  Atomically thin crystals of transition metal dichalcogenides (TMDs) host
excitons with strong binding energies and sizable light-matter interactions.
Coupled to optical cavities, monolayer TMDs routinely reach the regime of
strong light-matter coupling, where excitons and photons admix coherently to
form quasiparticles known as polaritons up to room temperature. Here, we
explore the two-dimensional nature of TMD polaritons with cavity-assisted
hyperspectral imaging. Using extended WS$_2$ monolayers, we establish the
regime of strong coupling with a scanning microcavity to map out polariton
properties and correlate their spatial features with intrinsic and extrinsic
effects. We find a high level of homogeneity, and show that polariton splitting
variations are correlated with intrinsic exciton properties such as oscillator
strength and linewidth. Moreover, we observe a deviation from thermal
equilibrium in the resonant polariton population, which we ascribe to
non-perturbative polariton-phonon coupling. Our measurements reveal a
promisingly consistent polariton landscape, and highlight the importance of
phonons for future polaritonic devices.
