Overwhelming Analogies between Plasmon Hybridization Theory and Molecular Orbital Theory Revealed: The Story of Plasmonic Heterodimers

Plasmon hybridization theory (PHT), an analogue of molecular orbital theory (MOT) for plasmonic molecules, has enjoyed tremendous success over the last decade in discerning the optical features of hybrid nanostructures in terms of their constituent monomeric nanostructures. Dimers of metal nanoparticles served as prototypes in elucidating many of the key aspects of plasmon hybridization. Employing quantum two-state model, in conjunction with the quasi-static approximation and the finite-difference time-domain simulations, we demonstrate that the analogy between PHT and MOT can be further propelled by a theoretical estimation of the plasmon-coupling strengths and the relative contributions of the unhybridized monomeric states toward the hybrid dimeric states in plasmonic Ag-Au nanorod heterodimers. The aspect ratio of the constituent nanorods and the gap size between the monomeric nanorods can further be used as handles to tune the relative contributions of (i) the bonding and the antibonding modes to the total extinction and (ii) the monomeric states toward the dimeric states, with meaningful implications for surface-enhanced spectroscopy. The tunability in light absorption properties of heterodimers in the 400-800 nm region arising as a result of broken symmetry is also suggestive of their potential role as plasmonic rulers for measuring distances.