Signed-Asymmetry-Controlled Radiative Leakage in an All-Dielectric Terahertz Quasi-BIC Metasurface

An ultrahigh numerical quality factor does not by itself establish a useful refractive-index sensor because radiation suppression, far-field accessibility, and analyte-field overlap are controlled by different mechanisms. Here, an all-dielectric terahertz metasurface comprising two mirror-related open-rectangular resonators is controlled by a signed opening mismatch, db = b′ − b. At db = 0, destructive interference produces a symmetry-protected bound state in the continuum (BIC); finite positive or negative db opens a magnetic-dipole-dominated quasi-BIC (QBIC) channel whose inverse radiative Q follows the expected quadratic asymptote. Signed continuation, complex-eigenfrequency and normalized-eigenfield-overlap tracking, M–Γ–X dispersion, multipolar decomposition, and driven-pole fitting verify branch identity. Momentum-space maps describe pointwise selected modes, supplemented by local field comparisons. At n = 1.0, the intrinsic BIC is isolated and has a numerical eigenmode Q above 10⁹, with the ideal lossless limit tending to infinity; sweeping the same superstrate-index parameter over n = 1.33–1.43 yields a sensitivity of 800.6 GHz/RIU with an almost db-independent slope. Sweeps of analyte thickness, incidence angle, polarization, extinction coefficient, and fabrication parameters then evaluate practical robustness and identify material loss as the principal limitation on the attainable Q. This work establishes a compact design framework for combining ultrahigh-Q resonances, controllable far-field readout, and high-sensitivity terahertz refractometric sensing.