Second-order nonlinear optical behavior

Evaluation of the Second-Order Nonlinear Optical Behavior... 

One core chiral system that shows dramatic amplification of its chiral structure is the substituted helicenes of Katz and coworkers [83]. In essence, this research cuts the helix into a number of six-helicene subunits that self-assemble (Figure 10). Only when these subunits, which look like lock washers, are prepared in optically pure form the material associates into supramolecular helical columns [84]. The assemblies have been synthesized with different amounts of substitution around the exterior. Depending on the helicenes substitution, the material exhibits hexagonally ordered soft-crystalline [84] or liquid-crystalline phases [85]. The liquid-crystalline versions of these molecules switch when electric fields are applied to neat and solution-phase samples and have been characterized as a dielectric response [85-87]. Upon association, these materials have enormous changes in their CD intensities and optical rotations [74]. In addition, this supramolecular chirality also significantly enhances the second-order nonlinear optical behavior of these materials in Langmuir-Blodgett films [88]. 

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Other related kinds of medium-dependent behavior have been observed, such as second-sphere coordination effects (e.g., with crown ethers, cyclodextrins ), and solvatochromic medes have been used as probes of their environment in polymers, micelles, " zeolites, inorganic glasses, and surfaces, etc. They may be used in this way either by virtue of their response to the electric field experienced in a particular environment, or because of specific interactions between the probe and the environment, e.g., hydrogen bonding. Solvatochromism is also a useful predictor of nonlinear optical behavior (see Chapter 9.14), because the same properties which give rise to strong solvatochromism are also necessary for large, second-order, nonlinear optical coefficients (/3). ... 

Ferroelectric behavior has been detected for polymer 11, as a consequence of the "chiral smectic-C" stracture of the liquid crystalline phase (29). Polymer 12 is a second order nonlinear optical material, the polarizable side groups, in which can be poled in a 10-20 kV electric field to generate values in the region on 34-45 picometers per volt (30, 31), Finally, species 13 is a photochromic polymer, which undergoes a spiropyran-merocyanine transformation when exposed to ultraviolet light (32),... 

In the case of molecular organic materials, where inter-molecular interactions are weak, the nonlinear optical behavior is essentially determined by Eq. (21). In what follows we focus on the molecular hyperpolarizabilities of the first and second order (/8 and y). 

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