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.
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