Tunable photonic nanostructures: Semi-transparent metallic opal replicas and nanocomposites

摘要

We measured the reflectivity spectra of several metallic PBG structures. We observed Bragg stop bands in the visible spectral range and a metallicity gap at ω_p < ω_p in metallic PBG structures with a network topology. We found that the spectral range at which the metallicity gap opens depends on the lattice parameters and filling factor of the metallic components. In addition, we also observed a new reflectivity band in the visible spectral range that is related to the surface plasmons of these metallic mesh structures; it is more pronounced in metallic replicas. The position of this optical gap in the visible depends very weakly on the filling factor or impinging angle of the light beam. It roughly occurs at ω_p/(ε_r+1)~(1/2), where 豞p is the slightly modified plasma frequency of the bulk. This result allows to give some predictions on the optimal conditions for making metallic nanostructures, with maximized electrical conductivity and optical transparency. The concept of highly transparent and conductive metallic nanostructures can be qualitatively understood using the diagram in fig. 8. Moreover, the viability of this concept is evident by comparing the quite different optical reflectivity of usual metals (fig. 8(a)) with those we have observed in our metallic photonic crystals. Good metals haw negative ε < 0 below the plasma frequency ω_p, which is typically in the ultraviolet, so they reflect photons with ω < ω_p. In order to achieve high conductivity, high transparency and high internal surface area, one should fabricate network-type metallic inverted opal nanostructures, or in other words with much lower losses, that we have observed and assigned to the brakes in our metallic nanowires.