We
investigated the spectral and spatial characteristics of plasmons
induced in chemically synthesized triangular gold nano- and microplates
by aperture-type scanning near-field optical microscopy. Near-field
transmission images taken at plasmon resonance wavelengths showed
two-dimensional oscillating patterns inside the plates. These spatial
features were well reproduced by the square moduli of calculated eigen
functions confined in the two-dimensional triangular potential well.
From the irreducible representations of the eigen functions, it was
found that both the out-of-plane modes and in-plane modes were clearly
visualized in the near-field images. We compared near-field transmission
images of a triangular nanoplate to those of a truncated one with
a similar dimension and revealed that the fine details of the geometrical
shape of the apex on the plate strongly influence the experimentally
observed eigen mode structures. We also performed near-field transmission
measurements of micrometer-scale triangular plates and found that
wavy patterns were observed along the edges of the plates. The wavy
features can be interpreted as the superposition of eigen modes with
similar eigen energy. These findings prove that near-field transmission
imaging enables one to directly visualize plasmonic eigen modes confined
in the particle and provide fruitful information not only for a deeper
understanding of plasmons but also for the application of the design
and active control of plasmonic optical fields
