High-Order Sideband Generation in Quantum Wells Driven by Intense THz Radiation: Electron-Hole Recollisions

Non-linear mixing of optical beams with intense terahertz beams has been observed in semiconductor heterostructures both as changes to the absorption spectrum and as sideband generation. Sidebands are generated when a material has a non-linear dielectric response that mixes the electric field of the two beams and produces radiation at the optical frequency plus or minus multiples of the THz frequency. Perturbatively generated sidebands have been observed up to fourth order when the optical beam resonantly excites excitons in GaAs quantum wells. We present here our observation of high-order sideband generation (HSG) from excitons in InGaAs QWs. Sidebands of up to 18$^{th}$ order are observed. From the THz intensity dependence and THz polarization dependence (linear-circular) of the sideband generation, we show that the observed phenomenon is non-perturbative. We note that the exciton system driven by an intense THz field is analogous to an atomic system driven by intense optical fields, as is the case for high-order harmonic generation (HHG). The recollision model used to describe HHG in atoms is then extended to describe HSG in excitons. Experimental results indicate that the recollision model is a valid description of the nature of HSG in excitons.