  Dark excitons in transition metal dichalcogenides (TMD) have been so far
neglected in the context of polariton physics due to their lack of oscillator
strength. However, in tungsten-based TMDs, dark excitons are known to be the
energetically lowest states and could thus provide important scattering
partners for polaritons. In this joint theory-experiment work, we investigate
the impact of the full exciton energy landscape on polariton absorption and
reflectance. By changing the cavity detuning, we vary the polariton energy
relative to the unaffected dark excitons in such a way that we open or close
specific phonon-driven scattering channels. We demonstrate both in theory and
experiment that this controlled switching of scattering channels manifests in
characteristic sharp changes in optical spectra of polaritons. These spectral
features can be exploited to extract the position of dark excitons. Our work
suggests new possibilities for exploiting polaritons for fingerprinting
nanomaterials via their unique exciton landscape.
