Light enhancement in surface-enhanced Raman scattering at oblique incidence

Abstract

Surface enhanced Raman scattering (SERS) measurements have been carried out at different focusing conditions using objective lenses of different numerical apertures. The experimentally observed dependence of SERS intensity of thiophenol-coated Ag nano-islands shows a close-to-linear scaling with the collection aperture. The linear relationship breaks down for large numerical apertures, which suggests that the scattering is anisotropic. Numerical simulations of realistically shaped Ag nano-islands were carried out, and the spatial distribution of hot-spots has been revealed at different heights near the nano-islands. Local field enhancements of up to 100 times were estimated. The simulation also suggests an explanation for the anisotropy in the scattering observed for larger numerical aperture objectives. This appears to be due to a reduction in the local field enhancement as the electric field vector component in the plane of the shallow metal islands reduces at larger angles of incidence.

Additional Enhancement of Electric Field in Surface-Enhanced Raman Scattering due to Fresnel Mechanism

“…Note that there is considerable evidence that the SERS signal tends to be dominated by scattering from hot spots such as the nanogap considered in this model2425. Recent FDTD modeling has confirmed that this assumption remains valid for the case of silver island films formed by oblique island deposition10.…”

“…Higher NA objectives will tend to introduce a perpendicular polarization component with respect to the dielectric surface. This polarization will not be effective in exciting hot spots between neighboring metal particles, thus causing a decrease in the average SERS intensity10. This effect may be further exaggerated due to diffraction in the far-side excitation geometry.…”

“…This EM mechanism can be used to explain several facets of SERS including the distance dependence, metal specificity, nanoparticle shape and size dependence and the generation of SERS ‘hot spots'78910. Computer-based modeling of the EM mechanism not only provides a quantification of the effect but can also guide efforts to engineer the surface geometry in order to maximize the signal yield6.…”

Surface Enhanced Raman Scattering Substrates Made by Oblique Angle Deposition: Methods and Applications

“…It was also observed that the s polarised light gave greater Raman intensities, a result that was consistent with the FDTD calculations. FDTD has also been used to investigate the anisotropic effects of using different objectives and apatures for SERS measurements [45]. Here, the linear relationship of larger apature with Raman intensity was observed to breakdown for large (solid angle > 1) numerical apatures.…”

Fabrication of nanoparticles for generation of force and torque at nanoscale

“…This requires a very accurate fabrication capability in order to control the particle layout and shape in ordered or semi-random patterns, as has been shown in several instances in light harvesting and sensing applications, [3][4][5][6] and in the development of light-driven nano-motors. 7 When such tiny objects are in close proximity of metal edges and grooves, especially of regions where field enhancement hot-spots are located, they selectively interact with the light near-field increasing or reducing the trapping force.…”