We study the very early processes of photogenerated carrier transport in GaAs and Si using THz nonlinear spectroscopy with 10 fs time resolution. This ultrahigh time resolution allows us to clearly separate the instantaneous creation of polarized electron-hole pairs and the subsequent carrier motion for the first time. In addition, our results give the first clear demonstration of the distinct phases of carrier transport; ballistic transport, electron velocity overshoot, and steady-state drift.

• Received 24 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.1689