  Two-dimensional (2D) semiconducting microcavity, where exciton-polaritons can
be formed, constitues a promising setup for exploring and manipulating various
regimes of light-matter interaction. Here, the coupling between 2D excitons and
metallic cavity photons is studied by using first-principles propagator
technique. The strength of exciton-photon coupling is characterised by its Rabi
splitting to two exciton-polaritons, which can be tuned by cavity thickness.
Maximum splitting of 128 meV is achieved in phosporene cavity, while remarkable
value of about 440 meV is predicted in monolayer hBN device. The obtained Rabi
splittings in WS$_2$ microcavity are in excellent agreement with the recent
experiments. Present methodology can aid in predicting and proposing potential
setups for trapping robust 2D exciton-polariton condensates.
