Valley-polarized exciton-polaritons in a monolayer semiconductor

Single layers of transition metal dichalcogenides are two-dimensional (2D) direct-bandgap semiconductors with degenerate, but inequivalent, 'valleys' in the electronic structure that can be selectively excited by polarized light. Coherent superpositions of light and matter, exciton-polaritons, have been observed when these materials are strongly coupled to photons, but these hybrid quasiparticles do not harness the valley sensitivity of the monolayer semiconductors. Here, we observe valley-polarized exciton-polaritons in monolayers of MoS2 embedded in a dielectric microcavity. These light-matter quasiparticles emit polarized light with spectral Rabi splitting and anticrossing indicative of strongly coupled exciton-polaritons in the topologically separate spin-coupled valleys. The interplay of intervalley depolarization and cavity-modified exciton dynamics in the high-cooperativity regime causes valley-polarized exciton-polaritons to persist at room temperature, distinct from the vanishing polarization in bare monolayers. Achieving polarization-sensitive polaritonic devices operating at room temperature presents a pathway for manipulating novel valley degrees of freedom in coherent states of light and matter.