Photoexcited GaAs surfaces studied by transient terahertz time-domain spectroscopy

The transmission characteristics of an air-GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are i nvestigated subsequent to excitation by a femtosecond laser pulse. We find that the total phase change and transmission of a terahertz (THz) probe pul se are dominated by interface effects. This observation has important impli cations in the interpretation of THz time-domain spectroscopy data of absor bing media. We also observe that the THz pulse apparently arrives at the de tector as much as 60 fs earlier when it is transmitted through an optically excited GaAs wafer. This effect is fully explained in terms of a frequency -dependent transmission and phase shift at the air-GaAs interface and is no t associated with superluminal propagation. (C) 2000 Optical Society of Ame rica.