Optical effects associated with aggregates of clusters

The effect of aggregation on the optical properties of nanometer-sized part icles is studied. It is shown that for small noble metal clusters as well a s for pigments of Fe2O3, TiN, or ZrN, the aggregation leads to changes in t he color of the colloidal systems which are caused by electromagnetic coupl ing among the clusters in the aggregates. The model of interacting particle s is shown to be helpful also for interpretation of optical properties of o rganic dyes with incorporated metal clusters and for interpretation of the reflectance of magnetooptical cluster systems. For soot particles it is sho wn that scattering and absorption are enhanced over the whole visible spect ral region compared to isolated carbonaceous clusters. Finally, it is shown that the model of interacting clusters can also be applied for data interp retation in scanning near-field optical microscopy.