Effect of symmetry breaking on the mode structure of trimeric plasmonic molecules

Symmetry breaking enriches the spectrum of interactions of surface plasmons with light by introducing phenomena that are not observed in nanostructures with high symmetry. We investigate plasmonic spectra of nanoparticle trimers as a function of a gradual change in their geometry, which implies a smooth transition between different symmetry groups. Three homologous series are presented, starting from an equilateral trimer and ending with either a linear chain of three nanoparticles, a dimer, or a monomer. Degeneracy of the plasmon modes of highly symmetric clusters is lifted, and the energy splitting of the new modes and the corresponding resonance intensities strongly depend on the cluster geometry. Single-cluster optical plasmon spectroscopy is correlated with TEM imaging of the clusters and numerical simulations. The results are understood with the aid of symmetry correlation tables and plasmon-hybridization theory, which reveal the origins of transformations observed in the plasmon spectra. The changes in mode energies and characters that are most crucial for the correct interpretation of mode evolution along the series are found to occur in a narrow range of cluster geometrical parameters, involving in some cases an avoided crossing between two modes.