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-ind uced surface field dynamics by a midband gap probe beam from a femtosecond fiber laser. By measuring the ultrafast surface field variation, we are abl e to derive the undistorted terahertz wave form radiated from the semicondu ctor surface that is excited by the femtosecond laser pulses. The derived w ave form agrees well with the directly measured terahertz radiation at the far field. The peak frequency of terahertz radiation is found to increase w ith the carrier density, which can be explained in terms of field dynamics. (C) 2000 American Institute of Physics. [S0003-6951(00)01944-6].