Ultrafast electron and lattice dynamics in semiconductors at high excited carrier densities

We directly measure ultrafast changes in the dielectric function of GaAs over the spectral range from the near-IR to the near-UV caused by intense 70-fs laser excitation. The dielectric function reveals the nature of the ultrafast phase transformations generated in the material, including a semiconductor-to-metal transition for the strongest excitations. Although the electron and lattice dynamics are complex when large carrier densities are excited - between 1 and 20% of the valence electrons - the dominant processes and their timescales can be deduced. (C) 2000 Elsevier Science B.V. All rights reserved.