Merging of quasi-bound states in the continuum and Wood anomalies in terahertz metasurfaces based on complementary periodic cross-shaped resonators

Metasurfaces provide an ideal platform for controlling the merging of multiple modes owing to trimmable periodic meta-atoms. In this work, a terahertz metasurface based on complementary periodic cross-shaped resonators (CPCRs) with mixed modes is demonstrated. The CPCRs support sharp quasi-bound states in the continuum (quasi-BICs) by introducing an offset parameter. The optical response of quasi-BICs such as frequency shift and Q -factor can be optimized by tuning offset parameters. As such parameter increases, a mixed mode originating from quasi-BICs and Wood anomalies appears. The merged resonance has a narrow bandwidth with steep edges and thus can serve as bandpass filters . Simulated electric field distributions reveal the special patterns for these modes. Owing to the effect of quasi-BICs, Wood anomalies exhibit different field patterns in contrast to those of pure modes. These results prove the possibility of multimode merging in metallic metasurfaces, which is beneficial for terahertz applications such as sensing and wave filters.