The apparent optical indices of spongy nanoporous gold

Abstract

Very thin spongy nanoporous gold films have a unique nanostructure and hence unusual properties. Our interest in these materials is also due to their wide range of potential applications [1,2]. An optical study for such nanostructured films is of fundamental interest for understanding how light interacts with such a spongy nanoporous structure. In general the gold either percolates or is very closely packed. Thus surface plasmons, and surface plasmon resonant effects, are expected to play a key role given the large surface area of metal and the metal backbone of the nanostructure. The topological complexity of the nano-void network is also expected to be a major influence. The optical response has, for a metal system, quite unusual dispersion relations for the effective complex refractive index components n*, k*. Once these are better understood, new optical engineering possibilities arise. We are not aware of any optical studies for spongy metal film nanostructures apart from a brief preliminary report of our own on one such film [3] whose nanostructure was different to the spongy nanoporous films presented here. We check the internal consistency and physical acceptability of the results with a Kramers-Kronig analysis of the spectrum of n*, k* values, because of their unusual spectral character.