  Strong coupling between lead halide perovskite materials and optical
resonators enables both the polaritonic control of the photophysical properties
of these emerging semiconductors and the observation of novel fundamental
physical phenomena. However, the difficulty to achieve optical-quality
perovskite quantum dot (PQD) films showing well-defined excitonic transitions
has prevented the study of strong light-matter coupling in these materials,
central to the field of optoelectronics. Herein we demonstrate the formation at
room temperature of multiple cavity exciton-polaritons in metallic resonators
embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD)
solids, revealed by a significant reconfiguration of the absorption and
emission properties of the system. Our results indicate that the effects of
biexciton interaction or large polaron formation, frequently invoked to explain
the properties of PQDs, are seemingly absent or compensated by other more
conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong
coupling enables a significant reduction of the photoemission linewidth, as
well as the ultrafast switching of the optical absorption, controllable by
means of the excitation fluence. We find that the interplay of the polariton
states with the large dark state reservoir play a decisive role in determining
the dynamics of the emission and transient absorption properties of the
hybridized light-quantum dot solid system. Our results open the route for the
investigation of PQD solids as polaritonic optoelectronic materials.
