Below-bandgap excited, terahertz emission of optically pumped GaAs/AlGaAs multiple quantum wells

We present terahertz (THz) emission of optically pumped 5-nm GaAs/AlGaAs multiple quantum wells (MQWs) even at excitation energies below the bandgap. The excitation energy corresponding to the peak THz emission is red-shifted with respect to the photoluminescence (PL) and photoluminescence excitation peak. This is attributed to a transient bandgap renormalization that occurs on the same time scale as the generation of the THz transients. Moreover, an emission shoulder at ∼40 meV below the THz emission peak was observed. Deep level transient spectroscopy results do not indicate that this is due to electron traps. However, an indistinct LO phonon-related, below-bandgap PL feature was seen at low temperature that coincides well with the observed THz radiation feature. It is proposed that the THz action spectrum may be sensitive to phonon-mediated processes in contrast to more conventional optical spectroscopy techniques, albeit at room temperature.