Dual physical origins of polarization-independent scattering in plasmonic nanoparticle dimers

Plasmonic nanoparticle dimers show strong polarization dependence due to the high asymmetry between their longitudinal and transverse axes. In this work, we theoretically and numerically demonstrate that nanosphere dimers exhibit obvious polarization-independent optical features at specific wavelengths. By mapping the complex polarizabilities of the hybridized cluster, we identify two size-dependent regimes underlying this behavior. In larger dimers where retardation matters, higher-order multipoles contribute noticeably and modify the longitudinal and transverse scattering channels. Polarization independence appears when the total multipolar scattering cross sections for these two orthogonal excitation directions become equal. For smaller dimers dominated by the dipole response, by contrast, the PIP comes from the spectral crossing of the longitudinal and transverse electric-dipole responses. This crossing requires a delicate balance of interparticle coupling and complex modal amplitudes, while its spectral evolution changes as the interaction transitions from near-field-dominated coupling at small gaps to retardation-influenced coupling at larger separations. These findings show that polarization-independent scattering can be achieved even in linear plasmonic dimers, but only under suitable structural conditions. By clarifying the roles of particle size, gap distance, and modal coupling, the work provides useful design guidance for polarization-insensitive plasmonic devices.