Figure 1. Ultrafast photodetector based on topological insulators.

(a) Circularly polarized photons of the pump pulse control surface currents in Bi₂Se₃. (b) The time-resolved photocurrent I_sampling is read-out by a silicon-based Auston-switch triggered by a probe pulse at a time-delay Δt. The time-integrated photocurrent I_photo is measured between the electronic contacts, which form two co-planar striplines. (c) Optical microscope image of a Bi₂Se₃ film with 75 nm thickness, contacted by two Au striplines. Scale bar, 5 μm.