MODE LOCALIZATION IN SELF-AFFINE FRACTAL INTERFACES OBSERVED BY NEAR-FIELD MICROSCOPY

Near-field optical microscopy with simultaneous topographic imaging sh ows that for a given excitation wavelength the electromagnetic optical fields excited in a self-affine film are highly localized at ''hot sp ots'' whose dimensions are often smaller than the wavelength. The patt ern of hot spots varies greatly with excitation wavelength and does no t correlate simply with the film topography. This observation is consi stent with a recent theory of the optical response of self-affine frac tal objects and also indicates that the locus of optical effects such as surface-enhanced Raman does not reside in special surface sites suc h as interstices. Additionally, we show that the conclusions of the th eory, which was based on a quasilocal approximation, are qualitatively valid for self-affine films considerably larger than the exciting opt ical wavelength.