17 September 2016 Surface-enhanced Raman scattering: effective optical constants for electric field modelling of nanostructured Ag films
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Abstract
Surface-enhanced Raman scattering (SERS) is drawing increasing interest in fields such as chemical and biomolecular sensing, nanoscale plasmonic engineering and surface science. In addition to the electromagnetic and chemical enhancements in SERS, several studies have reported a “back-side” enhancement when nanostructures are excited through a transparent base rather than directly through air. This additional enhancement has been attributed to a local increase in the electric field for propagation from high to low refractive index media. In this study, Mueller matrix ellipsometry was used to derive the effective optical constants of Ag nanostructures fabricated by thermal evaporation at oblique angles. The results confirm that the effective optical constants of the nanostructured Ag film depart substantially from the bulk properties. Detailed analysis suggests that the optical constants of the nano-island Ag structures exhibit uniaxial optical properties with the optical axis inclined from the substrate normal towards the deposition direction of the vapour flux. The substrates were functionalized with thiophenol and used to measure the wavelength dependence of the additional SERS signal. Further, a model based on the Fresnel equations was developed, using the Ag film optical constants and thickness as determined by ellipsometry. Both experimental data and the model show a significant additional enhancement in the back-side SERS, blue shifted from the plasmon resonance of the nanostructures. This information will be useful for a range of applications where it is necessary to understand the effective optical behaviour of thin films and in designing miniaturized optical fibre sensors for remote sensing applications.
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M. Nilusha M. N. Perera, M. Nilusha M. N. Perera, Daniel Schmidt, Daniel Schmidt, W. E. Keith Gibbs, W. E. Keith Gibbs, Saulius Juodkazis, Saulius Juodkazis, Paul R. Stoddart, Paul R. Stoddart, } "Surface-enhanced Raman scattering: effective optical constants for electric field modelling of nanostructured Ag films", Proc. SPIE 9921, Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XIV, 992125 (17 September 2016); doi: 10.1117/12.2236169; https://doi.org/10.1117/12.2236169
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