Mesoporous gold sponge as a prototype ‘metamaterial’

Mesoporous gold sponge has optical properties that can be intermediate between those of metals and insulators, with a flat spectral response that is unlike that of bulk gold. Films of different thicknesses were produced and an extension of the Lorentz–Drude (LD) model used to model their spectral behavior. We found that it was necessary to include an additional special oscillator centered at 1.4 eV in order to model the unusual spectral response. This is quite unlike bulk gold, which can be modeled using a standard two-oscillator LD model.