Two-Photon Luminescence Microscopy of Tunable Gold Nanostructures Randomly Distributed on Templates of Anodized Aluminum

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.