Anomalous infra-red absorption of nanocermets in the percolation range

We present a two-dimensional simulation of the optical absorption of gold granular films, using a real space renormalization procedure. This numerical effective-medium theory takes into account the actual morphology of the films. Results are compared both with experimental measurements and with other theoretical predictions asserting that the optical properties around the percolation threshold cannot be described by an effective dielectric function. Nevertheless, we obtain a good agreement between simulations and experimental data, whatever the morphology and the concentration. It is concluded on both theoretical and experimental arguments that the IR absorption can be attributed to classical surface plasmon modes, generally situated in the visible spectral range for spheroidal inclusions, broadened here towards higher wavelengths due to the formation of fractal clusters of various sizes and shapes.