Tailoring bound states in the continuum in symmetric photonic crystal slabs by coupling strengths

In this work, we investigate polarization-insensitive dual bound states in the continuum (BICs) at Gamma point in symmetric photonic crystal (PhC) slabs. Especially, BICs are tailored by tuning infra- and intercellular optical coupling strengths of PhC slabs. Based on four different approaches, we realize the transition from BIC to quasi-BIC resonances with various dispersion behaviors while maintaining the symmetry of slabs. Also, we show the two resonances are lowest-order even and odd eigenmodes that can match the symmetry of the incident plane wave, and their quality (Q) factors follow the inverse quadratic law except for cases with larger perturbations. Furthermore, multipolar decomposition reveals that even quasi-BICs are dominated by the toroidal dipole and magnetic quadrupole, while odd quasi-BICs are governed by the magnetic dipole and electric quadrupole. Interestingly, an anomalous increase of the Q factor is observed in one case, which is attributed to the mode transformation. Finally, anisotropic coupling adjustment is discussed, which enriches the degrees of freedom to manipulate BICs. This work introduces a novel perspective to tailor BICs at Gamma point in PhC slabs and has potential planar photonic applications for nonlinear enhancement and sensing. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

In this work, polarization-insensitive dual bound states in the continuum (BICs) at the Gamma point in symmetric photonic crystal slabs are investigated. The BICs are tailored by adjusting the optical coupling strengths of the slabs. The transition from BIC to quasi-BIC resonances is realized using four different approaches while maintaining the slabs' symmetry. The resonances are found to be lowest-order eigenmodes that match the symmetry of incident plane waves, and their quality factors follow a specific law. Additionally, the dominant modes of the BICs are identified using multipolar decomposition. Anisotropic coupling adjustment is discussed, providing more control over the BICs. The work introduces a new perspective for manipulating BICs in PhC slabs and has potential applications in nonlinear enhancement and sensing.