Optical studies of metal-infiltrated opal photonic crystals

We have studied the optical properties of metal infiltrated opal photonic crystals and metallic replicas made of Sb, Hg, Bi and Pb. We observed dramatic differences in the reflectivity spectra of two kinds of metallic structures, namely those with either network or cermet topologies. We found that all metallic photonic crystals have Bragg stop bands in the visible spectral range. Highly interconnected metallic photonic crystal structures, however, showed about one order of magnitude decrease in the effective plasma frequency compared to that of the bulk metals. Coherent backscattering measurements showed a much larger optical mean free path for the metal-infiltrated opals compared to the skin depth of the metals, in agreement with the reduction in the plasma frequency. We also found an extra band in the reflectivity spectra of network-type samples that is related to surface plasmons. A theoretical model that considers the metallic mesh inside the opal as a cubic wire network agrees well with the experimental results.