Plasmon 3D Electron Tomography and Local Electric-Field Enhancement of Engineered Plasmonic Nanoantennas

Plasmonic
nanoantennas are pushing the limits of optical imaging
resolution capabilities in near-field scanning optical microscopy
(NSOM). Accordingly, these techniques are driving the basic understanding
of photonic and optoelectronic nanoscale devices with applications
in sensing, energy conversion, solid-state lighting, and information
technology. Imaging the localized surface plasmon resonance (LSPR)
at the nanoscale is a key to understanding the optical responses of
a given tip geometry in order to engineer better plasmonic nanoantennas
for near-field experiments. In recent years the advancement of focused
ion beam technology provides the ability to directly modify plasmonic
structures with nanometer resolution. Also, scanning transmission
electron microscopy (STEM) with electron energy loss spectroscopy
(EELS) is an established technique allowing imaging of LSPR. Specifically,
the combination of these two techniques provides spectrally sensitive
two-dimensional (2D) image information to better visualize and understand
LSPR on the nanometer scale. This can be combined with electron tomography
to provide the three-dimensional LSPR distribution. Here we demonstrate
the fabrication of Au nanopyramids using helium ion microscopy, and
analyze the LSPR in 3D reconstructions produced by total variation
(TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally,
a boundary element simulation method was used to verify the experimentally
observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions
(PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field
enhancement under optical excitation making direct comparison to NSOM
experimental resolution difficult. However, the STEM-EELS results
show how LSPR modes are influenced by the tip characteristics, which
can inform the development of new nanoantenna designs.