Tunable symmetry-protected quasi-BICs in bulk Dirac semimetal metasurface for terahertz sensing

Abstract

Symmetry-protected bound states in the continuum (BICs) suppress radiative leakage because the preserved structural symmetry prevents the resonant mode from coupling to accessible free-space radiation channels. Their practical implementation, however, requires finite radiative accessibility together with post-fabrication spectral tunability. Here, we numerically demonstrate a terahertz metasurface composed of periodically arranged cross-shaped bulk Dirac semimetal (BDS) resonators that support a Fermi-energy-tunable quasi-BIC. A controlled width perturbation introduced into a single horizontal arm breaks the in-plane rotational symmetry and converts the otherwise nonradiating state into an observable high-Q quasi-BIC characterized by an asymmetric Fano line shape. Near-field mapping and Cartesian multipolar decomposition reveal that the resonance is dominated by a narrow electric-quadrupole (EQ) mode coherently interacting with a broadband electric-dipole (ED) background. Increasing the BDS Fermi energy from 0.2 to 0.5 eV continuously blueshifts the resonance from approximately 1.525–1.56 THz while simultaneously reconfiguring the transmission contrast. The spectral response exhibits pronounced polarization selectivity and remains stable for incidence angles up to 5°. For bulk superstrate refractive-index sensing, the metasurface achieves a sensitivity of 374.08 GHz RIU−1 and a maximum peak-to-dip spectral figure of merit of 59.8 RIU−1. These results establish a structure–material co-engineering strategy that combines symmetry-regulated radiation leakage with carrier-mediated dispersion control, providing a compact platform for tunable terahertz filtering, modulation, and high-resolution refractive-index sensing.