Abstract

Until recently it was believed that THz radiation from the surface of a semiconductor was generated by transient transport currents in the surface depletion field of the semiconductor after optical excitation with an ultrashort laser pulse. [1] In quantum wells however, transport current is inhibited significantly allowing us to prove finally a recently proposed additional explanation for the generation of THz radiation from the surface of bulk semiconductors. [2] The new explanation is based on the optical creation of e-h pairs in states in which they are already polarized. This leads to a time-dependent total polarization P that grows with the integrated pulse energy and hence radiates an electrical transient according to E~∂²P/∂t². This is demonstrated by the generation of THz radiation in a MQW sample consisting of 50 periods of 105 Å thick GaAs, separated by 107 Å thick Al_0.3Ga_0.7As barriers, after illumination with a 100 fs laser pulse (fig. 1).

© 1992 Optical Society of America