Polarization-insensitive near-infrared quasi-BIC perfect absorbers realized via Brillouin zone folding and temporal coupled-mode theory.

We present a high-Q near-infrared metasurface absorber enabled via Brillouin zone folding (BZF) modulated bound states in the continuum (BIC). The structure comprises an array of asymmetric silicon hole-pillar nanocylinders, a SiO2 spacer layer and a gold reflector. Full-wave simulations based on the finite element method (FEM) are carried out to explore its optical properties. Structural symmetry breaking excites quasi-BIC modes, which feature strong field localization and ultra-narrow resonant bandwidth. Numerical results demonstrate that the proposed device achieves an absorption efficiency of 99.98% at 1154.2 nm, with a Q factor reaching 1170.11. Multipolar decomposition verifies that the resonance is predominantly governed by the toroidal dipole (TD) mode, while the electric dipole (ED) acts as an auxiliary component. We also systematically discuss how lattice period, silicon height, polarization angle and incident angle affect the absorption performance. Temporal coupled-mode theory (TCMT) is employed to fit the spectra and validate the resonant mechanism of quasi-BIC. This work offers an effective design scheme for high-Q narrowband absorbers, which has great application potential in optical sensing and photodetection.