Quasi-bound states in the continuum in metal complementary periodic cross-shaped resonators at terahertz frequencies

We numerically and experimentally demonstrate quasi-bound states in the continuum (BICs) in free-standing metal complementary periodic cross-shaped resonators (CPCRs) at terahertz frequencies. Such induced quasi-BICs arise from the broken symmetry in CPCRs. By slightly breaking mirror symmetry through reducing the length of one arm in CPCRs, the measured Q -factor of the quasi-BIC can reach 102, which is significantly high compared with that of conventional modes. Further study shows that the high Q -factor of the quasi-BIC is mainly determined not by the radiative loss but the material loss, as the asymmetry of the structures is unremarkable. The sharp quasi-BICs realized in the proposed structure may immediately boost the exploration of ultra-narrowband filters and ultra-sensitive sensors at terahertz frequencies.