Intra-excitonic coherent nonlinear optics in quantum wells: the Autler-Townes effect and beyond

Excitons in quantum wells represent a quasi-hydrogenic system, scaled down in energy to the meV (or THz) range due to the effective mass of the electrons and the dielectric constant. We take advantage of a free-electron laser as a narrow-band, intense THz source and drive the intra-excitonic heavy-hole 1s-2p transition in an undoped GaAs/AlGaAs multiquantum well (MQW). Probing the near-bandgap absorption with broad-band light from a 10 fs Ti: sapphire laser, we demonstrate the Autler-Townes splitting of the 1s exciton, giving evidence for dressed states. While the basic features at relatively low intensities follow the predictions of a simple two-level model, strong deviations are observed at higher THz fields in the 10 kV/cm range. At such field strengths, the rotating-wave approximation is not valid anymore, and also the two-level approximation breaks down, as higher excitonic bound states and the continuum cannot be neglected. Striking features are a peak reversal and overall blue shift of the Rabi sidebands with increasing field strength and a saturation of the splitting, going along with a line broadening that may indicate the onset of field ionization. Relevant for possible applications, signatures of this AC Stark effect are visible up to room temperature, with a THz induced threefold (at 200 K) near-infrared transmission modulation on a picosecond time scale. The above results are corroborated by recent measurements on an InGaAs/GaAs MQW with narrower exciton linewidth and corresponding calculations based on the semiconductor Bloch equations.