The bound states in the continuum (BICs) in optical metasurfaces have attracted extensive attention due to their non-radiative nature with ultrahigh quality factors and their potential to establish new metadevices. Most prior demonstrations are limited to the realization of a single BIC with subwavelength monolayer microstructures, while the exploration of multiple BICs with multilayer structures opens many possibilities in light-matter interaction at the nanoscale and shows great potential for sensors and filters. In this work, the authors demonstrate numerically multiple ultranarrow BICs, both at-Γ and off-Γ, in monolayer and bi-layer all-dielectric metasurfaces. These metasurfaces consist of four square nanopores in one lattice in each transparent high-index silicon nitride layer sandwiched between transparent lower-index silicon dioxide, probed under p-polarized plane-wave illuminations. For normal incidence, four structural parameters-dependent at-Γ BICs can be implemented in a bi-layer metasurface, which can be tuned to coincide into two BICs, resembling the monolayer case. For the case of oblique incidence, two off-Γ BICs influenced by the size of the nanopores and the distance between layers can be achieved. Sensitivity of the BIC wavelength to changes in the refractive index of the surrounding medium is close to 150 nm/RIU, yielding an excellent figure of merit. The proposed metasurfaces that are designed for the visible frequency range provide a superior alternative for narrowband and high-quality metadevices.
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