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, twodimensional (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.

mechanical effect, [1] the origin of BICs is nowadays fully unraveled as a particular solution of wave equations, which has led to their exploitation in other fields where it is straightforwardly attributed to destructive interference mechanisms or symmetry mismatches. [2] The most active playground of BIC physics is in contemporary Photonics [3] since most of optoelectronic devices rely on resonances and their coupling mechanisms with environment. Indeed, the trapping of light through photonic BICs is a salient feature to enhance different light-matter interaction mechanisms, leading to various applications in microlasers, [4][5][6][7][8] sensors, [9] optical switches, [7] and nonlinear optics. [10][11][12][13] Moreover, on the fundamental side, photonic BICs in periodic lattices are pinned to singularities of farfield polarization vortex and can be considered as topological charges of nonHermitian systems. [14][15][16][17] This topological nature, together with modern technological feasibility to tailor photonic materials, makes photonic BICs a fruitful platform to engineer polarization singularities of open photonic systems. [5,[18][19][20] Another prominent area of investigation in modern Optics is represented by exciton-polaritons, elementary excitations arising from the strong coupling regime between confined light and semiconductor excitons. [21] As hybridized eigenmodes, these bosonic quasiparticles inherit original features of both Exciton-polaritons are mixed light-matter excitations resulting from the strong coupling regime between an active excitonic material and photonic resonances. Harnessing these hybrid excitations provides a rich playground to explore fascinating fundamental features, as out-of-equilibrium Bose-Einstein condensation and quantum fluids of light, plus novel mechanisms to be exploited in optoelectronic devices. The formation of exciton-polaritons arising from the mixing between hybrid inorganic-organic perovskite excitons and an optical bound state in a continuum (BIC) of a subwavelength-scale metasurface, are experimentally investigated at room temperature. These polaritonic eigenmodes, hereby called polariton BICs (pol-BICs) are revealed in reflectivity, resonant scattering, and photoluminescence measurements. Although pol-BICs only exhibit a finite quality factor bounded by the nonradiative losses of the excitonic component, they fully inherit BIC peculiar features: a full uncoupling from the radiative continuum in the vertical direction, which is associated to a locally vanishing farfield radiation in momentum space. Most importantly, the experimental results confirm that the topological nature of the photonic BIC is perfectly transferred to the pol-BIC. This is evidenced by the observation of a polarization vortex in the farfield of polaritonic emission. The results pave the way to engineer BIC physics of interacting bosons and novel room temperature polaritonic devices.