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 35 000 cm −1 (0.3 -20 µm). 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.

While the optical properties of thin metal films are well understood in the visible and near-infrared range, little has been done in the mid-and far-infrared region. Here we investigate ultra-thin gold films prepared on Si(111)(7 × 7) in UHV by measuring in the frequency range between 500 cm −1 and 7000 cm −1 and for temperatures between 300 K and 5 K. The nominal thickness of the gold layers ranges between one monolayer and 9 nm. The frequency and temperature dependences of the thicker films can be well described by the Drude model of a metal, when taking into account classical size effects due to surface scattering. The films below the percolation threshold exhibit a non-metallic behavior: the reflection increases with frequency and decreases with temperature. The frequency dependence can partly be described by a generalized Drude model. The temperature dependence does not follow a simple activation process. For monolayers we observe a transition between surface states around 1100 cm −1 .

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