Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

• Published in: Nature communications Vol. 7; no. 1; p. 13259
• Main Authors: Braun, Lukas, Mussler, Gregor, Hruban, Andrzej, Konczykowski, Marcin, Schumann, Thomas, Wolf, Martin, Münzenberg, Markus, Perfetti, Luca, Kampfrath, Tobias
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 31.10.2016; Nature Publishing Group; Nature Portfolio
• Subjects: 140/125; 639/301/119/2792; 639/624/400/561; 639/766/119/1001; 639/766/119/2792; Electrons; Humanities and Social Sciences; multidisciplinary; Population decline; Science; Science (multidisciplinary); Velocity; Germany
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms13259

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi 2 Se 3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents. Surface currents in topological insulators can be controlled by light, but the underlying mechanisms are not well understood. Here, Braun et al . report an ultrafast shift photocurrent at the surface of Ca-doped Bi 2 Se 3 , whereas injection currents are much smaller than expected from asymmetric depopulation of the Dirac cone.