Optical response of Cermet composite films in the microstructural transition region

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Measurements of the intrinsic optical constants $n$ and $k$ have been made on Au-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ Cermet thin films with metal-fill fractions above and below the percolation threshold. The simple Maxwell-Garnett effective-medium theory, which is successful at low-metal-fill fractions, is shown to be inadequate in the description of the system near this threshold, where the microstructure of the composite undergoes a radical change. A modified effective-medium theory, based on a probabilistic growth model recently introduced by Sheng [Phys. Rev. Lett. 45, 60 (1980)], is found to predict the optical response in the transition region with few adjustable parameters.