Chapter 6 - Surface-enhanced optical phenomena in nanostructured fractal materials

Abstract

This chapter considers surface-enhanced optical phenomena in objects with fractal morphology, such as aggregates of nanoparticles, self-affine surfaces, and random metaldielectric thin films. It is shown that optical modes of such nanostructured random objects consist of localized sharp peaks resulting in very inhomogeneous spatial distributions of local fields. In peaks (hot spots), local fields exceed the applied field by several orders of magnitudes. These peaks are localized in nm-sized areas and they result from the excitation of the fractal plasmon modes. The strongly fluctuating fields associated with the sharp peaks in various random parts of a fractal lead to giant enhancements of optical processes, especially, nonlinear ones that are proportional to the enhanced local field raised to a power higher than 1. The enhancement in these peaks is much larger (by several orders of magnitude) than the ensemble-average enhancement partially because the peaks are separated by distances much larger than the mode spatial sizes. Another important reason is related to the fact that the well-separated peaks represent often topologically disconnected parts of the same mode and therefore are correlated in phase. Destructive (in part) interference between the local field in different parts of a film results in the decreased average enhancements. It is shown experimentally that, in accordance with the theoretical predictions, the degree of localization of optical excitations increases toward longer wavelengths.