Efficient THz sideband generation in GaAs/AlGaAs multi quantum wells

We have investigated the third-order nonlinear mixing process between a near-infrared laser and a free-electron laser in an undoped symmetric GaAs/AlGaAs multi quantum well. AC THz electric fields which couple strongly with intraband excitations in semiconductors can lead to spectral sidebands of simultaneous interband excitation. In this nonlinear mixing process a near-infrared (NIR) laser beam is mixed with the THz beam to generate sidebands around the NIR frequency with a frequency spacing equal to the THz frequency or multiples of it: ω = ω_NIR ± n × ω_THz (with integer n). We have investigated the third-order nonlinear mixing process between a near-infrared laser and a free-electron laser (FEL) in an undoped symmetric GaAs/AlGaAs multi quantum well. The sample is studied in transmission under illumination by FELBE, the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. We use the tunability of the FEL to study the dependence of the mixing efficiency on THz wavelength. We find resonances related to heavy- to light-hole transitions, but also to the heavy-hole intra-exciton 1s-2p transition. It turns out that the conversion efficiency of the n=+2 sideband is largest (up to 0.1 %) for the 1s-2p intra-exciton transition, which is comparable to values previously reported for n=+1.