Exciton–Polariton Valley Hall Effect in Monolayer Semiconductors on Plasmonic Metasurface

Excitons in monolayer transition metal dichalcogenides (TMDs) possess the valley degree of freedom (DOF), which is regarded as a pseudospin (in addition to charge and spin DOF) and can be addressed optically by using polarized light. Incorporating monolayer TMDs into an optical microcavity in the strong coupling regime further enables the formation of valley polaritons that are half-light and half-matter quasiparticles with addressable spin and momentum through the spin-orbit interactions of light, in analogy with the spin-Hall effect in electronic systems. By placing monolayer TMDs on a plasmonic metasurface to enable strong coupling between excitons and surface plasmon polaritons (SPPs), we report here the observation of valley resolved polaritons in momentum space and a large separation in real space. The directional coupling of valley polaritons originated from the intrinsic spin-momentum locking associated with SPPs, resembling a photonic version of the valley Hall effect for polaritons. The spatially routed valley polaritons provide a unique pathway for transporting and detecting the valley DOF through circular polarization of light for valleytronic applications.