Highly Orientable Liquid Crystalline Polymers for Quadratic Nonlinear Optics

Abstract

During the last decade it was recognized, that some organic materials may exhibit second order nonlinear optical (NLO) properties which are much more pronounced than those of the conventional inorganic materials, provided that the arrangement of the noncentrosymmetric molecular units in the macroscopic structure is noncentrosymmetric¹. The molecular, non-centrosymmetric, charge transfer (CT) constituents can be considered as molecular diodes with enhanced polarizability in the CT direction. In consequence the first molecular hyperpolarizability tensor β_ijk has a component β_zzz enhanced along the CT axis (z). Such CT molecules consist of a conjugated π — electron core, terminated by an electron — donating group and an electron — withdrawing group. In such systems, the NLO response of the bulk material, which is important for device applications, is determined by the optical nonlinearities of the constituent molecules through their ensemble addition. In the simplest case of a single crystal the macroscopic second order NLO susceptibility χ⁽²⁾ _ijk in the laboratory reference frame (IJK) is related to the molecular first hyperpolarizability β_ijk describing the second order NLO properties of a given molecule in the molecular reference frame (ijk). Neglecting intermolecular interactions, χ⁽²⁾ _ijk is determined by the summation over independent molecules. Local field factors must also be included to account for the effect of the dielectric environment on the electric field strength at the molecular site.