By means of full-wavelength near-field plasmon mapping, gradual evolutions of multipole coupling modes in Ag nanoparticle homodimer and heterodimer hotspots are witnessed. According to the symmetry and distributions of the compressed surface charge lobes, various complicated high-order coupling modes are identified, which turns out to be beyond the prediction of general plasmon hybridization model. Specifically, different intermediate states between the bonding dipole-dipole coupling and bonding quadrupole-quadrupole coupling can be observed. Our results provide an essential understanding of the dimer hotspot systems and pave a new way for the exploration of multipole coupling effects in plasmonic complexes.
Zhou et al. (Thu,) studied this question.