OPTICAL RESONANCES OF 3-PHASE COMPOSITES AND ANOMALIES IN TRANSMISSION

Composite media prepared by coevaporation of two constituents are modelled, in this work, as three-phase composites to take account of the presence of void spaces, which are inevitably formed during growth. Previous work on the effective dielectric constant of regular arrays of coated cylinders revealed the unexpected phenomenon of the extension of core properties to and beyond the coating (or shell), at certain 'resonance wavelengths'. We extend this treatment first to the case of regular arrays of coated spheres and then to the case of disordered arrangements of cylinders and spheres. Our model uses the quasistatic approximation and is applicable to composite media when the size of the inclusions is much smaller than the wavelength of the incident radiation. The model is intended to predict if a three-phase composite exhibits resonances and, if so, what are the wavelengths of the incident radiation producing this effect. The results of the model are compared with experimental measurements on Ag-MgF2 cermets by treating the cermets as three-phase composites containing voids. The resonance wavelengths are identified with features in the transmission, reflectance and absorption spectra of the cermets.