We report in this paper the near-field microscopic studies on localized plasmon resonances in gold nanoparticles and
their assemblies (nanorods, triangular nanoplates, and assembled nanospheres). We have utilized near-field
measurements of linear transmission/scattering, as well as nonlinear two-photon excitation, to enable spectroscopic
imaging of local electric field or local density of electromagnetic states. We show that the wavefunction of the plasmon
excitation in the nanoparticle is visualized by the near-field methods. For single nanorod, many plasmon resonances
were observed in the near-field transmission spectrum. At each resonant peak wavelength, the near-field image of the
nanorod gave a characteristic spatially oscillating feature along the long axis of the nanorod. The feature is attributable
to the square modulus of the resonant plasmon-mode wavefunction. In the assembled nanoparticles, strong electric-field
enhancement localized in the interstitial sites ("hot spot"), which was theoretically predicted previously, was clearly
imaged by the near-field two-photon excitation method. Major contribution of the hot spots to surface enhanced Raman
scattering is also shown for the samples weakly doped with Raman-active dye molecules, by the near-field excited
Raman spectra and images.
KEYWORDS: Gold, Near field, Surface plasmons, Near field scanning optical microscopy, Nanoparticles, Plasmons, Near field optics, Electromagnetism, Ultrafast imaging, Nanorods
We investigated near-field optical properties and images of single gold nanorods by using a near-field optical
microscope. Observed transmission spectra show distinct transverse and longitudinal surface plasmon resonances.
Transmission images observed near the surface plasmon resonances agree qualitatively with calculated maps of optical
local density-of-states, and are assignable to plasmon wavefunctions. Ultrafast temporal responses in the single gold
nanoparticles were observed by combining a near-field microscope with time-resolved techniques. Observed transient
transmission images of the single nanorods show characteristic optical features, and are in good agreement with a
calculated map of variation of local density of states arising from the elevation of electronic temperature in the nanorod
induced by photoexcitation.
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