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

Black gold layers, prepared by an evaporation process in an inert atmospher e, are an efficient absorber in the far infrared spectral region. Experimen tal results are presented for their reflectance in the wavelength range bet ween lambda = 15 mu m and 1000 mu m, corresponding to a wave number k = 1/l ambda between 10 cm(-1) and 650 cm(-1). The black gold layers may electrica lly be either metallic or semiconducting, the metallic ones exhibiting high er absorbance. The layers have a percolated structure whose dielectric prop erties are described by the Bergman formalism for an effective medium. On t he basis of this model, the deviation of the optical properties of the laye rs from bulk gold are accounted for solely by the topology of the percolate d 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. (C) 1999 Elsevier Science B.V. All rights reserved.