Comparison of finite-difference time-domain simulations and experiments on the optical properties of gold nanoparticle arrays on gold film

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

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.

Surface enhanced Raman imaging: periodic arrays and individual metal nanoparticles

Surface enhanced Raman scattering (SERS) from Rhodamine 6G homogenously adsorbed on both periodic arrays of and individual gold nanoparticles is investigated using high-resolution Raman imaging with polarized excitation. Rectangular 50-nm-high nanoparticles of different sizes chosen to ensure the presence of localized surface plasmon resonances close to the 532-nm excitation wavelength are fabricated with electron-beam lithography on the surface of a smooth gold film and arranged both individually (i.e., placed sufficiently far apart) and in 740-nm-period arrays. Linear reflection spectra and high-resolution Raman images obtained from arrays of nanoparticles are compared revealing good correspondence in the spectral dependences of reflection and local SERS enhancements (measured at the top of nanoparticles). The latter are related to those observed with individual nanoparticles. The results obtained emphasize the importance and quantify the influence of particle dimensions, polarized excitation, collctive resonances and SERS locations.