Abstract
Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12-nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies. The samples are characterized using scanning electron microscopy images and linear optical spectroscopy. TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3-8-nm, near the percolation threshold. For some thicknesses, TPL measurements reveal super-cubic dependences on the incident power. We ascribe this feature to the occurrence of very strongly localized and enhanced electromagnetic fields due to multiple light scattering in random nanostructures that might eventually lead to white-light generation. Raman images exhibit increasing Raman signals when decreasing the film thickness from 12 to 6-nm and decreasing signal for the 3-nm-film. This feature correlates with the TPL observations indicating that highest FE is to be expected near the percolation threshold.
Highlights
Light interaction with nanostructures and nanostructured materials gives rise to various fascinating optical phenomena occurring at the nanoscale [1]
Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies
TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3–8-nm, near the percolation threshold
Summary
Light interaction with nanostructures and nanostructured materials gives rise to various fascinating optical phenomena occurring at the nanoscale [1]. The spectral position of resonances can be tuned through a variety of parameters such as geometry, composition of nanostructures or size and shape of NPs [12] These structures or NPs represented well-defined regular configurations exhibiting resonant FE at one or several wavelengths [5,6,7,8,9,10] or irregular random nanostructures featuring (spatially separated) resonant excitations covering a wide spectrum range [11]. One route for the fabrication of large-area structures with “hot spots” could be realized with the semi-continuous metallic films [27,28,29] These films can be obtained by evaporation of noble metals like gold or silver onto a dielectric or semiconductor substrate. We anticipate that the results reported in this manuscript will be interesting for applications of various surfaceenhanced spectroscopies, in general, and SERS applications (in bio- and molecular sensing and identification), in particular
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