Numerical study of optical absorption in two-dimensional metal-insulator and normal-superconductor composites

We analyze a random resistor-inductor-capacitor (RLC) lattice model for the optical properties of a two-dimensional normal-metal--insulator composite, using the Y-{Δ} transformation algorithm developed by Frank and Lobb. Within such a model, the surface plasmon resonances of a Drude-metal--insulator composite are modeled by the ac resonances of a random RLC network. The real part of the effective conductance is found to show a broad surface plasmon resonance peak below and above the metal percolation threshold, and a Drude peak above the threshold. An effective-medium-approximation (EMA) calculation is in excellent agreement with the results of the simulations. We also calculate the far-infrared absorption in a model composite of normal metal and superconductor, using a lattice model. The absorption shows a strong absorption below the superconducting energy gap. An approximate calculation based on the EMA is again in excellent agreement with these results.