Expansion of the electron-hole plasma in Si: A picosecond time-resolved Raman probe

The expansion velocity of the photoexcited electron-hole plasma in a Si crystal has been measured by time-resolved Raman scattering on a picosecond time scale with 1-m spatial resolution. The observed transient density-time profile of the plasma can be fit by a modified diffusion equation based on a hydrodynamic model, which takes into account the driftlike motion of the plasma and proper boundary conditions. The deduced drift velocity of the plasma is shown to depend strongly on lattice temperature as well as plasma density.