## Spectral Representation for the Effective Macroscopic Response of a
Polycrystal: Application to Third-Order Nonlinear Susceptibility

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S. Barabash and D. Stroud, Department of Physics,
The Ohio State University, Columbus, Ohio 43210

We extend the spectral theory used for the calculation
of the effective linear response functions of composites to the case
of a polycrystalline material with uniaxially anisotropic microscopic symmetry.
As an application, we combine these results with a nonlinear decoupling
approximation as modified by Ma {\it et al}, to calculate the third-order
nonlinear optical susceptibility of a uniaxial polycrystal, assuming that
the effective dielectric function of the polycrystal can be calculated
within the effective-medium approximation.