Valley-Polarized Exciton-Polaritons in a Monolayer Semiconductor Embedded in a Microcavity*

Two-dimensional transition metal dichalcogenides (TMDs) are semiconductors that exhibit degenerate, but inequivalent, valleys at their $K$-point band gaps which selectively couple to circularly-polarized light fields. Coherent hybrid states of light and matter, exciton-polaritons, have been observed when monolayer TMDs strongly interact with photon fields in a microcavity~\footnote{X. Liu, T. Galfsky, Z. Sun, F. Xia, E.-C. Lin, Y.-H. Lee, S. K\”{e}na-Cohen, and V. M. Menon. \textit{Nature. Photon.} \textbf{9}, 30 (2015)}. The degree of polarization is determined by the relative rates of exciton and intervalley relaxation, which can be modified for microcavity exciton-polaritons. Preservation of valley-polarization in a microcavity at room temperature is compared to the nearly zero polarization for bare monolayer MoS$_2$ on SiO$_2$, demonstrating cavity-modified relaxation dynamics of the coherent valley-specific exciton-polaritons. These results suggest promising opto-electronic applications for valley-based polaritonic and photonic devices integrating monolayer TMDs.