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.