We produce and characterize randomly distributed, highly enhancing, large-area gold nanostructures formed on
templates made after anodization of Al with either oxalic acid or phosphoric acid, producing nanoporous alumina films.
The interpore distance of the fabricated templates can be tuned continuously, and by a subsequent selective dissolution of
the upper film making up the porous Al2O3 layers, the remaining embossed barrier layer can be used as a template for
sputter deposition of gold. The density (and structure) of gold nanoparticles covering the template is adjusted by varying
the sputtering conditions. We directly correlate the strong and broad surface plasmon (SP) resonances investigated by
reflection spectroscopy, as well as the field intensity enhancement (FE) factor investigated by far-field two-photon
luminescence (TPL) scanning optical microscopy measurements, to the density of the randomly distributed gold
nanoparticles on the templates. The position of high enhancements in the TPL-images and the magnitude of the average
FE are dictated by the laser excitation wavelength. We relate this large-area massive enhancement to constructive
interference of SP polaritons scattered from the densely packed gold particles on the fabricated templates.
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