Optical and electrical properties of black gold layers in the far infrared

Black gold layers, prepared by an evaporation process in an inert atmosphere, are an efficient absorber in the far infrared spectral region. Experimental results are presented for their reflectance in the wavelength range between λ=15 μm and 1000 μm, corresponding to a wave number k=1/λ between 10 cm⁻¹ and 650 cm⁻¹. The black gold layers may electrically be either metallic or semiconducting, the metallic ones exhibiting higher absorbance. The layers have a percolated structure whose dielectric properties are described by the Bergman formalism for an effective medium. On the basis of this model, the deviation of the optical properties of the layers from bulk gold are accounted for solely by the topology of the percolated structure. The values derived for the electron collision time in the gold particles and for the percolation factor agree with those derived from the electrical conductivity of the layers and its temperature dependence.