Optical properties of discontinuous gold films

Abstract

We prepared discontinuous gold films by ultrahigh-vacuum deposition onto Corning 7059 glass substrates. Optical transmittance at normal incidence was recorded; an anomalous absorption band occurred in the visible range. We described the optical properties within the Maxwell Garnett formalism with proper account taken of a size-dependent dielectric permeability of the islands. These were represented by prolate spheroids with symmetry axis along the substrate; their morphology was characterized by mean eccentricity, log normal distributions of half axes, and average interisland separation. Interactions among the islands were accounted for by following a procedure by Bedaux and Vlieger. The computed transmittances were lowered significantly by evan narrow distributions of interparticle separations. This complex situation was simulated by one parameter (the only free parameter in our theory); reasonable values yielded computed transmittances in excellent agreement with measurements as long as the islands were not too irregular. Effects of substrate interaction and dielectric pellicles on the islands were shown to be less influential. Hence, the optical properties of discontinuous metal films seem to be governed entirely by the morphology of the islands, and possible effects of modified band structure and/or size quantizations are not manifest.