Multiband High- Q Terahertz Resonances in a Twisted Triangular Moiré Metasurface

Abstract

Bound states in the continuum (BICs) are nonradiative modes embedded within the continuous spectrum of propagating states that remain perfectly localized without leaking into radiation channels. Symmetry-protected BICs (SP-BICs) are induced by structural symmetry at the Γ point, whereas accidental BICs arise under specific structural parameters and incident conditions. By introducing a twist angle between two identical lattices, a moiré metasurface folds multiple resonant modes into the reduced moiré Brillouin zone, enabling several symmetry-protected and accidental BICs to coexist within it. Herein, we numerically demonstrate the existence of multiple BICs in a moiré metasurface formed by twisted bilayer triangular lattices with commensurate angle 2α of 6.01°. We further observe multiple robust flatbands in the band structure, one of which hosts a SP-BIC, so that a single state is localized both in real space by the flatband and in momentum space (k-space) by the BIC. In contrast to magic-angle flatbands, which emerge only within narrow parameter windows, the flatbands in our system persist across a wide range of moiré structural parameters owing to strong intersublattice coupling and arise from interband coupling of folded dispersive bands. In k-space, all BICs coincide with polarization singularities carrying well-defined topological charges, including higher-order charges (q = −2) enabled by the C6 symmetry of the triangular moiré lattice. Notably, the accidental BIC radiates with an anomalous, negative-refraction-like directionality. By combining robust moiré flatbands with symmetry-protected and accidental BICs on a single C6 platform, these results establish twisted triangular lattices as a route toward multiband, high-Q terahertz devices.