Effective optical constants of nanostructured thin silver films and impact of an insulator coating

Two distinct electrically percolated types of nanostructured silver (n-Ag) films on glass substrates have been optically and structurally studied and effective refractive indices determined. The first though well above the percolation threshold contains networks of elongated nano-voids over ∼15% of the film area. The second is at a thickness where the film is almost void free. A surprising qualitative shift in spectral response to Drude like behaviour in the n-Ag film with many voids has been found after overcoating with oxide dielectric (Al2O3). With no overcoat much incident light is being shunted through the voids and does not see the metal while with the overcoat the reverse occurs and light passes mainly through the metal. A recent effective medium model for a pair and chain of cylindrical voids is used to explore if quasistatic models can describe these resonant features. It is concluded that induced surface current related dynamic effects are present and impact strongly on effective indices in n-Ag containing nano-voids.