High-Q transparency band and high optical sensitivity of an all-dielectric THz metasurface based on the bound state in the continuum

The bound state in the continuum (BIC), as a novel eigenmode with an infinitely high-quality factor ( Q -factor), has received great attention in modern optical science. We design a metasurface composed of a two-rod and a square ring in one lattice in the terahertz band. Three symmetry-protected BICs are observed of the composed metasurface, with two BICs originating from the rod resonator and one from the square ring resonator. It demonstrates that the number of BICs in the considered waveband can be regulated by these resonators in one lattice of the metasurface. We can transfer the ideal BICs into quasi-BICs by adjusting the incident angle and structural parameters. One quasi-BIC is considered as electromagnetically induced transparency (EIT), associated with the destructive interference between the dark and bright modes derived from the rod resonator. Further, the metasurface can convert the Fano effect to EIT mutually, and the EIT only appears in the overcoupling state if the frequency detuning of the two modes becomes zero. Notably, the dual-EITs are established by modulating the distance between two resonators within a single lattice. Concurrently, the interference-based BIC is generated by the anticrossing phenomenon between two energy bands. Additionally, the sensitivity and group index of the metasurface reach a high level. These results are valuable for the development of terahertz optical devices.