Photogalvanic Effect in Topological Insulator Surface States Investigated by Time-resolved Two Photon Photoemission Spectroscopy

Abstract

We present energy-momentum mapping of excited surface electrons in the topological insulator Sb₂Te₃ by using time- and angle-resolved two-photon photoemission spectroscopy combined with polarization-variable midinfrared pulsed laser excitation. It is demonstrated that a direct optical transition from the occupied to the unoccupied part of the surface Dirac-cone permits the linear and circular photogalvanic effect, which thereby enables us to control the electric surface photocurrent by laser polarization. Moreover, our photocurrent analysis on excited electrons directly visualizes ultrafast current dynamics in the surface Dirac-cone dispersion in the time domain. Our observations pave the way for coherent optical control over surface Dirac electrons in topological insulators.