Polarization-dependent plasmon coupling between Au nanorods and Au nanospheres in core–satellite nanoassemblies

Abstract

When gold nanoparticles are in close proximity, their plasmons interact to create hybridized plasmon modes. The interaction between gold nanorods (AuNRs) and spherical gold nanoparticles (AuNSs) is particularly intriguing due to the anisotropic shape of AuNRs, which results in two distinct plasmon modes: longitudinal and transverse. These plasmon modes interact differently with the plasmon of AuNSs depending on their relative positions. We investigate the influence of excitation light polarization on these plasmon couplings. Experimentally, we assemble AuNR@AuNS core@satellite nanoassemblies and measure dark-field (DF) scattering spectra for individual nanoassemblies with varying polarization angles of incident light. We find it crucial to spatially block part of the polarized light before it enters the DF condenser to maintain pure polarization at the sample in DF microscopy. The obtained scattering spectra indicate that longitudinal plasmon coupling is most effectively induced by light polarized approximately along the long axis of the AuNR. Surprisingly, transverse plasmon coupling does not exhibit a strong preference for polarization in the transverse direction. Finite-difference time-domain simulations reveal that quadrupole plasmon modes of AuNSs are mixed in the transverse plasmon coupling, weakening its directionality, particularly when the AuNSs are positioned off-center on the side of the AuNR. This slight reduction in symmetry also causes the imperfect alignment of light polarization with the long axis of the assembly for longitudinal plasmon coupling. Our findings provide a fundamental understanding of light-nanoassembly interactions, establishing a firm basis for the development of optically tunable photonic devices.