THz time-domain spectroscopy of nanometric-thick gold layers

Using THz time-domain spectroscopy (THz TDS), we study the far infrared (0.1-1.8 THz) response of evaporated gold films whose thicknesses spread from 1.5 up to 30 nm. In this frequency domain, free carriers mainly dictate the optical properties of metallic materials. These ultra thin films are composed of islands that are in contact when the thickness is larger than a critical value, namely the percolation threshold, leading to a metallic response for those thicknesses. Well below this threshold, the islands are completely separated and no electric conduction is allowed: the films behave as dielectric layers. Around the critical thickness, percolation-type conductivity is observed. THz TDS permits to clearly observe these 3 regimes and to determine the complex refractive index of ultra thin gold layers in the far infrared.