Super bound states in the continuum: Classification, direct computation, and geometry in parameter space

Abstract

In periodic structures, resonant states near a bound state in the continuum (BIC) have quality ($Q$) factors scaling as $Q\sim 1/{\delta }^{2\mu }$, where $\delta$ is the wavevector detuning from the BIC. While generic BICs have $\mu =1$, super-BICs with $\mu \ge 2$ give rise to higher-$Q$ resonances over a broad wavevector range, making them attractive for enhancing light-matter interactions. Here, we develop a systematic theoretical and computational framework for studying super-BICs. We first classify them according to $\mu$ and symmetry. We then study their unfolding under structural variations, characterizing the bifurcations of nearby generic BICs and determining the minimum number $n$ of additional tunable structural parameters required to recover a super-BIC of the same class. We devise a direct computational method for locating super-BICs together with their associated structural parameters and efficiently tracing their families. Finally, we investigate the local geometry of the BIC set in the product space of structural parameters and the Brillouin zone. This geometric framework provides a unified interpretation of the unfolding of super-BICs and offers a new understanding of BIC merging and splitting phenomena. Our results establish a theoretical and computational foundation for the systematic study of super-BICs, with potential applications in resonant optics and photonic device design.