Inverse Design of Quasi-Bound States in the Continuum Absorber

In this study, we demonstrate the relationship between the absorbing frequency and the quasi-bound states in the continuum (quasi-BIC) frequency by employing the coupled-mode theory (CMT) and interference theory. The structure consists of a symmetric-protected BIC metal structure layer, a polyimide spacer layer, and a silicon substrate. The top layer contains two similar metal structures, which make the structure asymmetric by varying one of them slightly, thus producing a symmetrical broken quasi-BIC. When a metal reflecting plate is added to the bottom of the dielectric spacer layer, a quasi-BIC absorber is formed. This is the first theoretical calculation using the coupled mode equation to analyze the relationship between the absorption frequency of a quasi-BIC absorber and the quasi-BIC resonance frequency, which is related to the resonance frequency of a single structure in a unit cell. According to the relationship between the coupling strength and distance between the structures within a unit cell combined with the resonant frequency of quasi-BIC, the geometric parameters of the absorber within a frequency range can be inverse design.