Ultrafast spin-polarized electron dynamics in the unoccupied topological surface state of Bi2Se3

The three-dimensional topological insulator Bi2Se3 presents two cone-like dispersive topological surface states centered at the (Gamma) over bar point. One of them is unoccupied in equilibrium conditions and located 1.8 eV above the other one lying close to the Fermi level. In this work we employ time-and angle-resolved photoemission spectroscopy with circularly polarized pump photons to selectively track the spin dynamics of the empty topological states. We observe that spin-polarized electrons flow along the topological cone and recombine towards the unpolarized bulk states on a timescale of few tens of femtoseconds. This provides direct evidence of the capability to trigger a spin current with circularly polarized light.