Spin-momentum locked interaction between guided photons and surface electrons in topological insulator Bi₂Se₃

The propagation of electrons and photons can respectively have the spin-momentum locking effect (SML) which correlates the spin with the linear momentum. A direct connection between the electron and photon spin occurs in topological insulators (TIs) with lifted spin degeneracy, which results in circular photogalvanic effect. Here, we study the photogalvanic effects in exfoliated Bi₂Se₃ by measuring the dependence of photoresponse on crystal orientation. In contrast to the linear photogalvanic effect that shows 3-fold rotation symmetry on crystal orientation, an isotropic photoresponse is observed for the circular photogalvanic effect, indicates the corresponding optical transitions indeed involve surface bands. Furthermore, we demonstrate an optoelectronic device that integrates a TI with a photonic waveguide. Interaction between the photons in the waveguide, which carries optical spin, and the surface electrons in a Bi₂Se₃ generates a directional photocurrent. Because of SML, the device works in a non-reciprocal way such that changing the light propagation direction reverses the optical spin and thus the direction of the photocurrent in TI.