Centroid-Symmetry-Controlled High-Q Terahertz Quasi-BIC Resonances in an All-Dielectric Tetramer Metasurface

Terahertz (THz) metasurfaces provide compact platforms for narrowband spectral readout and sensing, but useful operation requires high modal Q, far-field accessibility, and angular robustness. Here we numerically investigate a Si tetramer metasurface on a SiO2 substrate. In the lossless model, balanced radiation centroids support a BIC-like eigenmode at 1.251582 THz with a computed Q of 4.08 × 109. A vertical aperture displacement dy breaks the centroid balance and converts the dark state into a radiatively coupled QBIC. The Q factor decreases continuously with |dy|, whereas the eigenfrequency branch remains smooth; the near-linear relation between 1/Q and dy2 is consistent with perturbation-induced radiative leakage. Multipolar spectra resolve an electric-dipole-like resonant contribution, and momentum-space maps place the high-Q region on a continuous eigenfrequency surface. At dy = 0.3 μm, loading the superstrate from n = 1.33 to 1.43 keeps the mode on the same branch and increases the radiation-limited Q from 1.27 × 106 to 1.46 × 106. Together, these results connect centroid-controlled radiative leakage to a far-field-accessible high-Q THz response and define a radiation-limited operating window for narrowband sensing and spectral filtering.