Optical Characteristic and Numerical Study of Gold Nanoparticles on Al2O3 coated Gold Film for Tunable Plasmonic Sensing Platforms

Substrate-based tuning of plasmon resonances on gold nanoparticles (NP) is a versatile method of achieving plasmon resonances at a desired wavelength, and offers reliable nanogap sizes and large field enhancement factors. The reproducibility and relative simplicity of these structures makes them promising candidates for frequency-optimized sensing substrates. The underlying principle in resonance tuning of such a structure is the coupling between a metal nanoparticle and the substrate, which leads to a resonance shift and a polarization dependent scattering response. In this work, we experimentally investigate the optical scattering spectra of isolated 60 nm diameter gold nanoparticles on aluminum oxide (Al2O3) coated gold films with various oxide thicknesses. Dark-field scattering images and scattering spectra of gold particles reveal two distinct resonance modes. The experimental results are compared with numerical simulations, revealing the magnitude and phase relationships between the effective dipoles of the gold particle and the gold substrate. The numerical approach is described in detail, and enables the prediction of the resonance responses of a particle-on-film structure using methods that are available in many available electromagnetics simulation packages. The simulated scattering spectra match the experimentally observed data remarkably well, demonstrating the usefulness of the presented approach to researchers in the field.