  The direct writing using a focused electron beam allows for fabricating truly
three-dimensional structures of sub-wavelength dimensions in the visible
spectral regime. The resulting sophisticated geometries are perfectly suited
for studying light-matter interaction at the nanoscale. Their overall optical
response will strongly depend not only on geometry but also on the optical
properties of the deposited material. In case of the typically used
metal-organic precursors, the deposits show a substructure of metallic
nanocrystals embedded in a carbonaceous matrix. Since gold-containing precursor
media are especially interesting for optical applications, we experimentally
determine the effective permittivity of such an effective material. Our
experiment is based on spectroscopic measurements of planar deposits. The
retrieved permittivity shows a systematic dependence on the gold particle
density and cannot be sufficiently described using the common Maxwell-Garnett
approach for effective medium.
