Action Spectra of GaAs/AlGaAs Multiple Quantum Wells Exhibiting Terahertz Emission Peak at Excitation Energies Below the Bandgap

Abstract

We present terahertz (THz) emission of optically pumped 5- and 12.5-nm GaAs/AlGaAs multiple quantum wells (MQWs) even at excitation energies below the bandgap. The effect of the externally applied magnetic field parallel to the MQW plain is to enhance or reduce the THz emission intensity for the up or down direction, respectively. For both MQW samples, the excitation energy corresponding to the peak THz emission is red-shifted with respect to the photoluminescence (PL) and PL excitation peak. This is attributed to an instantaneous bandgap renormalization that occurs at the same time scale as the generation of the THz transients. Moreover, an emission shoulder at ∼40 meV below the THz emission peak of the 5-nm MQW was observed. Deep level transient spectroscopy results do not indicate this to be due to electron traps. However, an indistinct LO phonon-related, below-bandgap PL feature was seen at low temperature, which agrees well with the observed THz radiation feature. These results suggest that the THz action spectrum is more sensitive to phonon-mediated processes as compared to more conventional optical spectroscopy techniques.