  The influence of the orientation of gold nanorods in different assemblies has
been investigated using the Finite Difference Time Domain (FDTD) simulation
method. To understand the relative orientation, we vary the size and angle in
dimer geometries. Significant effects of plasmon coupling emerged in
longitudinal resonances having end-to-end configurations of gold nanorods. The
effect of orientational plasmon coupling in dimers gives rise to both bonding
and anti-bonding plasmon modes. Effects of various geometries like primary
monomer, dimer, trimer, and tetramer structures have been explored and compared
with their higher nanorod ensembles. The asymmetric spectral response in a 4 *
4 gold nanorods array indicates a Fano-like resonance. The variation of gap
distance in ordered arrays allowed modulation of the Fano resonance mode. The
plasmon modes' resonance wavelength and field enhancement have been tuned by
varying the gap distance, angular orientation, size irregularity between the
nanorods, and nanorod numbers in an array. The integrated nanostructures
studied here are not only significant for fundamental research but also
applications in plasmon-based devices.
