  Systems supporting exciton-polaritons represent solid-state optical platforms
with a strong built-in optical nonlinearity provided by exciton-exciton
interactions. In conventional semiconductors with hydrogen-like excitons the
nonlinearity rate demonstrates the inverse scaling with the binding energy.
This makes excitons stable at room temperatures weakly interacting, which
obviously limits the possibilities of practical applications of the
corresponding materials for nonlinear photonics. We demonstrate experimentally
and theoretically, that these limitations can be substantially softened in
hybrid perovskites, such as MAPbBr3 due to the crucial role of the polaron
effects mediating the inter-particle interactions. The resulting
exciton-polaron-polaritons remain both stable and strongly interacting at room
temperature, which is confirmed by large nonlinear blueshifts of lower
polariton branch energy under resonant femtosecond laser pulse excitation. Our
findings open novel perspectives for the management of the exciton-polariton
nonlinearities in ambient conditions.
