%'e present experimental results which demonstrate that the effective third-order susceptibility of a composite optical material can exceed those of the materials from which it is constructed. In particular, we have formed a composite of alternating, sub-wavelength-thick layers of titanium dioxide and the conjugated polymer poly(p-phenylene-benzobisthiazole), and find that its nonlinear susceptibility exceeds that of its more nonlinear constituent by 35%. The enhancement of the nonlinear susceptibility, which under more ideal but still realistic conditions can be as large as a factor of lo, can be understood as a consequence of local field corrections.There is a great need for nonlinear optical materials with large nonlinear coefficients and fast response. Many applications of nonlinear optics that have been demonstrated under controlled laboratory conditions could become practical for technological uses if such materials were available. Nonlinear optical switching devices for use in photonics and real-time coherent optical signal processors are examples of applications of nonlinear optics that would benefit from the development of fast, low-loss materials with large values of the third-order nonlinear susceptibility Xt3l [1]. The most common approach to the development of new nonlinear optical materials involves the search for materials in which the constituent molecules possess an inherently large nonlinear response [2]. In contrast, in this Letter we describe a technique that can be used to increase the g(3~v alue of a nonlinear optical material by forming a composite of that material and another material having a different value of the linear refractive index.The two constituent materials can be optically lossless, and the response time of the composite is essentially the same as that of the nonlinear constituent. The two materials are intermixed on a distance scale of the order of 50 nm, which is much larger than an atomic dimension but much smaller than the wavelength of light used in our experiment. Consequently, the structural properties of each constituent material are essentially the same as those of a bulk sample of that material, but the propagation of light through the composite can be described by effective values of the linear and nonlinear optical susceptibilities that are obtained by performing suitable volume averages. The reason why this technique leads to an enhancement of~(~is that the electric field amplitude of an incident laser beam becomes nonuniformly distributed between the two constituents of the composite, and under suitable conditions the electric field strength within the more (3) jeff eff 2 2 err( 3) f-x. a nonlinear constituent will exceed the spatially averaged field strength. Under these conditions, the effective thirdorder susceptibility g, «of the composite can exceed that (3) of either of its constituents.We have previously performed detailed theoretical studies of composite materials of the sort mentioned above both for the case in which small inclusion particles are embedde...

By applying the Maxwell Garnett model to gold nanoparticles in water we deduce a value of Im i (3) ϭ 1.1 ϫ 10 Ϫ7 esu for the imaginary part of the cubic susceptibility for gold corresponding to a Fermi smearing mechanism. We also demonstrate a sign reversal in the nonlinear absorption for gold particles in 1, 1Ј, 3, 3, 3Ј, 3Ј-hexamethylindotricarbocyanine iodide. Although the imaginary part of (3) is positive for each component by itself, remarkably the imaginary part of (3) is negative for the colloid as a whole. We show that the nonlinearity of the host must be considered and that the sign reversal in (3) is a result of the fact that at the surface plasmon resonance the local field factor has an imaginary component that arises from a phase shift between the applied field and the local field inside the particle.

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We formed composite materials with geometries approximating that of the Bruggeman model by filling the pores of a porous glass with optically nonlinear fluids spanning a range of linear refractive indices. The effective linear refractive indices of the composites were measured with a Mach-Zehnder interferometer. The effective nonlinear refractive indices were determined by the Z-scan method. In both cases good agreement between the experimental data and theoretical predictions was found.

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