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 $\overline{\Gamma }$ 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. (letter)