Optical Generation of Terahertz Pulses in Quantum Wells

We report on the generation of terahertz pulses from the optical excitation of excitons in GaAs/Al0.3Ga0.7As quantum wells. In general there are two contributions to the THz signals for nonzero field. The first arises from the creation of excitons in states in which they are already polarized. This leads to a time-dependent polarization that grows with the integrated pulse energy and radiates a short electrical transient. The second one arises from the coherent excitation of both light hole and heavy hole excitons. The time evolution of the coherent superposition of these two exciton states gives rise to charge oscillations that radiate a terahertz signal consisting of several oscillations of the electric field. By applying a field to the sample we can tune the oscillation frequency from 1.4 to 2.6 THz.