  Percolation of gold films of ~15 nm thickness was controlled to achieve the
largest openings during Au deposition. Gold was evaporated on 300-nm-thick
films of nanostructured porous and columnar SiO2, TiO2 and MgF2 which were
deposited by controlling the angle, rotation speed during film formation and
ambient pressure. The gold films were tested for SERS performance using
thiophenol reporter molecules which form a stable self-assembled monolayer on
gold. The phase retardation of these SERS substrates was up to 5% for
wavelengths in the visible spectral range, as measured by Stokes polarimetry.
The SERS intensity on gold percolation films can reach 10^3 counts/(mW.s) for
tight focusing in air, while back-side excitation through the substrate has
shown the presence of an additional SERS enhancement via the Fresnel near-field
mechanism.
