Dielectric properties of ultrathin metal films around the percolation threshold

Optical reflection measurements of thin Au films at and around the percolation threshold (film thickness 3--10 nm) are performed in an extremely broad spectral range up to 35 000 cm^-1. Combining spectroscopic ellipsometry, Fourier-transform infrared spectroscopy, and dc measurements, 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: a Drude component starting at the percolation threshold in the low-frequency range and by plasmons in the near-infrared region, which shift 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. The results are compared to predictions from effective-medium approximations and percolation theory.