Grain and void resonances in granular silver films near the percolation threshold: Experiment and simulation using an entropic model

Abstract

Granular metal films with low metal fractions exhibit the well-known resonant absorption, due to the excitation of surface plasmons in the metallic grains, that is not satisfactorily described by the effective medium theories (EMT) close to the percolation threshold. We have recently developed an optical model, which is based on the entropic analysis in the actual image of a heterogeneous medium. This model accounts well for the optical properties of silver granular films around the percolation threshold. In addition to the so-called plasmon resonant absorption, the model predicts a weak absorption, on the edge of the main one, which we attribute to the dielectric void resonance. Both absorptions are effectively observed in granular films close to the percolation threshold. Our model gives a fair account of the position of these two resonant absorptions and of their variation when the index of the dielectric medium is experimentally changed by embedding the film in a glycerin layer.