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This study deals with the ultrafast electron-hole plasma expansion of photo-excited carriers in GaAs. This work applies a novel experimental scheme for optical pump-terahertz probe (OPTP) measurements where the state of the photo-excited layer close to the sample surface is probed by means of an internal reflection of a picosecond terahertz (THz) pulse. This approach allows direct probing of the photo-excited layer from its back side. In conjunction with the possibility of phase-sensitive detection of THz pulses, the authors are able to observe the motion of the interface separating the excited and non-excited part of the sample.