Hybrid Plasmonic Symmetry‐Protected Bound state in the Continuum Entering the Zeptomolar Biodetection Range

Abstract Photonics bound states in the continuum (BICs) are peculiar localized states in the continuum of free‐space waves, unaffected by far‐field radiation loss. Although plasmonic nano‐antennas squeeze the optical field to nanoscale volumes, engineering the emergence of quasi‐BICs with plasmonic hotspots remains challenging. Here, the origin of symmetry‐protected (SP) quasi‐BICs in a 2D system of silver‐filled dimers, quasi‐embedded in a high‐index dielectric waveguide, is investigated through the strong coupling between photonic and plasmonic modes. By tailoring the hybridizing plasmonic/photonic fractions, a trade‐off is selected at which the quasi‐BIC exhibits both high intrinsic Q‐factor and strong near‐field enhancement because of dimer‐gap hotspot activation. Not only radiation loss is damped but in a configuration sustaining a lattice of plasmonic hotspots. This leads to an advantageous small modal volume for enhancing light‐matter interaction. The layout of nearly embedded dimers is designed to maximize the spatial overlap between the optical field and the target molecules, enhancing reactive sensing efficiency. The architecture is evaluated for its ability to detect transactive response DNA‐binding protein 43. The refractometric sensitivity outperforms current label‐free biosensing platforms, reaching the zeptomolar range. The approach highlights the potential of combining plasmonic and dielectric nanomaterials for advanced sensing technologies.