Nonlinear optical properties of nanocomposite materials

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Abstract
This paper reviews some of the authors' recent research aimed at obtaining an understanding of the physical processes that determine the linear and nonlinear optical properties of nanocomposite materials. One result of this research is the prediction and experimental verification that under proper conditions two materials can be combined in such a manner that the nonlinear susceptibility of the composite exceeds those of the constituent materials. This paper also presents a survey of the various geometrical structures of composite materials. A common approach to the development of nonlinear optical materials entails searching for materials that possess, at the molecular level, desirable nonlinear optical properties. An alternative approach, which will be explored in this paper, entails combining known materials into a composite material. Under proper conditions, this composite material might combine the more desirable properties of the starting materials, or ideally, might possess properties superior to those of the starting materials. Some of the commonly encountered structures of composite materials are shown in figure 1. The Maxwell Garnett (1) geometry consists of small inclusion particles embedded in a host material. The Bruggeman (2) geometry consists of two intermixed components. These two model geometries are the structures most often encountered in theoretical discussions of composite materials. Two additional structures are that of porous silicon and that of layered materials. Recent research (3) has shown that an electrochemical etching procedure can be used to turn silicon into a porous structure. The resulting structure then contains 'worm holes' which can be modelled as cylindrical columns in which the silicon has been eaten away from the host material. When still more material has been eaten away, the resulting structure can be modelled as cylindrical columns of silicon surrounded by voids. In either case, the voids can be filled with a second material to form a composite structure. These composite materials can be thought of as a two-dimensional version of the Maxwell Garnett structure. Research on porous silicon is still quite new and will not be discussed further in this paper. The final structure illustrated in figure 1 is the layered geometry, consisting of alternating layers of two materials with different linear and nonlinear optical properties. In all of the structures shown in figure 1, we assume that the two materials are intermixed on a distance scale much smaller than an optical wavelength. Under these conditions, the propagation of light can be described by effective values of the optical constants that are obtained by performing a suitable volume average of the local optical response of the material. In fact, performing such an average can be rather subtle for situations involving the nonlinear optical response, because it is the nonlinear polarization that must be averaged, and the nonlinear polarization depends on the spatially inhomogeneous electric field amplitude