Abstract

Until recently, it was believed that the generation of 1 terahertz pulses from the surface of a bulk semiconductor was caused by transient transport currents in the surface depletion layer of the material. We have generated terahertz pulses from an Al_0.3Ga_0.7As/ GaAs MQW sample after illumination with a 100 fs laser pulse, with an amplitude comparable to the one from a bulk GaAs sample. Since transport current in a quantum well is inhibited significantly, this proves that the radiation is generated by a recently proposed additional mechanism used to explain the creation of terahertz pulses from the surface of semiconductors. This explanation is based on the optical creation of electron-hole 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. In a wide MQW sample consisting of 15 periods of 175-Å GaAs wells separated by 150-Å Al_0.3Ga_0.7 As barriers, we observe an oscillatory terahertz signal lasting for several picoseconds when we coherently excite both the light hole and the heavy hole excitons. The oscillation frequency matches the light hole-heavy hole energy splitting, and we attribute the signal to a beating between the two.

© 1992 Optical Society of America