The propagation of electrons and photons can respectively have thespin-momentum locking effect which correlates the spin with the linearmomentum. For the surface electrons in three-dimensional topological insulators(TIs), their spin is locked to the transport direction. For photons in opticalfibers and photonic waveguides, they carry transverse spin angular momentum(SAM) which is also locked to the propagation direction. A direct connectionbetween the electronic and the optical spins occurs in TIs with lifted spindegeneracy, which leads to spin-dependent selection rules of opticaltransitions and results in phenomena such as circular photogalvanic effect(CPGE). Here, we demonstrate an optoelectronic device that integrates a TI witha photonic waveguide. Interaction between the photons in thetransverse-magnetic (TM) mode of the waveguide, which carries transverse SAM,and the surface electrons in a Bi2Se3 layer generates a directional,spin-polarized photocurrent. Because of optical spin-momentum locking, thedevice works in a non-reciprocal way such that changing the light propagationdirection reverses the photon spin and thus the direction of the photocurrentin the TI. This novel device provides a directional interface that directlyconverts the photon propagation path to the direction and spin polarization ofthe photo-excited surface current in the TI. It represents a new way ofimplementing coupled spin-orbit interaction between electrons and photons andmay lead to significant applications in opto-spintronics and quantuminformation processing.
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