Electrodynamics of ultrathin gold films at the insulator-to-metal transition

Gold films with a thickness (d = 3 to 10 nm) around the percolation threshold are investigated in a broad spectral range. Their optical conductivity is dominated by two contributions: a Drude-component present above a critical thickness and a plasmon in the near-infrared region, which shifts down in frequency as d decreases. The interplay of both components leads to a maximum in the dielectric constant ε1(d) observed right below the insulator-to-metal transition. For even lower coverage, we find an additional dielectric anomaly where the reflectivity for Au on Si/SiO2? completely vanishes in the infrared region.