Bound state in the continuum (BIC) excited by the metasurface breaking in-plane symmetry and quasi-BIC for terahertz sensing

Bound state in the continuum (BIC), generally realized by structures with in-plane symmetry, has been under the spotlight in the control of the electromagnetic field. This Letter presents a terahertz metasurface that possesses broken in-plane symmetry for exciting BIC. The switch from a BIC to a quasi-BIC can be achieved through parameter modulation. According to multipolar decomposition, the BIC is constructed by the destructive interference of far-field radiation originating from the electric dipole and magnetic quadrupole. More importantly, considering that the metal material has finite conductivity, we analyze the relationship between radiation and non-radiation losses, based on the time domain coupled mode theory, which provides substantial theoretical support for the investigation of BIC phenomena. Furthermore, the proposed metasurface exciting quasi-BIC is highly sensitive to the changes in the dielectric environment, resulting in a high sensitivity of 135 GHz/RIU and a distinction between sucrose and glucose with the same solid content. As a result, our research provides a theoretical explanation of the evolution of BIC excited by the structure with broken in-plane symmetry and promotes the development of the high-performance meta-device used for sensing.