Cloaked Quasi Bound States in the Continuum in Semiconductor Metasurfaces with Out‐of‐Plane Symmetry Breaking

Bound states in the continuum (BICs) have attracted much attention in Nanophotonics for their (formally) infinite Q factors, quasi‐BICs (qBICs) being the symmetry‐broken version allowing for in‐ and out‐coupling at the expense of finite (but large) Q factors. Here, it is demonstrated that dark and asymmetric qBICs arise in the optical domain in Si tilted nanodisk metasurfaces, due to the spectral overlap of in‐plane and out‐of‐plane dipolar resonances in these meta‐atoms, similar to so called Brewster qBICs in the microwave regime for single‐magnetic‐resonance, tilted microdisks, arising at the tilt angle. Using a coupled dipole model, it is shown that optical dark qBICs occur for tilted nanodisks at modified Brewster angles that can differ significantly from the nanodisk tilt angles, , due to the hybridization of in‐plane and out‐of‐plane dipolar resonances. If light is incident at , qBIC excitation is forbidden and the metasurface is transparent; counterintuitively, for , the qBIC is indeed excited but the metasurface remains also fully transparent, so that such qBIC excitation is cloaked. Numerical calculations confirm the asymmetric character of these qBICs, demonstrating that the cloaked qBIC largely enhances near‐fields and emerges in the extinction only when absorptive losses are present in the nanodisks. Finally, a practical metasurface design is proposed, amenable to fabrication, supporting cloaked qBICs. The rich phenomena associated with such cloaked qBICs make them highly suitable for tuning or switching nano‐optical devices (between on/off qBIC states with negligible reflection), offering promising applications for enhanced light–matter interactions at the nanoscale.