Regensburg 2013 – scientific programme
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O: Fachverband Oberflächenphysik
O 14: Topological Insulators 2 (jointly with DS, HL, MA, TT)
O 14.6: Talk
Monday, March 11, 2013, 16:45–17:00, H10
Observation of terahertz photocurrents in the topological insulator Bi2Se3 — •Lukas Braun1, Luca Perfetti2, Martin Wolf1, and Tobias Kampfrath1 — 1Physikalische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany — 2Laboratoire des Solides Irradiés, Ecole Polytechnique, Palaiseau cedex, France
Recent experiments have indicated that optical excitation of topological insulators (TIs) with circularly polarized light can induce spin-polarized electron currents along the TI surface. The direction of this photocurrent can be controlled by varying the circular polarization of the driving light from right- to left-handed. So far, only DC photocurrents have been detected [J. W. McIver et al. Nature Nanotechnology 7, 96 (2012)]. Since electrons moving through a solid typically undergo scattering on sub-picosecond time scales, it is highly desirable to generate and detect TI photocurrents with femtosecond time resolution.
Here, we drive ultrashort current bursts in n-doped Bi2Se3 by excitation with a laser pulse (10fs, 800nm, 10nJ). The photocurrent gives rise to the emission of a terahertz (THz) electromagnetic pulse whose transient electric field E(t) is detected by means of electro-optic sampling with a time resolution of 10fs. We observe extremely broadband THz emission covering the range from 10 to 30THz, and the THz intensity is found to depend strongly on the helicity of the pump pulses. A method is presented that allows us to extract the transient current j(t) from the measured E(t). We finally discuss the origin of j(t) and implications for the dynamics of photoexcited TI electrons.