We use an optical pump - terahertz probe technique to study relaxation dynamics of photoexcited carriers in semiconductors. The optical pump pulse (400/800 nm, 100 fs) from an amplified Ti:sapphire laser generates free carriers within the optical penetration depth of the semiconductor surface, and the transmission of the terahertz (THz) probe pulse is monitored as a function of delay time between pump and probe. In particular, we investigate carrier relaxation dynamics in radiation-damaged silicon-on-sapphire (RD-SOS). We measure pump-induced changes in the transmission of the amplitude of the THz pulse, which proves to be a valid technique for these low-mobility samples due to negligible phase shifts in the transmitted THz pulse. Using a simple thin-film Drude model, single-exponential relaxation times of 4 to 6 ps are observed and transient mobilities of about 420 cm2/Vs are obtained for moderately damaged RD-SOS> Picosecond carrier relaxation dynamics in high-implant-dose RD-SOS and sub-picosecond transients in amorphous silicon thin films are also observed.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.