Analogy to Exciton Coupling in Molecular Aggregates

The polarization and orientation dependence of plasmon coupling described above is very similar to absorption spectra shifts observed in organic molecules upon their dimerization or aggregation. Organic chromophores dimerized head-to-tail with respect to their transition dipole moment direction (known as J-dimers) show an absorption band that is red-shifted compared to that of the isolated 

For the head-to-tail case, the interaction is attractive for dipoles aligned symmetrically. As a result, we see a red-shift in the resonance of the dimer. The configuration corresponding to the dipoles aligned anti-symmetrically is higher in energy, but is optically forbidden, since the dipoles, being equal and opposite, 

In a heterodimer, where the two interacting particles are nonidentical in terms of their transition dipole moments (for instance, two nanorods of dissimilar aspect ratio), it is expected that both the symmetric and anti-symmetric modes would be present in the optical spectrum. Of course, the anti-symmetric mode, otherwise dark in the homodimer, would be significantly weaker in intensity compared to the symmetric mode. 

Plasmon coupling in nanoparticles assemblies can also be visualized 

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