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 $10\hspace{0.5em}{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^2<F<\mathrm{}400\mathrm{}\hspace{0.5em}{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^2,$ 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 ${10}^{22}\hspace{0.5em}{\mathrm{cm}}^{-3},$ 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.

• Received 20 November 1998

DOI:https://doi.org/10.1103/PhysRevB.61.2643