Nontrivial Plasmonic Bound States Unlocked by Weak Extrinsic Perturbations

Bound states in the continuum (BIC) provide an exceptional route to extreme light manipulation, yet their implementation in metallic metasurfaces has been hindered by high losses, limited tunability, and restricted design freedom. Here, we introduce a plasmonic BIC platform that supports multiple BICs across parameter space, by arranging spatially varied low-index nanoparticles inter- or intra-unit cells to form a periodic dual-BIC lattice. This architecture induces nonmonotonic <i>Q</i>-factor modulation and <i>Q</i>-factor enhancement under large perturbations, thereby revealing a new degree of freedom in BIC nanophotonic design. Despite their weak resonant response, these low-index nanoparticles in our configuration can serve as effective weak-perturbation tunning, allowing precise control of BIC modes through their spatial positioning. This mechanism likewise exhibits universality in dielectric platforms, unlocking new design opportunities in BIC nanophotonic, opening avenues for tunable nanophotonics, ultrasensitive detection, BIC-enabled lasing, and light-matter interaction.