Robust Bound States in the Continuum in Mirror-Symmetric Terahertz Metasurfaces

Conventional C ₂-symmetric metasurfaces supporting symmetry-protected bound states in the continuum (SP-BICs) are often hindered by sensitivity to fabrication errors. Here, we demonstrate that for metasurfaces composed of resonant dimers possessing local inversion symmetry, the SP-BIC mode remains robustly sustained at the Γ-point as long as in-plane mirror symmetry is preserved, even if the dimers undergo spatial displacement within the lattice that breaks global periodic inversion symmetry. This characteristic endows the design with a unique offset-robustness, significantly relaxing the stringent requirements for fabrication precision. Build on this, we introduce an asymmetry parameter (α) to break the mirror symmetry (and consequently the local inversion symmetry), enabling the transformation of the BIC into a quasi-BIC (QBIC). Experimental and simulation results show that the radiative loss of the QBIC can be tuned via α, with the quality factor following a power-law scaling of Q ∝ α ^-1.95. Momentum-space and near-field analyses indicate that this transition is accompanied by the splitting of topological singularities and the breakdown of multipole interference equilibrium. This study demonstrates that local inversion symmetry is the physical origin of BIC formation, whereas mirror symmetry ensures the critical tolerance to displacement, providing a theoretical basis and design approach for high-performance, process-tolerant terahertz photonic devices.