Plasmon resonant metal nanoparticles on substrates have been considered for use in several nanophotonic applications
due to the combination of large field enhancement factors, broadband frequency control, ease of fabrication, and
structural robustness that they provide. Despite the existence of a large body of work on the dependence of the
nanoparticle plasmon resonance on composition and particle-substrate separation, little is known about the role of
substrate roughness in these systems. This is in fact an important aspect, since particle-substrate gap sizes for which large
resonance shifts are observed are of the same order of typical surface roughness of deposited films. In the present study,
the plasmon resonance response of 80 nm diameter gold nanoparticles on a thermally evaporated gold film are
numerically calculated based on the measured surface morphology of the gold film. By combining the measured surface
data with electromagnetic simulations, it is demonstrated that the plasmon resonance wavelength of single gold
nanoparticles is blueshifted on a rough gold surface compared the response on a flat gold film. The anticipated degree of
spectral variation of gold nanoparticles on the rough surface is also presented. This study demonstrates that nanoscale
surface roughness can become an important source of spectral variation for substrate tuned resonances that use small gap
sizes.
Optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field
enhancement factors that significantly exceed those observed in isolated nanostructures. While previous studies
demonstrated the existence of such cascaded field enhancement in coupled nanospheres with identical composition, this
effect has not yet been studied in systems containing multiple materials. Here, we investigate the polarization-dependent
optical response of multi-material trimer nanostructures composed of Au nanoparticles surrounded by two Ag
nanoparticles as a function of nanoparticle size and inter-particle spacing. We observe field enhancement factors that are
ten times larger than observed in isolated Au nanoparticles.
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.
Frequency control of plasmon resonances is important for optical sensing applications such as Surface Enhanced Raman
Spectroscopy. Prior studies that investigated substrate-based control of noble metal nanoparticle plasmon resonances
mostly relied on metal substrates with organic or oxide spacer layers that provided a fixed resonance frequency after
particle deposition. Here we present a new approach enabling continuous resonance tuning through controlled substrate
anodization. Localized Surface Plasmon tuning of single gold nanoparticles on an Al film is observed in single-particle
microscopy and spectroscopy experiments. Au nanoparticles (diameter 60 nm) are deposited on 100 nm thick Al films
on silicon. Dark field microscopy reveals Au nanoparticles with a dipole moment perpendicular to the aluminum surface.
Subsequently an Al2O3 film is formed with voltage controlled thickness through anodization of the particle coated
sample. Spectroscopy on the same particles before and after various anodization steps reveal a consistent blue shift as the
oxide thickness is increased. The observed trends in the scattering peak position are explained as a voltage controlled
interaction between the nanoparticles and the substrate. The experimental findings are found to closely match numerical
simulations. The effects of particle size variation and spacer layer dielectric functions are investigated numerically. The
presented approach could provide a post-fabrication frequency tuning step in a wide range of plasmonic devices, could
enable the investigation of the optical response of metal nanostructures in a precisely controlled local environment, and
could form the basis of chemically stable frequency optimized sensors.
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