Near-field optical studies of local photomodification in nanostructured materials

Fractal aggregates of silver nanoparticles are studied experimentally using atomic force microscopy and photon scanning tunnelling microscopy. Large c hanges in the near-field optical response of fractal aggregates are observe d after the irradiation of samples with nanosecond laser pulses. The thresh old energy density for photomodification using a 532 nm laser is measured t o be 9 mJ cm(-2). It is shown that photomodification-induced changes in the local optical response can be two orders of magnitude larger than changes in far-field absorption.