Optical properties of aggregates of small metal particles

Many-particle systems exhibit unique optical properties which strongly depend on the arrangement of the individual particles in the sample. Novel methods were applied to produce gold particle samples with their arrangements systematically varying from statistical disorder to densely packed particle aggregates of various mean sizes and shapes. Their optical extinction spectra vary from narrow bands in the VIS to broad multiple band structures extending into the IR, the peak structure proving to be more pronounced if the particles are larger. Numerical calculations of explicit extinction spectra were performed on corresponding prototypic particle aggregates taking both interior and exterior retardation effects into account. In special, for aggregates of 2 to 5 particles, respectively, definite attachments of measured extinction peaks to special aggregate types were achieved from these calculations and confirmed by TEM micrographs of the investigated samples.