The optical-frequency magnetic and
electric properties of cyclic
aromatic plasmon-supporting metal nanoparticle oligomers are explored
through a combination of scanning transmission electron microscopy
(STEM)/electron energy-loss spectroscopy (EELS) simulation and first-principles
theory. A tight-binding-type model is introduced to explore the rich
hybridization physics in these plasmonic systems and tested with full-wave
numerical electrodynamics simulations of the STEM electron probe.
Building from a microscopic electric model, connection is made at
the macroscopic level between the hybridization of localized magnetic
moments into delocalized magnetic plasmons of controllable magnetic
order and the mixing of atomic p
z
orbitals
into delocalized π molecular orbitals of varying nodal structure
spanning the molecule. It is found that the STEM electrons are uniquely
capable of exciting all of the different hybridized eigenmodes of
the nanoparticle assemblyincluding multipolar closed-loop
ferromagnetic and antiferromagnetic plasmons, giant electric dipole
resonances, and radial breathing modesby raster scanning the
beam to the appropriate position. Comparison to plane-wave light scattering
and cathodoluminescence spectroscopy is made. The presented work provides
a unified understanding of the complete plasmon eigenstructure of
such oligomer systems as well as of the excitation conditions necessary
to probe each mode.