Valley-polarized exciton–polaritons in a monolayer semiconductor

Single layers of transition metal dichalcogenides are two-dimensional 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-sensitive excitations of monolayer transition metal dichalcogenides. Here, we demonstrate evidence for valley polarized exciton-polaritons in monolayers of MoS$_2$ embedded in a dielectric microcavity. Unlike traditional microcavity exciton-polaritons, these light-matter quasiparticles emit polarized light with spectral Rabi splitting. The interplay of cavity-modified exciton dynamics and intervalley relaxation in the high-cooperativity regime causes valley polarized exciton-polaritons to persist to 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.