  Gold nanoparticle assemblies show a strong plasmonic response due to the
combined effects of the individual nanoparticles' plasmon modes. Increasing the
number of nanoparticles in structured assemblies leads to significant shifts in
the optical and physical properties. We use Finite-Difference Time-Domain
(FDTD) simulations to analyze the electromagnetic response of structurally
ordered gold nanorods in monomer and dimer configurations. The plasmonic
coupling between nanorods in monomers or dimers configurations provides a
unique technique for tuning the spectrum intensity, spatial distribution, and
polarisation of local electric fields within and surrounding nanostructures.
Our study shows an exponential coupling behavior when two gold nanorods are
assembled in end-to-end and side-by-side dimer configurations with a small
separation distance. The maximum electric field in the gaps between adjacent
nanorods in end-to-end dimer configuration describes a more significant
enhancement factor than the individual gold nanorod. Our FDTD simulation on
dimer in end-to-end assembly for small separation distance up to ~ 40 nm can
well explain the observed experimental growth dynamics of gold nanorods.
