  Monolayer transition metal dichalcogenides, known for exhibiting strong
excitonic resonances, constitute a very interesting and versatile platform for
investigation of light-matter interactions. In this work we report on a strong
coupling regime between excitons in monolayer WSe2 and photons confined in an
open, voltage-tunable dielectric microcavity. The tunability of our system
allows us to extend the exciton-polariton state over a wide energy range and,
in particular, to bring the excitonic component of the lower polariton mode
into resonance with other excitonic transitions in monolayer WSe2. With
selective excitation of spin-polarized exciton-polaritons we demonstrate the
valley polarization when the polaritons from the lower branch come into
resonance with a bright trion state in monolayer WSe2 and valley depolarization
when they are in resonance with a dark trion state.
