OPTICAL RESONANCES OF 3-PHASE COMPOSITES AND ANOMALIES IN TRANSMISSION

Composite media prepared by coevaporation of two constituents are mode lled, in this work, as three-phase composites to take account of the p resence of void spaces, which are inevitably formed during growth. Pre vious work on the effective dielectric constant of regular arrays of c oated cylinders revealed the unexpected phenomenon of the extension of core properties to and beyond the coating (or shell), at certain 'res onance wavelengths'. We extend this treatment first to the case of reg ular arrays of coated spheres and then to the case of disordered arran gements of cylinders and spheres. Our model uses the quasistatic appro ximation and is applicable to composite media when the size of the inc lusions 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 radiat ion producing this effect. The results of the model are compared with experimental measurements on Ag-MgF2 cermets by treating the cermets a s three-phase composites containing voids. The resonance wavelengths a re identified with features in the transmission, reflectance and absor ption spectra of the cermets.