  Exciton-polaritons, elementary excitations arising from the strong coupling
regime between photons and excitons in insulators or semiconductors, represent
a promising platform for studying quantum fluids of light and realizing
prospective all-optical devices. Among different materials for room temperature
polaritonic devices, two-dimensional (2D) layered perovskites have recently
emerged as one of the promising candidates thanks to their prominent excitonic
features at room temperature. Here we report on the experimental demonstration
of exciton-polaritons at room temperature in resonant metasurfaces made from a
subwavelength 2D lattice of perovskite pillars. These metasurfaces are obtained
via spincoating, followed by crystallization of the perovskite solution in a
pre-patterned glass backbone. The strong coupling regime is revealed by both
angular-resolved reflectivity and photoluminescence measurements, showing
anticrossing between photonic modes and the exciton resonance with a Rabi
splitting in the 200 meV range. Moreover, we show that the polaritonic
dispersion can be engineered by tailoring the photonic Bloch mode to which
perovskite excitons are coupled. Linear, parabolic, and multi-valley
polaritonic dispersions are experimentally demonstrated. All of our results are
perfectly reproduced by both numerical simulations based on a rigorous coupled
wave analysis and an elementary model based on a quantum theory of
radiation-matter interaction. Our results suggest a new approach to study
exciton-polaritons and pave the way towards large-scale and low-cost integrated
polaritonic devices operating at room temperature.
