Abstract.
We present a scanning plasmon near-field microscope (SPNM) which operate in tapping mode of atomic force microscope (AFM). We interpret the observed maximum in light scattering intensity during a tip approach to (and withdrawal from) a surface as an electromagnetic (em) resonance in a tip-surface (sphere-plane) structure. This em resonance is of greater intensity when the tip and the surface are noble metals. At the em resonance in such a structure, the dimension of the light field localization is of the order of $L=(2dR)^{1/2}$, where d is the tip-surface distance and R is the tip radius. Therefore at d<<R the resolution of the near-field images is less than tip radius (L<R). We propose to record the light signal at the second harmonic of tapping frequency to pick out the signal associated with the sphere-plane em resonance. Different registration modes of the light signal are considered and the nature of negative contrast of surface hillocks in certain of the registration modes is analyzed. Near-field images of rough silver surfaces are presented and it is shown that distribution of the near-field intensity on the surface is the result of the interference between scattering plasmons and the initial plasmon beam.

Fig.1 Tip-surface structure considered in theoretical calculations (a), and one in a real experimental situation (b).

Fig.2 Schematic of the experimental setup: PMT - photo-multiplier tube, SoD - set of diaphragms; scattered rays S1-S2 and angle $\Delta\varphi$ between them lie in the plane which is perpendicular to the plane of the figure.

Fig.3 The distance-dependent intensity of the light signal in "external scattering registration mode". On insets in dashed-line envelops the mechanism of the $2\Omega$ frequency generation at the tip-surface distance modulation by $\Omega$ frequency is illustrated.

Fig.5 AFM (a) and SPNM (b) images of some surface hillock on silver surface (scan size is 700~nm$\times$700~nm).