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Nonlinear optical response of layered composite materials.

We present a theoretical formalism that describes the nonlinear phase shift acquired by a laser beam in passing through an anisotropic material oriented at an arbitrary angle with respect to the beam. We use this theory to analyze the results of z-scan measurements made on layered composite materials. Samples are constructed from alternating, subwavelength-thick layers of titanium dioxide and the conjugated polymer poly ( p-phenylenebenzobisthiazole!. Effective-medium theory predicts an enhancement of 35% in the third-order susceptibility when the electric field is polarized normal to the layers. Good agreement between theory and experiment is observed. I. INTRODUCTION The practical development of nonlinear optical devices is often hindered by the lack of materials with the proper combination of desirable properties such as large nonlinear response, short response times, and high damage thresholds. For this reason, much effort has recently been directed toward the development of materials possessing these desirable properties. The most common approach for doing so is to search for materials, or to synthesize new materials, that possess these desirable attributes as inherent properties. An alternative approach is to fabricate composites from one or more constituents in such a manner that the composite material possesses attributes superior to those of its constituent materials. The second approach underlies the work reported