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Vibrational coupling in plasmonic molecules

Significance In this work, we provide evidence of vibrational coupling between the constituent atoms in a plasmonic molecule mediated by coherent substrate phonons. A simple coupled harmonic oscillator model provides fundamental insight into the optomechanical selection rules for the launching of vibrational modes. The coupling strength-dependent vibrational frequency shifts can be directly manipulated by precise tuning of the structure of the plasmonic molecule by using top-down lithography. This tunability paves the way for new strategies to design optomechanical devices based on metallic nanostructures. Plasmon hybridization theory, inspired by molecular orbital theory, has been extremely successful in describing the near-field coupling in clusters of plasmonic nanoparticles, also known as plasmonic molecules. However, the vibrational modes of plasmonic molecules have been virtually unexplored. By designing precisely configured plasmonic molecules of varying complexity and probing them at the individual plasmonic molecule level, intramolecular coupling of acoustic modes, mediated by the underlying substrate, is observed. The strength of this coupling can be manipulated through the configuration of the plasmonic molecules. Surprisingly, classical continuum elastic theory fails to account for the experimental trends, which are well described by a simple coupled oscillator picture that assumes the vibrational coupling is mediated by coherent phonons with low energies. These findings provide a route to the systematic optical control of the gigahertz response of metallic nanostructures, opening the door to new optomechanical device strategies.