Novel Organic Crystals for Nonlinear and Electro-Optics

Abstract

There have been significant advances in understanding and optimizing classical π-conjugated donor-acceptor chromophores with large second-order molecular hyper-polarizabilities in the area of organic nonlinear optics in the last few years [1, 2, 3, 4, 5, 6]. However, there are only few chromophores with very large molecular hyper-polarizabilities such as donor-acceptor stilbenes and tolanes that form potentially useful crystalline materials. Our interest in molecular crystals stems from the fact that the potential upper limits of macroscopic nonlinearities and long-term orientational stability of molecular crystals are significantly superior to those of polymers [1]. In addition to a large molecular hyperpolarizability, the second-order macroscopic nonlinearities are strongly dependent of the relative arrangement and orientation of the π-conjugation chromophores in the crystalline solid [1, 3]. Unlike the third-order nonlinear optical effects, chromophores are required to arrange noncentrosymmetrically in the crystalline state so as to exhibit non-vanishing macroscopic second-order nonlinear optical responses such as second harmonic generation, frequency mixing, the linear electro-optic effect, and photorefractive effects. To be an efficient as well as useful second-order nonlinear optical crystalline material, the orientation of the chromophores in the bulk also needs to be optimized.