excitation of transverse phonons, resulting in a high temperature. Our results are in good agreement with experiments done on surfaces of silicon and GaAs.