Abstract

We investigate the field dynamics in GaAs after femtosecond injection of free charge carriers on a subpicosecond time scale with two different optical methods: Reflective Electro-Optic Sampling (REOS) and Differential Transmission Spectroscopy (DTS). The first method is applied to bare (100) orientated GaAs surfaces for the observation of ultrafast screening of built-in surface fields via free carrier transport. DTS is applied to an externally biased GaAs film. The field evolution is monitored via the Franz- Keldysh (FK) effect. We compare the experimental results with elaborate numerical calculations for the spatial carrier distributions, the linear electro-optic (EO) effect and the FK effect including excitonic contributions. The comparison gives a detailed information on the carrier density dependence of the screening dynamics and carrier drift velocities on a subpicosecond time scale. Screening effects due to an instantaneous polarization are not significant for the interpretation of the experimental results.

© 1993 Optical Society of America