Plasmon resonance shift and near electric field of silver nanoparcticles: Theoretical analysis of size, shape, and aggregation effects.

We develop theoretical method to analyze optical response of nanoparticles by combining with discrete dipole approximation. To clarify microscopic level mechanism of plasmon resonance and near field effect, the induced dipole is decomposed into contributions from the electric field by incident light (an incident light term) and that by the induced dipole oscillations (a dipole-dipole interaction term). We examine our theoretical approach to silver nanoparticles and analyze size, shape-dependence of optical response as well as effect of aggregation. The analysis quantitatively reveals that the incident light term dominates the plasmon resonance along the direction of the small diameter whereas the dipole-dipole interaction term is dominant in that of the large diameter. To show effect of aggregation to optical response, we calculate total optical response of interacting two nanospheres. The plasmon resonance along the direction, on which two spheres are aligned, is caused by an interparticle interaction, resulting in the similar behavior to the prolate nanospheroid. These results indicate usefulness of modeling of the optical response of multi-nanoparticles by that of a shape-anisotropic nanoparticle.