Operation of scanning plasmon near-field microscope with gold and silver tips in tapping mode: demonstration of subtip resolution

We present a scanning plasmon near-field microscope which operate in tappin g mode of atomic force microscope. We interpret the observed maximum in lig ht scattering intensity during a tip approach to land withdrawal from) a su rface as an electromagnetic (em) resonance in a tip-surface (sphere-plane) structure. This em resonance is of greater intensity when the tip and the s urface are noble metals. At the em resonance in such a structure, the dimen sion of the light field localization is of the order of L similar or equal to (2dR)(1/2), where d is the tip-surface distance and R is the tip radius. Therefore at d much less than 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 wit h the sphere-plane em resonance. Different registration modes of the light signal are considered and the nature of negative contrast of surface hilloc ks in certain of the registration modes is analyzed. Near-field images of r ough silver surfaces are presented and it is shown that distribution of the near-field intensity on the surface is the result of the interference betw een scattering plasmons and the initial plasmon beam. (C) 2000 Published by Elsevier Science B.V.