Observation of below-bandgap excited terahertz emission in the action spectra of GaAs/AlGaAs multiple quantum wells

We present terahertz emission of optically pumped GaAs/AlGaAs multiple quantum wells (MQWs). The action spectra exhibit emission even at excitation energies below the bandgap. Results show that the radiation could not have come from the bulk layers and is inferred to be due to shallow states under the band edge.