Near-field optical microscopy of surface second harmonic generation

We report the implementation of near-field scanning optical microscopy (NSOM) to study surface SHG. The SHG has been excited with a Nd:YAG laser operating at 1064 nm. The local SH field distribution has been probed with an uncoated adiabatically tapered fiber tip. To determine the contribution of near-field processes to the SHG, we have studied the dependence of the SH signal from a rough silver surface on the tip-sample distance for S- and P-polarized excitation light. This dependence for S-polarized light is shown. A rather strong increase in the signal occurs within about 500 nm from the surface. We believe that this behavior results from the relaxation of phase-matching conditions and polarization selection rules in the near-field region of the surface. We have studied near-field SHG from various samples: rough metal surfaces, nanometer size gold colloid particles, LiNbO₃ waveguide structures, piezoceramics, etc. An important application of this technique is in magnetic and electric domain imaging. The high spatial resolution of NSOM SH microscopy may allow it to become a new surface characterization technique complimentary to magnetic force and electric force microscopies