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 $10 {\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}<F<400 {\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2},$ 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 ${10}^{22} {\mathrm{cm}}^{\ensuremath{-}3},$ 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.