Investigation of quasi-bound states in the continuum in terahertz metal complementary periodic cross-shaped resonators

We numerically and experimentally demonstrate quasi-bound states in the continuum (BICs) in the free-standing metal complementary periodic cross-shaped resonators (CPCRs). 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-BICs can reach 102, which is significantly high compared with that of the conventional modes and could even be improved by further reducing the structural asymmetry factor. The sharp quasi-BICs realized in the structure may immediately boost the exploration of ultra-narrowband filters and ultra-sensitive sensors at terahertz frequencies.