Influences of dipole-quadrupole strong interactions on optical properties of plasmonic nanodimers

Abstract

An extended analytical method based on the dipole-quadrupole electromagnetic interaction is proposed to investigate the optical properties of strongly interacting plasmonic nanoparticles for which the known coupled dipoles approximation (CDA) is inaccurate. The introduced simple and novel method used here, namely coupled dipole-quadrupole approximation (CDQA), is used to elaborate on the optical interactions of individual modes including dipole-dipole, dipolequadrupole and quadrupole-quadrupole. A simple and versatile formula is presented for the modified dipole-polarizability by considering an adjacent quadrupole effect, leading to accurate prediction of remarkable features in the optical properties of nanoparticle clusters in simple or complex forms. Interestingly, in a nanodimer configuration, it is shown that the quadrupole strongly affects the dipolar resonance energy, though the dipole impact on quadrupole properties are negligible. The findings are verified by the approximated methods, numerical computations and generalized Mie theory.