Photocurrents in a topological insulator - an inside view

Topological insulators have been in the focus of condensed-matter research in recent years.  In particular, much effort was dedicated to optically excite and control currents involving the topological surface states. We image the unoccupied bandstructures of a topological insulator as it evolves following an optical excitation, and observe the signature of a photo-induced current. By applying a time-mapping analysis we gain a complete view on the occupied and unoccupied electronic states, and how they are coupled by the optical excitation. This enables us to determine that photocurrents are excited only via the resonant optical transitions, paving the way to optical control of currents in topological insulators.