Bound States in the Continuum in the Visible Emerging from out-of-Plane Magnetic Dipoles

Bound states in the continuum (BICs) are electromagnetic modes with a dispersion inside the light cone and infinite lifetimes. This exceptional property has led to intensive research and the demonstration of BICs in the gigahertz, teraherz, and near-infrared, up to the visible region. In this study, we design and experimentally demonstrate optical BICs using a subdiffraction lattice of Si nanodisks. The out-of-plane magnetic dipole resonance in the dielectric nanodisks couples with the subdiffraction lattice and defines a symmetry-protected BIC at normal incidence. This mode becomes a quasi-BIC as the angle of incidence is increased. The spectral position of the BIC can be controlled with the diameter of the nanodisks, which governs the out-of-plane magnetic dipole. The investigated BIC is robust to shape irregularities of the individual nanodisks. This robustness makes the design strategy of the BICs presented here very attractive for applications in which high fabrication precision can not be achieved.