Optical Properties of Nanostructured Random Media
This book reviews recent advances in one of the most prominent fields of physics. The optics of random media displays a rich variety of effects, and some of these effects are hardly intuitive. Localization of various sorts of optical excitations occur and
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Universit´e de Versailles Saint Quentin, Laboratoire de Magn´etisme et d’Optique, CNRS UMR 8624, F-78035 Versailles, France [email protected] Universit´e Pierre et Marie Curie, Laboratoire d’Optique Physique ESPCI, CNRS UPR 5, F-75231 Paris, France [email protected]
Abstract. The surfaces of percolating random 2-D metal–dielectric films consist of several spectral resonances, which have been calculated and afterward observed by near-field optical microscopy. These films show anomalous optical properties which are investigated in the first section. Nonlinear electrical and optical properties of metal–dielectric film percolation composites, though recognized very early, were not well understood. It is only recently that calculation of local fields in semicontinuous films allows us to define the enhancement factors of optical nonlinearities. These calculations are outlined from basic principles in the second section and compared with experimental results. An insightful approach to the same problem is to use a network description to represent the random system and discretize the equations satisfied by the scalar potential of the electrical field. We recall in the third section how such discretization leads to a Hamiltonian which is paradigmatic in the theory of Anderson localization. The imaging and spectroscopy of localized optical excitation in gold-on-glass percolation films was performed using near-field optical microscopy (SNOM), and the fourth section recalls the basic features of the experimental technique and describes the first experimental observation of “hot spots” in a nanometer-scale area.
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Introduction
In a significantly wide range close to the percolation second-order transition, granular metal thin films are known to manifest electromagnetic properties that are absent for both components: bulk metal and dielectric. Two-dimensional metal–dielectric films consist of a planar distribution of nanometer-sized metal grains randomly distributed on the surface of an insulating substrate; each metallic grain has about the same height (thickness) above the substrate. If the edges do not dominate the shape of the grains, we can assume that the metallic grains are not too far from cylinders, and then the ratio between the metal covered surface and the total surface of the V. M. Shalaev (Ed.): Optical Properties of Nanostructured Random Media, Topics Appl. Phys. 82, 185–215 (2002) c Springer-Verlag Berlin Heidelberg 2002
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Patrice Gadenne and Jean C. Rivoal
substrate can be regarded as representing the metal filling factor p, or metallic concentration of the film. When this filling factor increases, coalescence between initially isolated metallic grains occurs, resulting in the formation of very irregularly shaped clusters. When p increases and reaches the value p = pc at the percolation threshold, one extended metal cluster spans the entire sample, and electrical transition from insulating to dc conducting behavior occurs. The films in this concentration region show very remarkable linear and nonlinear optic
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