Hindawi Publishing Corporation International Journal of Optics Volume 2012, Article ID 519509, 9 pages doi:10.1155/2012/519509 Research Article Surface Plasmon States in Inhomogeneous Media at Critical and Subcritical Metal Concentrations Katyayani Seal 1 and Dentcho A. Genov 2 1 Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA 2 College of Engineering and Science, Louisiana Tech University, Ruston, LA 71270, USA Correspondence should be addressed to Katyayani Seal, sealk@ornl.gov Received 30 September 2011; Accepted 14 December 2011 Academic Editor: Ali Passian Copyright © 2012 K. Seal and D. A. Genov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Semicontinuous metal-dielectric films are composed of a wide range of metal clusters of various geometries—sizes as well as structures. This ensures that at any given wavelength of incident radiation, clusters exist in the film that will respond resonantly, akin to resonating nanoantennas, resulting in the broad optical response (absorption) that is a characteristic of semicontinuous films. The physics of the surface plasmon states that are supported by such systems is complex and can involve both localized and propagating plasmons. This chapter describes near-field experimental and numerical studies of the surface plasmon states in semicontinuous films at critical and subcritical metal concentrations and evaluates the local field intensity statistics to discuss the interplay between various eigenmodes. 1. Introduction Random metal-dielectric composites, such as nanoscaled semicontinuous metallic structures, exhibit unique optical and electrical transport properties [1–26]. One of the most remarkable properties of such composites is the enhance- ment of electric and magnetic fields in the visible and infra- red spectral ranges [1, 8–22]. These materials show promise in wide ranging photonics technologies including applica- tions in negative index materials [27–29], biosensing [30, 31], and nonlinear optics [10] spectroscopy [32, 33] broad- band optical amplifiers and optical absorbers [10, 34, 35]. The synthesis of metal-dielectric composites typically involves the deposition of metal particles onto a dielectric substrate [14, 21–23]. At low metal concentrations, mutually separated nanometer-sized grains are formed at random locations. Self-similar clusters form as the metal surface concentration increases [1, 3, 10]. At a certain metal concen- tration ( p c ), that is, the percolation threshold, the metal clusters interconnect to form an infinite “backbone cluster”, setting off an insulator-to-metal phase transition. This is marked by a sharp drop in dc resistivity [14, 23] and anomalous absorption at visible and near-infrared wave- lengths [1, 10, 11, 14–19, 21–23]. At even higher metal con- centrations, the sample becomes mostly metallic with dielec- tric voids, ultimately resulting in a uniform metal film. From both theoretical predictions and experimental corroboration, it has been observed that under illumination with light at optical and near infrared frequencies, strong local field enhancement of up to 10 4 can be achieved on the surface of two-dimensional metal-dielectric composites [1, 10–22, 36]. This light trapping effect is facilitated by the excitation of collective electron oscillations, that is, surface plasmons (SP). Surface plasmon modes are morphological resonances governed by the structural inhomogeneities in- herent in the fractal clusters formed within the random metal-dielectric films. The observed local fields appear to be strongly localized [1, 10, 11, 17–19] with relatively high Q-factors dependent on the intrinsic dissipation in the metal. This phenomenon has been observed in rough and semicontinuous metal films [8, 9, 15, 16, 23]. Notably, due to the geometry of the clusters, the spatial distribution of the field maxima is sensitive to the polarization, wavelength and angle of incidence of the applied field [1, 2, 10]. Thus the