Recent developments in linear and nonlinear near-field microscopy on single plasmonic nanoparticles

Plasmonic circuits constituted of several interacting optical elements require new means to characterize their response down to the single particle level, both in amplitude and phase. To this aim, scanning near-field optical microscopy (SNOM) allows for sub-diffraction resolution, but requires a deep understanding of the physical processes at the basis of image formation. In this paper, we briefly review recent developments in the linear and nonlinear near-field characterization of single plasmonic resonators. On the one hand, linear scattering processes are governed by interference between light propagating out of the SNOM aperture and that radiated by the plasmonic scatterer, thus bearing signatures of both amplitude and phase of the particle response. On the other hand, nonlinear investigations are dominated by local field enhancements, and are thus best suited to image the spatial distribution of the plasmon modes sustained by the resonator. The combined use of the two techniques, therefore, allows for a complete analysis of both the scattering response and the local field distribution near resonant plasmonic nanoparticles. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim