Evidence for surface-generated photocurrent in (Bi,Sb)$_{2}$Se$_{3}$ and (Bi,Sb)$_{2}$Te$_{3}$ thin films*

Illumination with circularly polarized light is known produce a helicity-dependent photocurrent in topological insulators such as Bi$_{2}$Se$_{3}$ [e.g. Nature Nanotech. \textbf{7}, 96 (2012)]. However, the exact origin of this effect is still unclear since it is observed with photons well above the bulk band gap. We report measurements of the polarization-dependent photocurrent in a series of (Bi,Sb)$_{2}$Se$_{3}$ thin films with different carrier concentrations and find that the photocurrent is enhanced as we increase the population of the surface states. This finding is supported by a study of helicity-dependent photocurrents in back-gated (Bi,Sb)$_{2}$Te$_{3}$ thin films, where the chemical potential is varied electrostatically. By illuminating our samples at different wavelengths, we show that the helicity-dependent photocurrent is enhanced when the photon energy approaches the energy difference between the lowest and first excited (unoccupied) topological surface states. This leads us to attribute the helicity-dependent photocurrent in topological insulators to optical excitations between these two spin-textured surface states. We will also discuss experiments imaging the spatial variation of these helicity-dependent photocurrents.