Numerical study of optical and EELS response of coupled metallic nanoparticles

Localized surface plasmon resonances (LSPR) govern the optical properties of metallic nanoparticles at the nanoscale level and depend strongly on their shape, size and environment. When particles are a few nanometers apart, new plasmon modes, that can be either bright or dark, arise from the electromagnetic coupling of the plasmon modes of the individual nanoparticles. In the case of 3 gold nanorods assembled in a dolmen-like structure, a transmission window has been observed experimentally from 0.9 and 1.3 eV. It has been attributed to Fano interferences between a broad bright mode coming from the monomer and a narrow dark mode coming from the dimer. Because it is optically inactive, the latter has to be probed locally with an electron beam. This is why we investigate numerically the energy electron loss (EEL) response of coupled metallic nanorods together with their optical properties to get insight into the origin of these Fano resonances. To achieve this, calculations are performed in the frame of the discrete dipole approximation both for optical and EEL excitations.