Twin and merging bound states in the continuum in rotated bilayer gratings

Bound states in the continuum (BICs) have recently been studied intensively in photonics owing to their unique properties. We here demonstrate two less-explored BICs in rotated bilayer gratings: twin BICs and merging BICs. The former is a symmetry-protected BIC (SP-BIC) pair at Γ point, although arising from the near-field coupling of two isolated SP-BICs, which remain against variations in coupling strength (i.e., the grating separation), and obey the familiar inverse-square law with respect to rotation-angle-dependent asymmetry parameter. During rotation process, two accidental BICs (A-BICs) carrying opposite topological charges, emerge successively and reemerge simultaneously at off-Γ points when the rotation angle is 45 ∘ . By increasing the grating separation, the two A-BICs form an off-Γ merging BIC, validated through topological charge evolution and Q factor divergence rate analysis. The proposed at-Γ twin BICs/off-Γ merging BICs expand the current BIC landscape, and will boost the development of BIC-based applications.