Controlling valley excitons in monolayer MoS₂ with a metasurface

Excitons in monolayer transition metal dichalcogenides (TMDs) are formed at K and K’ points at the boundary of the Brillouin zone and inherent valley degree of freedoms. Using circularly polarized excitation lasers, excitons can be created in a specific valley. In MoS₂, it was found that the photoluminescence preserves the circular polarization at low temperature but not at room temperature, suggesting that the exciton valley depolarization occurs on longer or similar time scale in comparison to exciton recombination times at low temperature. Here, we demonstrate that by placing monolayer MoS₂ onto a metasurface, valley polarized exciton can couple to surface plasmon polaritons and be spatially separate even at room temperature. In addition, we found that emission from excitons in opposite valley can be separated in K space, i.e., excitons from opposite valleys emit photons to different directions.