Quantitative Analysis of Dipole and Quadrupole Excitation in the Surface Plasmon Resonance of Metal Nanoparticles

Metal nanoparticles have received increasing attention for their peculiar capability to control local surface plasmon resonance (SPR) when interacting with incident light waves. In this article, we calculate the optical extinction spectra of silver nanocubes with the edge length ranging from 15 to 200 nm by using the discrete dipole approximation method. An increasing number of SPR peaks appear in the optical spectra, and their positions change when the nanocube size increases. We have developed a method to quantitatively separate the contributions of the individual dipole component and quadrupole component of the optical extinction cross sections. This allows us to specify unambiguously the physical origin of each SPR peak in the spectra. We have also extensively analyzed the distribution patterns of electric fields and electric charges within and around the silver nanoparticle. These patterns clearly show the dipole and quadrupole excitation features at the SPR peaks. The near-field analyses are consistent with the far-field extinction spectra analyses. This suggests that the combination of far-field spectra and near-field pattern analysis can greatly help to uncover the intrinsic physics behind light interaction with metal nanoparticles and excitation dynamics of local surface plasmonic waves.