Effects of Aspect Ratio Heterogeneity of an Assembly of Gold Nanorod on Localized Surface Plasmon Resonance

We examine the effects of aspect ratio (AR) heterogeneity of an assembly of gold nanorods (GNRs) in a colloid on the total cross-section for its light scattering via localized surface plasmons at visible wavelengths. We observe the extraordinary broadening of the extinction spectrum of light through an assembly of GNRs, a colloidal mixture of those having two different ARs. The interparticle distance estimated as similar to 1.2-1.3 mu m, being greater than the incident wavelength, allows the radiative dipolar coupling to govern the long-range interaction between GNRs. We find that the coupling enhanced local fields can activate the nonresonant polarization of GNRs to turn into a quasi-resonant one. These higher-order effects for GNR polarization can produce the deviation of total cross-section of GNRs assembly beyond the simple sum of an individual cross-section of GNRs that are assumed to have no such long-range coupling. The extraordinary properties of the extinction spectrum need to be taken into account for modulating the spectral distribution of electromagnetic field in photonic devices where an assembly of GNRs is utilized for field enhancement such as those for surface-enhanced spectroscopy, highly efficient photovoltaics, photothermal nanotherapy, and ultrathin absorption filters.