Optoelectronic transport in topologically protected quantum systems

Topological insulators are a novel quantum phase of matter. In the bulk, a topological in- sulator behaves like an ordinary insulator with a band gap. At the surface, gapless states exist, which have a spin-helical Dirac dispersion. This peculiar spin structure renders topo- logical insulators promising materials for spintronic applications. However, The character- ization and control of these surface states via transport experiments is often hindered by residual bulk contributions. In this thesis, the optoelectronic transport at the surface of three-dimensional topological insulators is investigated by photocurrent microscopy. Employing time-resolved photocurrent spectroscopy, we show that surface currents in the topological insulator Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermo- electric and drift currents in the bulk. Furthermore, we study the optoelectronic transport in Bi2Se3 and (Bi0.5Sb0.5)2Te3 films grown by molecular beam epitaxy. In spatially resolved experiments, we find reproducible, submicron photocurrent patterns generated by long-range chemical potential fluctuations, occurring predominantly at the topological insulator/substrate interface. We fabricate nano- plowed constrictions which comprise single potential fluctuations. Hereby, we can quantify the magnitude of the disorder potential to be in the meV range. The results further suggest a dominating photo-thermoelectric current generated in the surface states in such nanoscale constrictions. By spatially resolved photocurrent microscopy at low temperatures, we uncover spin- polarized one-dimensional transport channels in these thin topological insulator films. The transport is quantized at a single conductance quantum 1e2/h without a magnetic field, and it can be switched on and off by an electrostatic field-effect. The transport channels are optically induced, and they originate from a lateral quantum confinement of non-topological surface states with strong Rashba-splitting at the nanofabricated edges of the films. Within the topological classification scheme, the quantum Hall state is regarded as a two-dimensional topologically non-trivial insulator. We use AlGaAs/GaAs-based quantum point contacts as mesoscopic detectors to locally analyze the flow of photogenerated charge carriers along the emerging quantum Hall edge states. We demonstrate that photogenerated electrons can be directly injected into an edge channel, transported across several tens of micrometers and read-out on-chip by the quantum point contact.