Near-field optical study of selective photomodification of fractal aggregates

Selective photomodification of fractal aggregates of silver nanoparticles is studied experimentally by photon-scanning-tunneling microscopy. Near-field optical images of the aggregates before and after photomodification show changes in the distribution of local electromagnetic fields in the near zone at subwavelength scale. These changes are much stronger than those measured in the far field. Results from numerical modeling of photomodification are in qualitative agreement with the experimental observations.