Homogenized Lorentz–Drude optical response in highly nanoporous conducting gold layers produced by de-alloying

Nanoporous gold films produced by de-alloying AuAl2 with void densities between 45% and 65% retain high infra-red reflectance and good conductivity. They act optically like a homogeneous Lorentz–Drude metal with a unique plasma frequency ωp∗ and an inter-band transition energy unchanged from that of dense gold. The link between ωp∗ and ωp in dense gold is found using a simplification of the Bergman expansion for permittivity valid at infra-red wavelengths. The carrier relaxation time of the “effective metal” becomes the actual relaxation time in the Au network and the complex refractive indices (n, k  ) found using normal incidence spectrophotometry and oblique incidence ellipsometry agree closely with each other. The single pole approximation for the ratio ωp∗/ωp in the infra-red allows estimates of void content and the apparent shift in carrier effective mass. It is then possible to model with no adjustable parameters, the full UV–visible–NIR spectral response, giving excellent agreement with data. A range of films with these properties are presented.