Topological control of polaritonic flatbands in anisotropic van der Waals metasurfaces.

Structuring ReS2 into a resonant metasurface splits the integer topological charge of quasi-bound states in the continuum into two half-integer polarization singularities, enabling directional far-field flatbands.

• Why it matters: Direction-dependent van der Waals excitons and topological metasurface band engineering are promising, but linking topological charge changes to polarization-resolved flatband polaritons is still needed.
• What they did: The authors design an anisotropic ReS2 resonant metasurface and tune it into resonance with linearly polarized excitonic transitions, then analyze how the photonic band topology transforms to form flatbands.
• The result: The resulting flatband states hybridize into distinct directionally polarized exciton-polaritons, suggesting routes to polarization-resolved quantum emitters, flatband-enhanced nonlinear optics, and low-threshold polariton devices.

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