Terahertz dynamics in confined magnetoexcitons

We have investigated the linear and nonlinear response of confined magnetoexcitons to intense terahertz (THz) radiation. By monitoring photoluminescence from THz-driven GaAs quantum wells, we have observed for the first time internal transitions in direct excitons. The spectrum of excitations is enriched by the complexities of the valence band and is well explained by an effective-mass theory. At high THz-field strengths, the emission properties of the driven quantum wells are completely dominated by new near-band-gap features, or optical sidebands, which appear at frequencies ω_NIR ± 2nω_THz, where ω_NIR is the exciton-creation frequency, ω_THz is the driving frequency, and n is an integer. The intensity of the sidebands exhibits pronounced enhancement when ω_THz coincides with one- and two-photon exciton internal transitions, providing new and accurate information on the internal dynamics of excitons.