Near-field imaging of optical field and plasmon wavefunctions in metal nanoparticles

Scanning near-field optical microscopes enable optical measurements and observations with a spatial resolution far beyond the diffraction limit of light. This allows us to spatially resolve the positions in nanoparticles and at the same time to investigate spectroscopic characteristics point by point. In this feature article, we focus on near-field microscopic studies on surface plasmons in anisotropic gold nanoparticles and aggregates (nanorods, triangular nanoplates, and aggregated nanospheres). We have utilized linear transmission/scattering as well as nonlinear two-photon excitation measurements in the near field, to observe the local electric field or electromagnetic local density of states. We show that the wavefunction of the plasmon excitation in the nanoparticle resonant with the incident radiation field is visualized by the near-field methods. In the aggregated nanoparticles, strong electric-field enhancement localized in the interstitial sites, which was theoretically predicted previously, is clearly imaged, and its major contribution to surface enhanced Raman scattering is also shown by the near-field images.