  When two or more metallic nanoparticles are in close proximity, their
plasmonic modes may interact through the near field, leading to additional
resonances of the coupled system or to shifts of their resonant frequencies.
This process is analogous to atom-hybridization, as had been proposed by
Gersten and Nitzan and modeled by Nordlander et al. The coupling between
plasmonic modes can be in-phase (symmetric) or out-of-phase (anti-symmetric),
reflecting correspondingly, the "bonding" and "anti-bonding" nature of such
configurations. Since the incoming light redistributes the charge distribution
around the metallic nanoparticles, its polarization features play a major role
in the nonlinear optical probing of the energy-level landscape upon
hybridization. Thus, controlling the nature of coupling between metallic
nanostructures is of a great importance as it enables tuning their spectral
responses leading to novel devices which may surpass the diffraction limit.
