Influence of the close sphere interaction on the surface plasmon resonance absorption peak.

Effects of agglomeration in optical properties of noble metal colloids (gold) have been calculated by an integral equation formalism valid for small (around 6 nm) nanoparticles in the spectral region corresponding to surface plasmon resonance (SPR). These calculations are characterized by their accuracy and for the generality of their solutions, which do not depend on the nature of the materials nor the wavelength of the incident electric field. Chains of two (dimers), three (linear trimers) and four (linear tetramers) spheres have been taken as geometric models. Results indicate that for distances between spheres larger than 0.3 times the diameter, only dipolar interactions are relevant. For smaller distances between spheres, multipolar interactions generate low frequency modes that transform into a continuous band in the limiting case of touching spheres, band asymmetry and SPR broadening. Additionally, strong and sharp oscillation corresponding to surface charge density and electric field appear in the close neighborhood of sphere contact. These features are very difficult to determine by ad-hoc simulations but they should be taken into account when dealing with nearly touching nanosphere systems.