Dark asymmetric modes in semiconductor metasurfaces with out-of-plane symmetry breaking: cloaked quasi bound states in the continuum

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. very all-dielectric metasurfaces Here we demonstrate 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, we show 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, for both electric and magnetic dipole resonances. If light is incident at such Brewster angle, qBIC excitation is forbidden and the metasurface is transparent; counterintuitively, for mirror-symmetric incidence, 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 manifests through largely enhanced near fields, and emerges in the extinction when absorptive losses are present in the nanodisks. Finally, we propose a practical metasurface design that supports dark qBICs, either uncoupled or cloaked, achievable through oblique epitaxial growth to create inclined, rather than merely tilted, cylinder arrays. The rich phenomena associated with such asymmetric 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.