From anapole to polariton: A fundamental transition in the mechanism underlying robust flatband generation in metasurfaces
Phys. Rev. B 113, 035401 – Published 2 January, 2026
DOI: https://doi.org/10.1103/w65x-lwfy
Abstract
Flat photonic bands promise extreme control of light-matter interactions but are often degraded by loss and angular dispersion. We demonstrate robust polaritonic flatbands in dimer metasurfaces by strongly coupling an engineered quasibound state in the continuum (-BIC) to intrinsic excitons. The photonic mode, realized via suppression of the first-order Fourier harmonic and originating from an anapolelike interference between electric dipole and magnetic quadrupole moments, forms an ultraflat band with an angle-conserved quality factor. Upon strong coupling, distinct upper and lower polariton branches emerge with a vacuum Rabi splitting of 190 meV. Crucially, both branches inherit the dispersionless character of the parent -BIC, maintaining nearly constant resonance energy, linewidth, and amplitude over a wide angular range, even when anapole conditions are broken by material loss. This identifies a mechanism transition from interference-based to inheritance-based dispersion control, in which lattice-determined photonic dispersion and energy-selective coupling ensure resilience against loss. The resulting angle-invariant polaritonic response, compatible with substrate-supported implementations and external-cavity-free architectures, opens avenues for wide-angle sensing and ultralow-threshold polariton lasing, establishing a versatile platform for flatband engineering in the strong-coupling regime.
