  We report on optical reflection measurements of thin Au films at and around
the percolation threshold (film thickness 3 to 10 nm) in an extremely broad
spectral range from 500 to 35000 cm-1 (0.3 - 20 *10-6 m). Combining
spectroscopic ellipsometry and Fourier-transform infrared spectroscopy, the
dielectric properties of the films can be described over the whole frequency
range by Kramers-Kronig consistent effective dielectric functions. The optical
conductivity of the films is dominated by two contributions: by a
Drude-component starting at the percolation threshold in the low frequency
range and a plasmon in the near-infrared region, which shifts down in frequency
with increasing film thickness. The interplay of both components leads to a
dielectric anomaly in the infrared region with a maximum of the dielectric
constant at the insulator-to-metal transition.
