This paper describes the development and optimization of chiral, non-polar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum- mechanical sum-over-states theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second rank tensor hyper-polarizabilities (beta) can be aligned so as to result in large susceptibilities, (chi) <SUP>(2</SUP>), in structures that lack polar order. We found that (Lambda) -shaped chromophores with C<SUB>2v</SUB> or similar symmetry are good candidates for these materials as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral non-polar symmetry groups, D<SUB>(infinity</SUB> ) and D<SUB>2</SUB>. The microscopic chromophore alignment schemes that optimize the NLO response in such materials are deduced from general symmetry consideration for both molecules and bulk.
We describe a new approach to second-order nonlinear optical materials, namely quadrupoling. This approach is valid in the regime of Kleinman (full permutation) symmetry breaking, and thus requires a
two- or three dimensional microscopic nonlinearity at wavelengths away from material resonances. This "quadrupolar" nonlinearity arises from the second rank pseudotensor of the rotationally invariant representation of the second-order nonlinear optical tensor. We have experimentally investigated candidate molecules comprised of chiral camphorquinone derivatives by measuring the scalar invariant associated with the rank two pseudotensor using hyper-Rayleigh scattering. We have found sizable scalar figures of merit for several compounds using light for which the second harmonic wavelengths are greater than 100 nm longer than the absorption peak location. At these wavelengths, the quadrupolar scalar is as large as the polar (EFISH) scalar of p-nitroaniline. Prospects for applications are discussed.