The field of
plasmonics has seen enormous research activity over the past decade for applications
as diverse as bio-sensing, optical cloaking, tumour therapy and sub-wavelength
optics. Electron energy-loss spectroscopy (EELS) has played a key role in this
field. Using EELS, not only can the plasmon responses from individual
nanoparticles be measured, but a map of resonances excited at different
electron beam positions within a nanoparticle can be generated, revealing
important connections between geometry and plasmon resonances. Silver and gold triangular
nanoplates have been observed to generate strong plasmonic resonances when
excited by both light and electron beams. Experimental electron energy-loss
spectra (EELS) and maps were acquired from Ag nanoplates using an aberration
corrected JEOL-ARM microscope. The corner, edge and centre modes that are often
observed in such triangular structures were observed in these measurements.
However, in addition, novel higher order internal modes were observed and were
found to be well-reproduced by the simulations. These modes are “dark modes” so
are not observed in the optical extinction spectra. They can be thought of as
propagating surface plasmons that are instead confined to the triangular shape.
They are entirely analogous to the laser modes found in triangular cavities. Furthermore,
a mode resulting from the hybridisation of the centre and edge modes was observed
and, because of symmetry considerations, this mode only appears in triangular
nanoplates with sharp tips. In addition, a brief introduction will be given to
a new computer code available for calculations of surface plasmons in simple
geometries.