  Circularly polarized photons are known to generate a directional
helicity-dependent photocurrent in three-dimensional topological insulators at
room temperature. Surprisingly, the phenomenon is readily observed at photon
energies that excite electrons to states far above the spin-momentum locked
Dirac cone and the underlying mechanism for the helicity-dependent photocurrent
is still not understood. We resolve the puzzle through a comprehensive study of
the helicity-dependent photocurrent in (Bi,Sb)_2Te_3 thin films as a function
of the incidence angle of the optical excitation, its wavelength, and the
gate-tuned chemical potential. Our observations allow us to unambiguously
identify the circular photo-galvanic effect as the dominant mechanism for the
helicity-dependent photocurrent. Additionally, we use an analytical calculation
to relate the directional nature of the photocurrent to asymmetric optical
transitions between the topological surface states and bulk bands. The insights
we obtain are important for engineering opto-spintronic devices that rely on
optical steering of spin and charge currents.
