Optical injection of spin in topologically protected surface states is a promising route towards realization of spintronics devices based on topological insulators (TIs).
The control over spin current requires the understanding of the electron and spin scattering processes, as well the direct imaging of the coupling between electrons and the optical excitation. Time and angle resolved photoelectron spectroscopy (tr-ARPES) represents the ideal experimental tool to shed light upon these aspects.
We have recently carried out a systematic study of the prototypical TIs, Bi2Se3, by combining tr-ARPES with high photon energy, spin resolution and by comparing its out-of-equilibrium electronic properties to one of several other TIs, characterized by different doping level. All together our results suggest that spatial diffusion of the excited electrons towards the bulk plays a critical role. This effect can be mediated by bulk states and by unoccupied surface resonance, whose presence have been identified by means of spin resolved tr-ARPES.