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.

单层过渡金属二卤化物是二维（2D）直接带隙半导体，在电子结构中具有简并但不等价的“谷”，可以被偏振光选择性地激发。当这些材料与光子强耦合时，已经观察到光和物质（激子-极化子）的相干叠加，但是这些杂化准粒子没有利用单层半导体的谷值敏感度。在这里，我们观察到MoS ₂单层中的谷极化激子-极化子嵌入介电微腔中。这些光物质准粒子发出具有光谱Rabi分裂和反交叉的偏振光，表明在拓扑上分离的自旋耦合谷中存在强耦合激子-极化子。在高合作态下，间隔极去极化和腔修饰的激子动力学相互作用，导致谷极化的激子-极化子在室温下持续存在，这与裸露的单层极化消失不同。实现在室温下工作的偏振敏感极化子器件为在光和物质的相干态中操纵新颖的谷自由度提供了一条途径。