Photorefraction orientational enhancement effect

The photorefractive performance of high-molecular-weight PVK-based composites can be enhanced by optimizing the composition of the PVK composites [73]. 2,4,7-Trinitrofluorenone is used as sensitizer material. The improved performance arises due to a large orientational enhancement effect with faster response speed in addition to samples with an appropriate glass transihon temperature. 

Because of these advantages as well as orientational enhancement effects and the superior compositional flexibility of pol5nners, the number of studies of polymers for photorefraction grew rapidly during the 1990s and into the current decade. It is now common to see a chapter on polymeric photorefraction in treatises that once emphasized only inorganic crystalline materials (for example, see References 23-27) and to see specific sections on photorefraction in overviews of nonlinear optical properties of pol5nners (for example. See references 28-31). 

Fig. 2. Oblique geometry tsqiically used for characterizing bulk photorefractive pol3miers. The magnified portion illustrates the orientational enhancement effect described in the text due to the modulated poling of the electro-optic chromophore in the combined Eb + Esc fields. The beam and grating geometry applies for both four-wave mixing (beams 1-4) and two-beam coupling (beams 1 and 2).

Ellipsometry. Because of the importance of the orientational enhancement effect, the electric-field-induced birefiringence is independently measured using a transmission elUpsometry technique. In this experiment, the photorefractive pol3rmer is placed between two crossed polarizers with the normal to the... 

Understanding the effects of orientational enhancements has been made more complex, however, by the need to account for supramolecular interactions such as dipolar aggregation, the tendency of dipoles to adopt align antiparallel with their neighbors (38). Several studies have explored the effects of glass transition temperature on polymeric photorefractive properties (46-48). 

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