The Dirac-cone surface states of topological insulators are characterized by a chiral spin texture in k-space with the electron spin locked to its parallel momentum. Mid-infrared pump pulses can induce spin-polarized photocurrents in such a topological surface state by optical transitions between the occupied and unoccupied part of the Dirac cone. We monitor the ultrafast dynamics of the corresponding asymmetric electron population in momentum space directly by time- and angle-resolved two-photon photoemission (2PPE). The elastic scattering times of 2.5 ps deduced for Sb2Te3 corresponds to a mean-fee path of 0.75 μm in real space.
K. Kuroda, J. Reimann, J. Güdde, and U. Höfer, "Momentum space view of the ultrafast dynamics of surface photocurrents on topological insulators," Proc. SPIE 10102, Ultrafast Phenomena and Nanophotonics XXI, 101020Q (Presented at SPIE OPTO: January 31, 2017; Published: 23 February 2017); https://doi.org/10.1117/12.2250295.
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