Multiple Bound States in the Continuum in Deep-Subwavelength Nonlocal Polar Dielectric Nanoparticles

The confinement of light in open deep-subwavelength nanostructures for enhanced light-matter interactions is a long-standing goal in nanophotonics. Bound states in the continuum (BICs) offer a promising route to achieve light confinement, but their performance in deep-subwavelength nanostructures is often limited. Here, we demonstrate that spatial dispersion (nonlocality) in polar dielectric nanoparticles enables the formation of multiple BICs, offering unprecedented confinement of light at the nanoscale. Through combined analytical and numerical studies of deep-subwavelength core-shell nanoparticles (radius ∼ ${\lambda }_0/500$ ), we show that nonlocal polar dielectric shell supports multiple longitudinal-transverse hybrid modes, manifesting as narrow Fano resonances in scattering spectra. Interestingly, by tuning geometric parameters, the radiative damping of these modes can be completely suppressed forming a series of BICs. Importantly, unlike conventional BICs in local-response nanoparticles that require extreme physical conditions, these nonlocality-induced BICs emerge across a broad parameter space. Our findings establish polar dielectrics as a versatile platform for multiple BICs for extreme subwavelength confinement with potential applications in sensing, nonlinear optics, and nanolasing.