Photorefractive Conjugated Polymer-Liquid Crystal Composites
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for dynamic
photorefractive liquid
holography
4 7 crystals. 4-9,1 -1
applications
is
This is due to the
efficiency of the nonlinear optical mechanism for photorefractivity in nematic liquid crystals coupled with the ease and cost effectiveness with which new samples can be prepared. The photorefractive effect itself is a lightinduced change in the refractive index of a nonlinear optical material, resulting from the creation of an electric field induced by directional charge transport over macroscopic distances. If the material is electro-optic, the electric (or space-charge) field can then modulate the refractive index of the material. In order to obtain a quantitative understanding of photorefractivity and fully exploit its optical signal processing possibilities, researchers usually induce the effect by means of a crossed laser beam technique.18,19 When two beams of identical energy and spatial profile are overlapped in a sample, a sinusoidal interference pattern of light and dark regions is produced as illustrated in Figure 1. Charge separation occurs in the illuminated regions,
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0./
0E
Figure
1.
The
phase
relationship
between the optical interference pattern and the space-charge field is illustrated. For liquid crystals, this example illustrates mobile anions migrating into the nulls of the interference pattern. The application of an applied electric field EA is usually required to observe a phase shifted photorefractive grating.
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Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
followed by charge migration. In liquid crystals, charge migration occurs through diffusion of ions until they ultimately reach the darker regions of the interference pattern. For a space charge field to form, the mobilities of the cation and anion must differ appreciably, or no space-charge field will form. This will permit one type of ion to remain preferentially in the illuminated region, while the more mobile ion diffuses into the dark region of the interference pattern. This produces a modulated space-charge field that can alter the refractive index of the material. The refractive index modulation, or grating, forms the basis for the optical signal processing applications of the photorefractive effect. Furthermore, the phase shift of the photorefractive grating relative to the optical interference pattern of approximately 7n/2, provides for asymmetric energy transfer from one excitation beam to the other. This leads to additional noise-free optical image amplification possibilities. Liquid crystals owe their photorefractive sensitivity to high birefringence and ease of molecular reorientation in a space-charge field. The powerful nature of this effect can also be seen from improvements in the photorefractive polymer field, where many of the best materials use a low glass transition polymer to induce an orientational response of the poled, nonlinear optical molecules. 1 The orientational enhancement effect is frequently much greater than the normal electro-optic effect. 2,3 In
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