SUBTIP RESOLUTION IN SCANNING PLASMON NEAR-FIELD MICROSCOPE WITH TIP-SURFACE PLASMON RESONANCE: NEW POSSIBILITIES FOR SINGLE MOLECULE DETECTION
Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region, 142190
A scanning plasmon near-field microscope with gold and silver tips, which operate in tapping mode of atomic force microscope is presented. 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 structure. 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 present near-field images of rough silver surfaces. Using the fast Fourier transformation (FFT) of near-field images it is shown that the distribution of the near-field plasmon 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 on specific surfaces .
The registration of the light signal at the second harmonic of the surface-tip distance modulation opens up several new possibilities for studies of surface with nanometer resolution. The most straightforward of them is the use of the huge field enhancement of the em field under the tip for registration of the nonlinear effects such as Raman scattering or second harmonic generation. Another possible implementation of the tip-surface resonance opens up if one will detect (in each point of the surface) the spacing between the tip and the surface at which the plasmon resonance occurs. Since this distance strongly depends on the intermediate media permittivity (dres ~ e02) such a registration will provide the information about e0 with a subtip resolution. For example, molecules with a resonance line at the laser frequency deposited on the surface may be visualized in such a manner.
 V.N. Konopsky, Optics Communications, 185, (2000), 83
 V.N. Konopsky et al., Ultramicroscopy, 88, (2001), 127