  In this letter, we address the chiral properties of valley exciton-polaritons
in a monolayer of WS2 in the regime of strong light-matter coupling with a
Tamm-Plasmon resonance. We observe that the valley polarization, which
manifests in the circular polarization of the emitted photoluminescence, is
strongly enhanced in comparison to bare WS2 monolayers, and can even be
observed under non-resonant excitation at ambient conditions. We study the
relaxation and decay dynamics of exciton-polaritons in our device, and present
a microscopic model to explain the wave vector-dependent valley depolarization
as an interplay of bright and dark states, electron-hole exchange interaction
and the linear polarization splitting inherent to the microcavity.
