Chiral interaction between spin-momentum locked photons and surface electrons in topological insulators

Siyuan Luo, Li He, Mo Li

Published 2017-07-14Version 1

The propagation of both electrons and photons becomes chiral when their momentum and spin are correlated in forms such as spin-momentum locking. For the surface electrons in three-dimensional topological insulators (TIs), their spin is locked to the transport direction. For photons in optical fibers and photonic waveguides, they carry transverse spin angular momentum (SAM) which is also locked to the propagation direction. A direct connection between chiral electrons and chiral photons occurs in TIs with lifted spin degeneracy, which leads to spin-dependent selection rules of optical transition and results in phenomena such as circular photogalvanic effect (CPGE). Here, we demonstrate an optoelectronic device that integrates a TI with a chiral photonic waveguide. Interaction between the photons in the transverse-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, the device works in a non-reciprocal way such that changing the light propagation direction reverses the photon spin and thus the direction of the photocurrent in the TI. This novel device provides a chiral interface that directly converts the photon propagation path to the direction and spin polarization of the photo-excited current in the TI. It represents a new way of implementing coupled spin-orbit interaction between electrons and photons and may lead to significant applications in opto-spintronics and quantum information processing.