Optical properties of mesoporous gold films

Mesoporous gold thin films on glass substrates were fabricated by sputtering of AuAl2 precursor films followed by a de-alloying etch. The resulting sponge-like Au films have very high internal surface area due to nanoscale pores and channels. Scattering is not significant and the optical properties for such nanostructured films were examined using ellipsometry and spectrophotometry. The complex refractive indices of the optically equivalent uniform smooth layer satisfy Kramers-Kronig (KK) self-consistency but have unusual dispersion relations and magnitudes for a film containing the amount of noble metal present. The reflectance at infrared wavelengths is neither metal-nor insulator-like, and the indices have unique dispersion curves.