Effect of film thickness on the far- and near-field optical response of nanoparticle-on-film systems

We study the near-field and far-field optical responses of nanoparticle-on-film systems using single-nanoparticle spectroscopy and numerical simulations. We find that the optical spectra contain three dominant modes: a transverse dipole, quadrupole mode, and a dominant vertical antenna mode. We vary the thickness of the metal film from 10 to 45 nm and find that the vertical antenna mode wavelength is nearly independent of the film thickness. In contrast, we find that the associated near-field enhancement in the gap between the particle and the film strongly depends on the film thickness. This trend is also observed in the far field where the vertical antenna mode strongly increases in amplitude relative to the quadrupole for the increasing film thicknesses up to the skin depth of gold. These findings are in good agreement with a numerical model and pave the way to study field-mediated processes such as fluorescence, surface-enhanced Raman spectroscopy, and localized chemistry at the same resonance wavelength but at varying degrees of field enhancement.