Spectroscopy of low-dimensional semiconductors with a terahertz free-electron laser

This talk reviews recent experimental studies which we carried out using the free-electron laser (FEL) facility FELBE in Dresden, Germany. Intense, nearly transform-limited ps pulses in the mid-infrared and terahertz (THz) regimes provide unique research opportunities to study novel materials and devices. In high-quality semiconductor quantum wells, we investigate the dynamics of excitons, i.e. two-dimensional, hydrogen-like electron-hole quasi-atoms. Tuning the FEL in resonance with the transtion between the excitonic 2s and 2p states (at about 2 THz) allows us to study the dynamics of intra-excitonic population transfer. Moreover, strong terahertz pumping results in a characteristic Rabi splitting of the 1s exciton state, which is a manifestation of the intra-excitonic Autler-Townes effect. In semiconductor quantum dots, resonant THz excitation between different sublevels is shown to produce an absorption contrast in aperture-less scattering scanning near-field optical microscopy (s-SNOM). This effect allows us to obtain functional s-SNOM images with deep sub-wavelength resolution, where far-infrared absorption by single electrons produces sufficient contrast to map individual quantum dots. In graphene, FEL-based pump-probe spectroscopy reveals different relaxation times for excitation energies above and below the optical phonons. At even smaller photon energies, this two-dimensional semiconductor material exhibits a transition from induced transmission to induced absorption, which is indicative for possible applications as an optical modulator.