Abstract.
A scanning plasmon near-field microscope with gold and silver tips, operating in tapping mode of atomic force microscope (AFM) is used to measure the distribution of the near-field intensity of surface plasmons on rough silver surfaces. Using the fast Fourier transformation (FFT) of near-field images it is shown that the distribution of the near-field intensity on the surface is the result of the interference between scattering plasmons and the initial plasmon beam. Multiple scattering effects such as backscattering enhancement of surface plasmons are also observed. It is shown that a nonuniformity in the registration of the scattered light leads to some artifacts in near-field images. Several registration modes of the light signal are considered and it is shown that recording the light signal at the second harmonic of the tapping frequency one can pick out the signal associated with an electromagnetic (em) resonance in a tip-surface (sphere-plane) structure. Possible implementations of this em resonance for studies of local permittivities and local nonlinear susceptibilities of intermediate media between the tip and surface with a subtip resolution are discussed.

Fig.4 AFM (a) and SPNM (b) images of the silver surface (scan size is 15800~nm$\times$16800~nm). (d) is FFT of SPNM image.

Fig.7 The backscattering enhancement effect: topography (a), the near-field image (b) of the surface and FFT of the near-field image (d) (the arrow length on FFT image corresponds the surface plasmon wavevector length $|{\mathbf k}_{\mathrm pl}|$). Scan size is 18000~nm$\times$18000~nm.