Study on optical properties of aggregated ultra-small metal nanoparticles

We study the optical properties of dimer and septamer aggregates (in forms of being separated or merged) of ultra-small silver nanoparticles (a few nanometers in diameter) by calculating their extinction spectra and optical field distributions using the discrete dipole approximation method. Characteristics of their extinction spectra are identified due to different resonance modes dependent on the incidence states of light. Specifically, as polarization of the incidence light is perpendicular to the center-to-center lines of the nanoparticles, interaction of neighboring nanoparticles is in the “repulsive coupling mode”, which results in blue-shift of the resonance peak with decrease of the nanoparticle interspacing. While, whenever there is a projection of the polarization of incident light on the center-to-center lines of the nanoparticles, the “attractive coupling mode” between the nanoparticles dominates in their resonant interaction, which results in red-shift of the resonance peak with decrease of the nanoparticle interspacing. It is also shown that optical interaction of the ultra-small nanoparticles is only effective when their interspacings are approximately less than 15 nm, and particularly prominent when Coulomb interactions of the electronic charges of plasmons are active within a few nanometers.