Two-Step Decomposition of Plasmon Coupling in Plasmonic Oligomers

Plasmon coupling in plasmonic oligomers gives rise to many unique optical properties including electromagnetically induced transparency and Fano resonances. Controlling over the optical properties of plasmonic oligomers requires a deep understanding of the physical mechanisms. Herein, we first examine the near and far field optical properties of silver pentamer by the coupled dipole approximation. It is shown that the Fano-like resonance in the pentamer is analogous to the spectrum of a heterogeneous plasmonic dimer. Therefore, we propose a subgroup decomposition method for predicting the spectrum profiles of the oligomers. In the decomposition procedure, the oligomer is divided into simple subgroups in terms of the arrangement of nanoparticles and incident polarization, and the optical response of the oligomer is determined by the coupling of the subgroups. We further investigate its validity in strongly coupled plasmonic oligomers through the generalized multiparticle Mie solution. The optical properties of the close-packed oligomers can also be easily modulated by means of the decomposition method, which provides a flexible tool for the optimal design of plasmonic nanostructures.