We report on THz emission from plasma oscillations in semiconductors excited by femtosecond optical pulses. Time-resolved correlation measurements are performed on $p-i-n$ and $n$-doped GaAs structures. In $p-i-n$ structures coherent oscillations of the hot photogenerated carrier plasma emit THz radiation. A fundamentally new emission process is proposed in $n$-doped GaAs structures. Here, the screening of the surface field starts plasma oscillations of the cold electrons in the GaAs bulk leading to an efficient emission of few-cycle THz radiation.

• Received 12 April 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.3038