Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy (SNOM). The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale areas, “hot spots,” where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot’s large local fields may result in local, frequency and spatially selective photomodification of percolation films.

• Received 13 April 2001

DOI:https://doi.org/10.1103/PhysRevB.64.165403