  Rhenium disulfide (ReS2) belongs to group-VII transition metal dichalcogenide
(TMDs) with attractive properties such as exceptionally high refractive index
and significant oscillator strength, large in-plane birefringence, and good
chemical stability. Unlike most other TMDs, the peculiar optical properties of
ReS2 persist from bulk to the monolayer, making this material potentially
suitable for applications in optical devices. In this work, we demonstrate with
unprecedented clarity the strong coupling between cavity modes and excited
states, which results in a strong polariton interaction, showing the interest
of such materials as a solid-state counterpart of Rydberg atomic systems.
Moreover, we definitively clarify the nature of important spectral features,
shedding light on some controversial aspects or incomplete interpretations and
demonstrating that their origin is due to the interesting combination of the
very high refractive index and the large oscillator strength expressed by these
TMDs.
