Valley-polarized exciton–polaritons in a monolayer semiconductor

Abstract

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. Exploitation of the valley electronic structure of transition metal dichalcogenides with exciton–polaritons is an elusive challenge. Now, valley-polarized exciton–polaritons in monolayers of MoS2 have been demonstrated.