  The near-field scanning optical microscopic (NSOM) imaging of Au
nanoparticles with size in the sub-wavelength limit (<wavelength/2N.A.) is
reported. The NSOM imaging technique can resolve the objects which is beyond
the scope of optical microscope using visible light (wavelength=500 nm) with
objectives having a numerical aperture (N.A.) close to unity. The role of
evanescent waves which is an exponentially decaying field with higher momenta
i.e., lower wavelengths compared to that of normal light, in the metal
dielectric interface is realized for imaging of noble metal nanostructures with
sub-wavelength dimension in the near field. However, the confined light with
components of evanescent waves, emanating from the NSOM probe, interacts with
the oscillating dipoles present in the sub-diffraction limited nanostructures
and produce propagating waves, which can be recorded by the far field detector.
The light-matter interactions of Au nanoparticles of diameters in the range of
10-150 nm probed by the NSOM technique with a visible excitation of 532 nm are
reported. The strong surface plasmon resonance (SPR) related absorption of Au
nanoparticles is envisaged for explaining the contrast variations in the
recorded NSOM images.
