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Twin and merging bound states in the continuum in rotated bilayer gratings
Abstract:
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.
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