Theory of far-infrared absorption in small-metal-particle<i>–</i>insulator composites

Abstract

We present a theory of the far-infrared absorption coefficient for metal--alkali-halide composites assumed to consist of finite metal-particle-cluster subcomposites, a geometry very likely to exist in such systems. Three different models for the cluster composite topology are considered: highly metallic clusters, clusters with near-percolation paths of metal, and clusters with weakly allowed tunneling between isolated metallic constituents. For each, the effective dielectric properties of the cluster are first derived and the effective dielectric function for the entire composite is then determined from an application of classical magnetic or electric dipole theory. Subsequent calculations of the absorption coefficient are in reasonable agreement with a number of experimentally observed absorption features, and thus resolve the long-standing discrepancy of orders-of-magnitude difference between theory and experiment in such systems.