Direct observation of localized dipolar excitations on rough nanostructured surfaces

Using a photon scanning tunneling microscope ~operating alternatively at the wavelengths of 594 and 633 nm! with shear-force feedback we image the topography of silver colloid fractals simultaneously with a near-field intensity distribution. We observe that near-field optical images exhibit spatially localized ~within 150–250 nm! intensity enhancement by one to two orders of magnitude. These bright light spots are found to be sensitive to the light wavelength, polarization, and angle of incidence. We relate the observed phenomenon to the localization of resonant dipolar excitations in random nanostructured aggregates. @S0163-1829~98!06941-0#