3D Imaging of Gap Plasmons in Vertically Coupled Nanoparticles by EELS Tomography

ABSTRACT:Plasmonic gap modes provide the ultimateconfinement of opticalfields. Demanding high spatialresolution, the direct imaging of these modes was onlyrecently achieved by electron energy loss spectroscopy (EELS)in a scanning transmission electron microscope (STEM).However, conventional 2D STEM-EELS is only sensitive tocomponents of the photonic local density of states (LDOS)parallel to the electron trajectory. It is thus insensitive tospecific gap modes, a restriction that was lifted with the introduction of tomographic 3D EELS imaging. Here, we show that by3D EELS tomography the gap mode LDOS of a vertically stacked nanotriangle dimer can be fully imaged. Besides probing thecomplete mode spectrum, we demonstrate that the tomographic approach allows disentangling the signal contributions from thetwo nanotriangles that superimpose in a single measurement with afixed electron trajectory. Generally, vertically couplednanoparticles enable the tailoring of 3D plasmonicfields, and their full characterization will thus aid the development of complexnanophotonic devices.