Direct characterization of terahertz radiation from the dynamics of the semiconductor surface field

We report on a time-resolved electro-optic sampling of the photocarrier-induced surface field dynamics by a midband gap probe beam from a femtosecond fiber laser. By measuring the ultrafast surface field variation, we are able to derive the undistorted terahertz wave form radiated from the semiconductor surface that is excited by the femtosecond laser pulses. The derived wave form agrees well with the directly measured terahertz radiation at the far field. The peak frequency of terahertz radiation is found to increase with the carrier density, which can be explained in terms of field dynamics. (C) 2000 American Institute of Physics. [S0003-6951(00)01944-6].