  Engineering light-matter interactions up to the strong-coupling regime at
room temperature is one of the cornerstones of modern nanophotonics. Achieving
this goal will enable new platforms for potential applications such as quantum
information processing, quantum light sources and even quantum metrology.
Materials like transition metal dichalcogenides (TMDC) and in particular
tungsten disulfide (WS$_2$) possess large transition dipole moments comparable
to semiconductor-based quantum dots, and strong exciton binding energies
allowing the detailed exploration of light-matter interactions at room
temperature. Additionally, recent works have shown that coupling TMDCs to
plasmonic nanocavities with light tightly focused on the nanometer scale can
reach the strong-coupling regime at ambient conditions. Here, we use ultra-thin
single-crystalline gold nanodisks featuring large in-plane electromagnetic
dipole moments aligned with the exciton transition-dipole moments located in
monolayer WS$_2$. Through scattering and reflection spectroscopy we demonstrate
strong coupling at room temperature with a Rabi splitting of $\sim$108 meV. In
order to go further into the strong-coupling regime and inspired by recent
experimental work by St\"uhrenberg et al., we couple these nanodisks to
multilayer WS$_2$. Due to an increase in the number of excitons coupled to our
nanodisks, we achieve a Rabi splitting of $\sim$175 meV, a major increase of
62%. To our knowledge, this is the highest Rabi splitting reported for TMDCs
coupled to open plasmonic cavities. Our results suggest that ultra-thin
single-crystalline gold nanodisks coupled to WS$_2$ represent an exquisite
platform to explore light-matter interactions.
