Optical Properties of Metal Coated Particles
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OPTICAL PROPERTIES OF METAL COATED PARTICLES
Joseph W. Haust, H.S. Zhout, I. Honmat, H. Komiyamat t Physics Dept., Rensselaer Polytechnic Institute, Troy, NY 12180-3590 and Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, JAPAN tDept. of Chemical Engineering, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, JAPAN ABSTRACT Metal-coated particles of nanometer dimensions can exhibit unusual optical properties due to the local-field enhancement mechanism. However, the properties are affected by uncontrollable factors that degrade the expected performance. In this paper we report theoretical results of coating thickness variations and diffuse interface effects on the optical absorption and the nonlinear optical response of particles with a CdS core and a silver coating. The linear and nonlinear optical properties are discussed within the context of the effective medium theory for small volume fractions. INTRODUCTION Metal particles immersed in a dielectric medium have an electromagnetic resonance effect, the socalled the surface plasmon resonance, that is relatively sharp in some materials, eg. gold and silver. The resonance is by no means an immutable property of the materials and it can be changed by using engineering degrees of freedom. For instance, the resonance observed in these metal particles depends on the shape, thus the optical properties can be made to be highly anisotropic when the particles are nonspherical; this makes them useful as polarizers. Recently, the surface plasmon resonance of metal particles has shown promise as a compact polarizer for integrated optics.1 In this paper another approach is theoretically investigated, namely, control of the resonance frequency by synthesizing metal coated particles. The advantage for nonlinear optical response is the local field enhancement in the nonmetallic, core material, which is designed to have a large nonlinear response without the coating. The local field enhancement has been predicted to give enhancements of the optical nonlinearity by several orders of magnitude.2',3 This prediction supposes an ideal particle with sharp interfacial region and no distribution in the core-shell sizes. The electromagnetic resonance effects can, in principle greatly enhance the optical nonlinearity of dielectric particles. A large enhancement of the conjugate reflectivity was observed on composites of Ag and Au. 4 They both have a strong surface plasmon resonance effect in the visible regime and therefore, coated particles have been suggested as a scheme for achieving very large effects. 2,3 The coated particle consists of a core and shell each with different material properties; those considered include metal or semiconductor as the core or the shell of the coated particle and the enhancement of the optical nonlinearity can be large for both cases. However, the major obstacle that remains is the synthesis of coated particles on a nanometer size scale. Generally, the predicted performance of
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