Near-field optical imaging of enhanced electric fields and plasmon waves in metal nanostructures

In this article, studies on noble metal nanostructures using near-field optical microscopic imaging are reviewed. We show that near-field transmission imaging and near-field two-photon excitation imaging provide valuable methods for investigation of plasmon resonances in metal nanostructures. The eigenfunctions of plasmon modes in metal nanoparticles are directly visualized using these methods. For metal nanowire systems, wavevectors of the longitudinal plasmon modes can be estimated directly from the wave-function images, and the dispersion relations are plotted and analyzed. Using ultrafast transient near-field imaging, we show that the deformation of the plasmon wave function takes place after photoexcitation of a gold nanorod. Such methods of plasmon-wave imaging may provide a unique basic tool for designing plasmon-mode-based nano-optical devices. We also demonstrate that the near-field two-photon excitation probability images reflect localized electric-field enhancements in metal nanostructures. We apply this method to gold nanosphere assemblies and clearly visualize the local enhanced optical fields in the interstitial sites between particles (hot spots). We also show the contribution of hot spots to surface enhanced Raman scattering. The methodology described here may provide valuable basic information about the characteristic enhanced optical fields in metal nanostructures as well as on their applications to new innovative research areas beyond the conventional scope of materials.