Semiconductor quantum structures in high THz fields

Electronic resonances in semiconductor quantum structures such as quantum wells and quantum dots are often accompanied by nonlinear phenomena. The intense, spectrally narrow radiation of a free-electron laser (FEL) is ideally suited for studying these nonlinear effects. We briefly discuss effects that can be treated in a perturbative way, such as the appearance of THz-sidebands around near-infrared absorption lines of quantum wells and quantum dots. Mainly we focus on an effect, which is beyond the perturbative description, namely the Autler-Townes splitting of excitonic absorption lines. Photogenerated electrons and holes in semiconductors form excitons, which exhibit a hydrogen-like energy spectrum. Due to the presence of the crystal lattice, however, the energy is scaled to the meV range. Excitation with intense THz pulses from an FEL "dresses" the excitonic states and leads to a splitting of energy levels. This splitting can be observed by probing the samples's transmission with broadband near-infrared radiation. While the main features, such as the dependence of the peak position on the THZ intensity, can be explained by a simple two-level model, other observations such as the relative strength of the peaks require more complex modelling. The mauin results presented here were obtained by M. Wagner, D. Stehr, H. Schneider and M. Helm on a sample grown by A. M. Andrews, S. Schartner, and G. Strasser.