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 - ar e much smaller than the wavelength of the light, the sample can be cha racterized in terms of an effective dielectric function. The Bergman r epresentation of the effective dielectric function takes into account the dielectric properties of the metal and of the host material as wel l as the interaction of the grains. The so-called spectral density occ uring in the Bergman representation describes the influence of the sam ple microstructure, which is important for the correct description of the interaction between the grains. A variety of simple mixing formula e - 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 wit h their own spectral densities depending only on the metal volume frac tion. From the knowledge of the fundamental Bergman representation we analyze the applicability of these simple formulae to metal-insulator systems. Their use may lead to severe misinterpretation of experimenta l data, because the specific microtopology is not accounted for in the right way. We show this for a cermet of Pt spheres in Al2O3 comparing our measurements with calculations on the basis of the simple mixing formulae. It is demonstrated that the measured effective DF is consist ent with the Bergman representation by an adjustment of a spectral den sity to the experimental data. We propose a procedure for the extracti on of the DF of one of the constituents from experimental data.