Effect of asymmetric morphology on coupling surface plasmon modes and generalized plasmon ruler

Plasmon coupling in aggregated noble metal systems can provide a path to manipulate the optical response purposefully and possesses a wide range of application. Previously, most studies focused on the coupling behavior of Ag-Ag dimers with the same shape. However, plasmon coupling between nanoparticles at different morphologies can provide a new way to modulate optical properties due to broken of symmetry. In this work, we investigate systematically the coupling modes of asymmetric Ag-Ag heterodimers consisting of different morphologies by the boundary element method (BEM). Herein nanoparticles with different surface curvatures (modified by rounding parameter e) are constructed and combined as asymmetric Ag-Ag heterodimers. Simulated electron energy loss spectroscopy (EELS) spectra and eigenmodes are combined to analyze the evolution of coupling modes. The mode energy degeneracy and degeneracy breaking phenomena are found, while the charge states are always not degenerate, for the first time by modulating symmetry of the morphology. It is also found that coupling gap mode G₂ can only be excited for Ag-Ag heterodimers with quite small separation distance, and will be greatly influenced by nanogap morphology. The rounded effect can also cause distinct blue shift of bounding dipolar modes. These results provide the possibility to modulate optical response by using different asymmetric dimers effectively. In contrast, optical response of high-order coupling modes is less sensitive to topographic effect than that of low-order coupling modes. Moreover, plasmon ruler for asymmetric Ag-Ag heterodimers is investigated and we demonstrate that a generalized plasmon ruler is applicable to predict the relative shift of coupling dipolar due to change of separation distance.