Prospects for chiral nonlinear optical media

This paper describes the development and optimization of chiral, nonpolar 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 hyperpolarizabilities β can be aligned so as to result in large susceptibilities χ⁽²⁾ in structures that lack polar order. We found that Λ-shaped chromophores with C_2v 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 nonpolar symmetry groups D_∞ and D₂. The microscopic chromophore alignment schemes that optimize the nonlinear optical response in such materials are deduced from general symmetry consideration for both molecules and bulk. We also speculate on the possible application of such materials as high-bandwidth spatial light modulators