Wavelength and refractive index dependence of the geometrical enhancement in surface-enhanced Raman scattering

The overall enhancement in surface-enhanced Raman scattering (SERS) is considered to originate from a combination of electromagnetic and chemical effects. However, various additional enhancements associated with SERS have been reported to arise from geometrical factors such as excitation direction and thin film interference. Here, we theoretically and experimentally study the wavelength and refractive index dependence of the geometrically enhanced SERS signal. Monolayers of thiophenol on thermally evaporated Ag-island films on glass and sapphire substrates were analysed as a model system. The effective optical constants of the nanostructured metal film, as determined by Mueller matrix ellipsometry, were used in an electric-field model that predicts the enhanced SERS signal arising at the boundary of the metal film-substrate interface. SERS experimental measurements were found to exhibit a geometrically enhanced signal in broad agreement with the theoretical model, with the additional enhancements peaking at more than three for both substrates. Designing plasmonic structures by carefully controlling their effective optical constants could lead to increased enhancement factors for SERS sensing on transparent substrates such as optical fibre probes or observation windows. Copyright (c) 2017 John Wiley & Sons, Ltd.