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. {} 1996 The American Physical Society.