Far infrared optical properties of metal-insulator composites

Measurements of the optical constants of metal-insulator composites in the far infrared provide information about the dielectric functions (DF) of the components and about the electromagnetic interaction of the different grains. If the metal particles — and their separations — are much smaller than the wavelength of the light, the sample can be characterized in terms of an effective dielectric function. The Bergman representation of the effective dielectric function takes into account the dielectric properties of the metal and of the host material as well as the interaction of the grains. The so-called spectral density occuring in the Bergman representation describes the influence of the sample microstructure, which is important for the correct description of the interaction between the grains. A variety of simple mixing formulae — each reflecting a specific microgeometry — have been published, e. g. the Maxwell-Garnett formula, the Looyenga formula or the Bruggeman formula. They all are special cases of the Bergman representation with their own spectral densities depending only on the metal volume fraction.