Tip-based measurement of the spectral and angular response of plasmonic thin films

Abstract

We have investigated the plasmonic effect of a gold thin film on the optical properties under a range of combinations of incident wavelengths, incident angles and polarization states, while assuming various film thicknesses. Theoretical calculation was performed with rigorous coupled-wave analysis based on the temperature-dependent Drude-Lorentz dispersion model. The calculation method considers the effects of absorption, which is converted to heat in a gold thin film and can affect material parameters such as permittivity. Experimentally, light absorption and field enhancement factor were directly measured using near-field scanning optical microscopy. We have also measured the near-field distribution and thermal effects in the gold thin film. Absorption and field enhancement experimentally measured using three incident wavelengths of 488, 532, and 721 nm for a thin gold film with thicknesses 20, 50, and 70 nm showed good agreement with calculated data. Also observed was the disparity between the incident angles that correspond to maximum absorption and highest field enhancement. The results can help understand the thermal effects on optical properties of plasmonic nanostructures for applications in biological imaging and sensing techniques.