Terahertz optical mixing in biased GaAs single quantum wells

The nonlinear mixing of near-infrared (NIR) and terahertz (THz) laser beams is investigated experimentally in a square GaAs quantum well structure, where the symmetry of the sample can be controlled by applying an electric field along the growth direction. The mixing produces sidebands, which appear at omega(sideband)=omega(NIR)+nomega(THz), where n=+/-1,2,.... For a given THz frequency, the intensity of the n=+/-1 sidebands displays two main resonances as a function of the NIR frequency. These resonances are separated by the THz frequency. Their intensity is found to depend strongly on the electric bias and on THz frequency. The n=+/-1 sideband intensity is zero when the sample is unbiased and increases significantly with bias. The sideband is strong when the THz laser frequency is tuned in resonance with an intersubband transition and also at low THz frequencies. The main features of our results are explained qualitatively, except at very high THz intensities, by a perturbative model of the nonlinear susceptibility chi((\n/+1)). At high THz intensities, the resonances are observed to red shift and broaden.