Speaker
Description
We demonstrate how in-plane material anisotropy reshapes the topology of photonic quasi-bound states in the continuum (qBICs) in a metasurface, enabling control over the formation of extended far-field photonic and polaritonic flatbands [1].
Starting from an isotropic metasurface with rotational symmetry - ensuring double degeneracy of the qBIC - we show, within the resonant-state expansion (RSE) formalism, that in-plane anisotropy lifts this degeneracy, giving rise to two distinct linearly polarized resonances. Furthermore, symmetry breaking splits polarization singularities of integer topological charges into pairs of half-integer singularities (photonic Dirac points) in momentum space, connected by a flat photonic dispersion. This topological landscape reshaping is also captured within the RSE framework, which provides the conditions for the emergence of photonic Dirac points and flatbands. Next, by employing nonlinear RSE [2], we incorporate an excitonic resonance as a pole in the dielectric function and theoretically demonstrate two distinct regimes of directionally hybridized exciton–polariton flatbands.
Finally, using the intrinsically anisotropic van der Waals material ReS2 [3], we experimentally confirm the predicted anisotropy-induced photonic and polaritonic transformations, paving the way for topology-driven control of distinct light-matter interaction dispersion regimes.
[1] C. Heimig et al., arXiv:2509.01258, 2025;
[2] E. Muljarov et al., Phys. Rev. B 93, 075417, 2016;
[3] B. Munkhbat et al., ACS Photonics, 9, 7, 2398–2407, 2022.