Dual quasi-bound states in the continuum in terahertz metasurfaces for high-Q resonance and multi-modal behavior

Bound states in the continuum (BICs) suppress radiative leakage and enable high-Q resonances for terahertz (THz) photonics. Here, we propose a metallic quadrumer metasurface in which dual quasi-BICs are efficiently excited via translational-symmetry perturbations, while preserving the overall C4v symmetry, leading to polarization-insensitive operation. The two resonances originate from distinct BIC mechanisms, namely, a Γ-point symmetry-protected BIC and a Friedrich–Wintgen BIC arising from modal interference, and evolve into coexisting Fano and electromagnetically induced transparency (EIT)-like responses within the same design. Notably, the resonance frequencies remain nearly unchanged across the perturbation range, indicating strong spectral robustness due to negligible variation of the metasurface’s effective refractive index. Terahertz time-domain spectroscopy (THz-TDS) measurements show consistent overall trends with simulations, while discrepancies in Q-factor and spectral contrast are mainly attributed to metallic loss and fabrication/measurement non-idealities. This work provides a compact route toward polarization-robust, multi-mode high-Q THz metasurface devices for filtering, sensing, and dispersion control.