of $\mathcal{F} / \mathcal{C} \simeq 40\%$ can be reached at RT for the strongly coupled WS$_2$ monolayer. As mentioned above, we understand this surprisingly robust contrast by invoking the fact that under strong coupling conditions, valley relaxation is outweighted by the faster Rabi energy exchange between the exciton of each valley and the corresponding plasmonic OSO mode, as described in the Supporting Information (Sec. A). From the polaritonic point of view, the local dephasing and scattering processes at play on bare excitons -that erase valley contrasts on a bare WS2 flake as observed in the Supporting Information (Sec. G)- are reduced by the delocalized nature of the chiralitonic state, a process akin to motional narrowing and recently observed on other polaritonic systems [@Whittaker1996; @Tartakovskii2016; @Chen2017; @Kleemann2017].

![(a) Differential PL dispersion spectrum for left and right circularly polarized excitations. The shaded regions in all panels are removed by the laser line filter, and the cross-cuts are taken at $2$ eV. (b) Differential angle-resolved reflection spectrum for left and right circularly polarized light. (c), (e) Angle-resolved spectrum of the normalized coefficient $S_1^{\textrm{\tiny out}}|_{\textrm{\tiny TM(TE)}}/S_0$ of the PL Stokes vector for a TM(TE) polarized excitation (see text for details). Note that we have 