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.