  Engineering the properties of quantum materials via strong light-matter
coupling is a compelling research direction with a multiplicity of modern
applications. Those range from modifying charge transport in organic molecules,
steering particle correlation and interactions, and even controlling chemical
reactions. Here, we study the modification of the material properties via
strong coupling and demonstrate an effective inversion of the excitonic
band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground
state. In our experiments, we harness the strong light-matter coupling between
cavity photon and the high energy, spin-allowed bright exciton, and thus
creating two bright polaritonic modes in the optical bandgap with the lower
polariton mode pushed below the WSe2 dark state. We demonstrate that in this
regime the commonly observed luminescence quenching stemming from the fast
relaxation to the dark ground state is prevented, which results in the
brightening of this intrinsically dark material. We probe this effective
brightening by temperature-dependent photoluminescence, and we find an
excellent agreement with a theoretical model accounting for the inversion of
the band ordering and phonon-assisted polariton relaxation.
