Topological control of polaritonic flatbands in anisotropic van der Waals metasurfaces

Abstract Anisotropic van der Waals materials exhibit direction-dependent optical and electronic properties, enabling tailored directional light–matter interactions. Rhenium disulfide (ReS 2 ) stands out among them for its strong in-plane anisotropy and thickness-independent direct-bandgap excitons. These excitons can hybridize with light to form exciton–polaritons, rendering ReS 2 a compelling material for nanophotonic integration. Metasurfaces are arrays of subwavelength resonators and can support ultrasharp quasi-bound states in the continuum resonances. Topological transformations of their band structure can give rise to flatbands, which host dispersionless states with quenched kinetic energy and vanishing group velocity. Here we show that structuring ReS 2 into a resonant metasurface reshapes its photonic band topology, splitting the integer topological charge of the quasi-bound states in the continuum into two half-integer polarization singularities and driving the formation of directional far-field flatbands. Tuned into resonance with the linearly polarized excitonic transitions of ReS 2 , these flatband states hybridize into distinct directionally polarized exciton–polaritons. This points towards a scalable route to polarization-resolved quantum emitters, flatband-enhanced nonlinear optics and low-threshold polariton devices.