Two-photon luminescence microscopy of
large-area gold nanostructures on templates of anodized aluminum

Abstract

Using linear reflection spectroscopy and far-field two-photon luminescence (TPL) scanning optical microscopy, we characterize highly enhancing, large-area gold nanostructures formed on porous templates made by anodization of aluminum with either oxalic acid or phosphoric acid. These templates are formed by a newly developed, stepwise technique making use of protective top oxide layers facilitating continuously tunable interpore distances. The upper, porous alumina layers are subsequently removed and the remaining embossed barrier layer is used as template for the sputtered gold, where the density of gold particles covering the sample is adjusted by regulating the sputtering conditions. We observe spatially averaged field intensity enhancement (FE) factors of up to ~5.210(2) and bright spots in the TPL-images exhibiting maximum FE factors of up to approximately 1410(2) which is the largest estimated FE from any hitherto examined structures with our setup. We relate this large-area massive FE to constructive interference of surface plasmon (SP) polaritons scattered from the densely packed, randomly distributed gold particles and directly correlate this particle density with the strong and broad SP resonances as well as the magnitude of the FE factors. The average FE and the position of high enhancements in the TPL-images are dictated by the excitation wavelength, and the structures could evidently serve as versatile structures facilitating practical molecular sensing.

Experimental study on polarized surface enhanced resonance Raman scattering of rhodamine 6G adsorbed on porous Al2O3 substrates

“…Large-area self-organizing hexagonally ordered Au nanostructures can be fabricated by sputter-coating the embossed Al template after removing the porous oxide layer created by hard anodization (HA) in oxalic acid. 1,18 The hexagonally ordered porous Al 2 O 3 used in the present study was prepared using annealed (500 • C) and electropolished high purity (99.999%) Al foils. The 7.5 min of pre-anodization at 40 V in 0.30 M oxalic acid (step 1, Figure 1) was carried out in order to form a protective oxide layer (200-400 nm) on the Al surface.…”

Hemispherical Shell Nanostructures from Metal-Stripped Embossed Alumina on Aluminum Templates

“…When using higher voltages with oxalic acid or other electrolytes (so-called hard anodization condition), an escalating risk of electrolyte breakdown and substrate burning will follow. Further details of Al/Al 2 O 3 template preparation have been reported earlier. ,, …”

“…Self-organizing porous alumina (Al 2 O 3 ) films on aluminum (Al) now attract increasing attention as templates for large-area ordered or randomly distributed metallic nanoparticles − incorporated in, for example, sensors, solar cells, , and metamaterials . By carefully tailoring the temperature, anodization time, t , and anodization voltage, U , for a given electrolyte, the dimensions of the hexagonally ordered porous oxide can be controlled over a wide continuous range. − …”

Surface‐enhanced Raman microscopy of hemispherical shells stripped from templates of anodized aluminum

“…Nanostructured templates for large‐area ordered or randomly distributed metallic nanoparticles incorporated in, e.g. sensors, solar cells, and metamaterials can be fabricated by utilizing self‐organizing porous alumina (Al 2 O 3 ) films on aluminum (Al).…”

“…Focusing and localizing SPs close to nanometer‐sized metallic objects and in sharp grooves can lead to extreme light concentration in nanometer‐sized regions creating thereby electromagnetic hot‐spots. This phenomenon has been successfully utilized to enhance a number of linear and nonlinear optical effects and sharp grooves formed by touching, parallel (kissing) nanowires can potentially be exploited for plasmonic light‐harvesting devices over the whole visible spectrum . Conducting detailed optical characterization of electromagnetic hot‐spots on well‐defined nanopatterned surfaces, including mapping their spatial locations, facilitates understanding and optimization of the structures for use within, e.g.…”