Exciton-Polariton Dynamics of a Monolayer Semiconductor Coupled to a Microcavity*

Strong light-matter interactions, evidenced by exciton-polariton states, have been observed in the two-dimensional limit with monolayer transition metal dichalcogenides (TMDs) embedded in a microcavity\footnote{~X. Liu, \textit{et al}. \textit{Nature Photon.} \textbf{9}, 30 (2015)}. Because of the valley degree of freedom in monolayer TMDs, these hybrid light-matter states can exhibit valley polarization as in a bare monolayer, with strongly-coupled dynamics determined by the relative rates of exciton relaxation and intervalley scattering, which can be highly modified in on-resonant cavities\footnote{~Y-J. Chen, \textit{et al}. \textit{CLEO: Science and Innovations.} STu3F--2 (2016)}. Here, we test this intuitive picture of the polarized exciton-polariton dynamics with monolayer MoS$_2$ coupled to detuned cavities. Upper and lower polariton branches exhibit distinct decay rates indicative of different cavity dynamics. As with on-resonant, strongly-coupled exciton-polaritons, the weakly-coupled regime causes exciton-polariton valley polarization to persist at room temperature, demonstrating that dynamics of valley-polarized excitations can be controlled by engineering light-matter interactions.