Plasmon resonance and enhanced near-field of anisotropic nanoparticle systems: unified analysis by factorization of light-excited dipole distribution.

We develop a simple factorization scheme to analyze the mechanism of dipole-plasmon resonance, which is controlled by the particle shape or the gap distance of neighboring particles. The method focuses on extracting the motion of local induced dipoles based on the discrete dipole approximation (DDA) and is applied to silver nanoparticles. Our analysis clarifies that the particle shape effect is characterized quantitatively by the oscillation of a small number of collective dipoles when the inhomogeneity of the distribution of induced dipoles is weak. Our factorization scheme is also applicable to a system consisting of neighboring nanoparticles and explains the relationship between the gap distance of neighboring nanoparticles and near-field enhancement. Our theoretical approach is useful for understanding the optical response of anisotropic- and multi-nanoparticle systems in a unified manner, and it provides a convenient view for the design of optical materials of nanoparticles.