  In the strong light-matter coupling regime realized e.g. by integrating
semiconductors into optical microcavities, polaritons as new hybrid
light-matter quasi-particles are formed. The corresponding change in the
dispersion relation has a large impact on optics, dynamics and transport
behaviour of semiconductors. In this work, we investigate the strong-coupling
regime in hBN-encapsulated MoSe$_2$ monolayers focusing on exciton-polariton
diffusion. Applying a microscopic approach based on the exciton density matrix
formalism combined with the Hopfield approach, we predict a drastic increase of
the diffusion coefficients by two to three orders of magnitude in the strong
coupling regime. We explain this behaviour by the much larger polariton group
velocity and suppressed polariton-phonon scattering channels with respect to
the case of bare excitons. Our study contributes to a better microscopic
understanding of polariton diffusion in atomically thin semiconductors.
