  We employ ultra-broadband terahertz-midinfrared probe pulses to characterize
the optical response of photoinduced charge-carrier plasmas in high-resistivity
silicon in a reflection geometry, over a wide range of excitation densities
(10^{15}-10^{19} cm^{-3}) at room temperature. In contrast to conventional
terahertz spectroscopy studies, this enables one to directly cover the
frequency range encompassing the resultant plasma frequencies. The intensity
reflection spectra of the thermalized plasma, measured using sum-frequency
(up-conversion) detection of the probe pulses, can be modeled well by a
standard Drude model with a density-dependent momentum scattering time of
approx. 200 fs at low densities, reaching approx. 20 fs for densities of
approx. 10^{19} cm^{-3}, where the increase of the scattering rate saturates.
This behavior can be reproduced well with theoretical results based on the
generalized Drude approach for the electron-hole scattering rate, where the
saturation occurs due to phase-space restrictions as the plasma becomes
degenerate. We also study the initial sub-picosecond temporal development of
the Drude response, and discuss the observed rise in the scattering time in
terms of initial charge-carrier relaxation, as well as the optical response of
the photoexcited sample as predicted by finite-difference time-domain
simulations.
