Multipolar scattering and collective mode engineering in SiO₂@C core-shell nanoparticles and clusters.

The scattering properties of nanoparticles are crucial in applications like optical sensing, photonics, and medical diagnostics. Despite extensive research, the interaction of multipolar resonances in metal-free nanophotonic structures remains underexplored. This study investigates the effects SiO2 nanoparticle size, carbon shell thickness, and cluster topology on the excitation and evolution of multipolar resonances. We used the finite-element method in COMSOL Multiphysics to investigate the scattering of SiO₂@C core-shell nanoparticles and their clusters, validating the simulations against Mie theory. Optimal performance was observed in SiO2 nanoparticles with a 200 nm radius, where multipolar resonances were most pronounced, achieving a balance between enhanced scattering intensity and well-defined resonance peaks. The carbon shell thickness, varied from 5 to 100 nm, induced significant spectral shifts and intensity modulation, particularly enhancing the electric dipole (ED), magnetic dipole (MD), and electric quadrupole (EQ) modes. Notably, the magnetic quadrupole (MQ) mode emerged as the dominant contributor to scattering, underlining the crucial role of shell thickness in controlling optical properties. For nanoparticle clusters ranging from dimers to heptamers, the MQ mode remains the dominant contributor, with sharp oscillation peaks intensifying as cluster size increases. However, the introduction of a central nanoparticle in pentamer and heptamer clusters leads to reduced scattering intensity due to destructive interference and disrupted phase coherence. These findings establish the governing influence of particle arrangement and symmetry on scattering in SiO₂@C core-shell nanoparticle clusters, providing a robust framework for designing low-loss, spectrally tunable nanophotonic structures for dielectric metasurfaces and refractive-index sensing.