On Percolation and Circular Bragg Phenomenon in Metallic, Helicoidally Periodic, Sculptured Thin Films

The concept of local homogenization is invoked in the visible and the sub-visible frequency regimes to estimate the permittivity dyadics of a thin-film helicoidal bianisotropic mediums (TFHBMs)~---~a canonical class of sculptured thin films with helicoidally periodic microstructure~---~made of a metal. Examination of the components of the permittivity dyadics, as well as of the co- and the cross-polarized reflectances of axially excited, metallic TFHBM halfspaces, reveals that three different absorbing-dielectric-to-metal transitions will occur as the metallic volumetric fraction is increased, but only two will be optically sensed using normally incident plane waves. The first transition is show to virtually eliminate the circular Bragg phenomenon characteristically displayed by axially excited, dielectric TFHBMs.