Multiple bound states in the continuum in a grating-slab-coupled structure.

Bound states in the continuum (BICs) are a special type of resonant state that remains localized even when coexisting with the radiation continuum. Owing to their different formation mechanisms, BICs are generally classified into symmetry-protected BIC (SP-BIC) at the Γ point (the Brillouin zone center with a zero wavevector), accidental BIC (A-BIC) and Friedrich-Wintgen BIC (FW-BIC) at the off-Γ point with nonzero wavevectors. Many approaches have been proposed to achieve BICs, but it remains a great challenge to design all three types of BICs simultaneously. In this work, we propose a simple grating-slab coupled structure, in which the interplay between two parts can emerge as a promising degree of freedom for BIC engineering. We showed that the structure supported multiple BICs, including two SP-BICs, one A-BIC, and one FW-BIC, which could be further manipulated using structural parameters. In a composite structure, the quality () factors of the two SP-BICs were demonstrated to follow the well-known quadratic scaling law with asymmetry parameters. In addition, the A-BIC could be fine-tuned by varying the slab thickness, exhibiting a typical topologically protected feature, and the FW-BIC was accompanied by an avoided crossing owing to the coupling between the two wave states hosted by the grating and slab. Moreover, the two off-Γ BICs exhibited different field patterns and multiple components but similar topological features, such as the same topological charge. The proposed structure provides an alternative platform for engineering multiple BICs, which could be combined with other degrees of freedom to achieve even more abundant and powerful light manipulation.