Spectral switching of the preferred transmission direction in absorbing anisotropic composites

Transmittance in inhomogeneous composite structures in which the arrangement or shape of an absorbing component is anisotropic, depends on the incident direction in relation to the axes that define the anisotropy. There are easily distinguished preferred transmittance directions in a given plane of incidence. However a complete reversal in the preferred direction, which is at first sight counter intuitive, has been observed in several anisotropic thin films containing small quantities of silver inclusions in partially ordered arrays. This switching occurs at a particular wavelength and is studied in terms of transmittance at equal angles of incidence either side of the normal. A filter which is low pass or high pass depending on direction of incidence may be possible based on this phenomenon. A theoretical explanation is given in terms of a two-component effective dielectric response of the composite. In the samples studied the splitting of the resonant absorption spectra for the two principal directions is seen to be the underlying cause of switching in the preferred transmittance direction. The experimentally observed optical data, and the observed microstructures, demand the use of advanced effective medium models which incorporate strong interaction effects.