Generation of dense electron-hole plasmas in silicon

Generation of dense electron-hole plasmas in silicon with intense 100-fs laser pulses is studied by time-resolved measurements of the optical reflectivity at 625 nm. For fluences F between 10 mJ/cm^2<F<400 mJ/cm^2, plasma generation is dominated by strong two-photon absorption, and possibly higher-order nonlinearities, which lead to very steep spatial carrier distributions. The maximum carrier densities at the sample surface are in excess of 10^22 cm^-3, and therefore, the reflectivity shows a mainly Drude-like free-carrier response. Within the Drude model, limits for the optical effective mass and the damping time are determined.