Valley-selective optical Stark effect of exciton-polaritons in a monolayer semiconductor

Microcavity exciton-polaritons based on transition metal dichalcogenide monolayers (TMDs) are a promising platform for coherent valleytronics, exhibiting valley-dependent phenomena at room-temperature. Using polarization-dependent transient reflectance, we demonstrate the valley-exclusive nature of the optical Stark effect in WS2 exciton-polaritons. We observe a simultaneous shift of both polariton branches when pump and probe are co-polarized and no appreciable shift when they are cross-polarized, demonstrating a polarization-selective stark shift in exciton-polaritons. This work highlights how the unique features of TMD exciton-polaritons can give rise to new polaritonic phenomena.