  The interaction between a mobile quantum impurity and a bosonic bath leads to
the formation of quasiparticles, termed Bose polarons. The elementary
properties of Bose polarons, such as their mutual interactions, can differ
drastically from those of the bare impurities. Here, we explore Bose polaron
physics in a two-dimensional nonequilibrium setting by injecting $\sigma^-$
polarised exciton-polariton impurities into a bath of coherent $\sigma^+$
polarised polaritons generated by resonant laser excitation of monolayer
MoSe$_2$ embedded in an optical cavity. By exploiting a biexciton Feshbach
resonance between the impurity and the bath polaritons, we tune the interacting
system to the strong-coupling regime and demonstrate the coexistence of two new
quasiparticle branches. Using time-resolved pump-probe measurements we observe
how polaron dressing modifies the interaction between impurity polaritons.
Remarkably, we find that the interactions between high-energy polaron
quasiparticles, that are repulsive for small bath occupancy, can become
attractive in the strong impurity-bath coupling regime. Our experiments provide
the first direct measurement of Bose polaron-polaron interaction strength in
any physical system and pave the way for exploration and control of many-body
correlations in driven-dissipative settings.
