Mode localization in self-affine fractal interfaces observed by near-field microscopy

Near-field optical microscopy with simultaneous topographic imaging shows that for a given excitation wavelength the electromagnetic optical fields excited in a self-affine film are highly localized at “hot spots” whose dimensions are often smaller than the wavelength. The pattern of hot spots varies greatly with excitation wavelength and does not correlate simply with the film topography. This observation is consistent with a recent theory of the optical response of self-affine fractal objects and also indicates that the locus of optical effects such as surface-enhanced Raman does not reside in special surface sites such as interstices. Additionally, we show that the conclusions of the theory, which was based on a quasilocal approximation, are qualitatively valid for self-affine films considerably larger than the exciting optical wavelength.