Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

11Optical Spectroscopy of SurfacePlasmons in Metal NanoparticlesStephan Link and Mostafa A. El-SayedGeorgia Institute of Technology, Atlanta, Georgia, U.S.A.I. INTRODUCTIONMetal nanoparticles have been used in applications long before the science ofnanometer-size materials have attracted much attention. Novel propertiesthat are different from those of individual atoms or bulk materials areinteresting not only from a fundamental research standpoint but also becauseof a wide variety of different potential applications [1–6]. Gold and silvernanoparticles were already used as coloring pigments in stained glass back inthe Middle Ages. Faraday [7] first recognized that the red color is due to a‘‘different form’’ of gold and postulated correctly that the color is caused bysmall gold particles without having any of the modern tools such astransmission electron microscopy (TEM). The intense color of metal colloidsis due to the coherent excitation of the conduction-band electrons and isknown as the surface plasmon absorption [8–10]. Mie [11] was able totheoretically model the plasmon absorption about a century ago by applyingthe Maxwell equations to describe the interaction between a metal nanoparticlewith a known dielectric function and an electromagnetic field.Mie’s theory can explain the size dependence of the surface plasmonabsorption for metallic nanoparticles larger than f20 nm. For smaller particles,an enhanced electron-surface scattering [12,13] leads to a broadening ofthe plasmon absorption. The enhanced surface scattering is due to the factthat the electron mean free path becomes longer than the dimensions of theparticle. The size and shape dependence of the plasmon absorption togetherCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.