Higher order plasmonic modes excited in Ag triangular nanoplates by an electron beam

Ag triangular nanoplates are known to generate strong plasmonic resonances when excited by both light and electron beams. Experimental electron energy-loss spectra (EELS) and maps were acquired using an aberration-corrected JEOL-ARM microscope. The corner, edge and centre modes that are often observed in such structures were also observed in these measurements. In addition, novel higher order internal modes were observed and were found to be well reproduced by theoretical cal- culations using boundary element method (BEM). These modes are "dark modes" so are not observed in the optical extinction spectra. They are confined surface propagating modes and are analogous to laser cavity modes