| Summary: | A bound state in the continuum (BIC) is a localized state in a continuous spectrum of radiating waves. In this study, the coupled-mode theory is introduced to more precisely interpret a symmetry-protected BIC and the quasi-BIC in a facile trimeric metasurface, compared with Fano formula, which is introduced to generate a high-quality factor resonance in a trimeric metallic slit metasurface. The multipole decomposition method and the near field distribution of the structure further illustrate that the underlying physics of the high-<i>Q</i> is mainly originated from the interference between the electric quadrupole mode and the magnetic toroidal mode. Physical mechanism shows that the resonance arises from the perturbation of symmetry-protected BICs. The result may play a role in the applications of lasers, optical sensors, and low-loss fibers.
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