Spin currents in polariton waveguides driven by bright and dark excitons

The optical spin Hall effect enables the spatial separation of four domains of circularly polarized light from a linearly polarized source through photonic spin-orbit coupling, which arises from non-degenerate transverse-electric and transverse-magnetic modes. This mechanism is ineffective for incoherent photoluminescence. In transition metal dichalcogenide monolayers, bright excitons possess in-plane electric dipoles and strong oscillator strengths, whereas dark excitons have out-of-plane dipoles. Here, by embedding WS₂ monolayers in waveguide microcavities, we demonstrate strong coupling between bright excitons and photonic modes, forming exciton-polaritons. Under non-resonant pumping, dark excitons become strongly populated and feed both transverse-magnetic- and transverse-electric-like exciton-polariton modes, each with an out-of-plane electric-field component. We observe the optical spin Hall effect in waveguide polaritons, leading to the formation of eight polarization domains and spin-polarized polariton currents propagating up to 92 ± 1 μm at room temperature. These results establish transition metal dichalcogenide-based polaritonic waveguides as a promising platform for room-temperature spin-optoelectronic devices.