Optical response of fractal aggregates of polarizable particles

We theoretically investigate the optical response of ensembles of polarizable metallic nanoparticles (NPs) that form (i) submonolayer films considered as 2D systems, and (ii) thin 3D films where NPs are embedded in a dielectric matrix. In both cases we find that short-range clustering leads to a broadening and a spectral shift of the absorption band related to the surface plasmon resonance in inividual NPs. We show that clustering can help achieving spectrally broad surface plasmon resonance (SPR) bands, especially if NPs aggregate into fractal clusters, which can be interesting for some applications, such as SERS. In particular, submonolayer films on NPs generated using the dillusion-limited aggregation algorithm, produce sizable and spectrally broad absorption, which can be tuned to the visible range by choosing an appropriate substrate. Calculated results for thin 3D films are compared to experimental data obtained for Au=TiO₂ nanocomposite layers produced by reactive co-sputtering.