Metasurface Bound States in the Continuum Spectrum Fitting Based on Coupled Mode Theory

In this paper, a method based on coupling theory is proposed which can be used to fit bound states in the continuum (BIC) or quasi-Bound states in the continuum (quasi-BIC) curve. Compared with the preceding theory (the coupling of the bright mode and one dark mode), we use two bright modes and phase calculation so that this method can calculate a relatively high value of Q. We fitted three different Q values of lower Q, middle Q and higher Q and found that an excellent fitting effect could be obtained. And our method is more beseem for BIC or quasi-BIC, which can accurately predict the scattering spectra of quasi-BIC.