Charge transport, photorefraction

Charge transport is one of the important processes that control the speed of the PR index grating formation and the PR sensitivity. According to the standard theory of photorefraction [21], the response time for the formation and erasure of the space-charge field [xr in Eq. (21)] is proportional to the dielectric relaxation... 

As shown previously, nematic liquid crystals reorient easily in weak electric fields and their high birefringence provides an efficient electro-optic mechanism that makes them excellent candidates for photorefractive materials. However, charge transport relies on the generation of mobile anions or cations. These mobile charges obey the current density (/) equations given by [82,83]... 

In earlier investigations by the author [1,2] an additional nonlinear optical chromophore, (IV), and charge transport agent, (V), respectively, were prepared and used in photorefractive applications. 

While polymeric photorefractive materials can be prepared by adding separate molecules for photo-generation, charge transport and non-linear optical response into an inert polymer, most systems studied use polymers where one or more of these components are covalently bonded to the polymer. Polymer... 

Table 3. Selection of charge transport parameters from some pure charge-transporting components of organic photorefractive materials.

Steenwinckel DV, Hendrickx E, Persoons A. 2001. Large dynamic ranges in photorefractive NLO polymers and NLO pol5mier dispersed liquid crystals using a bifunctional chro mophore as a charge transporter. Chem Mater 14(4) 1230 1237. 

Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport, or photoconductivity, and good electro-optical activity. 

A series of photorefractive poly(methacrylate)s, containing a Disperse Red-type chromophore and carbazole as the charge transport agent with various spacer lengths, was synthesized and characterized. The photorefractive effect of these materials was studied by four-wave mixing and two beam coupling at 780 nm after sensitization with 1% (2,4,7-trinitroflu-... 

Within a relatively short time, many polymeric materials were developed that do exhibit the photorefractive effects. These materials consist of polymers with charge transport agents and photosensitizing dyes. The quantity of change in a material s refractive index that is altered by alterations in an applied electric field is expressed as the electrooptic coefficient r. Lithium niobate has an r value of 30 pm / volt. Several polymers are already known that possess coefficients (at 1.3 pm) greater than 30 pmW and the aim now is to develop materials with r values of 50 pm / volt or even higher. It is also known that the photorefractive effect can also be observed in many organic materials. 

One photorefractive polyimide 59 was formed by linking thee units, a substituted porphyrin (charge generator), 4-amino-4 -nitrostilbene derivative (the nonlinear optics molecule), and a charge transporting agent, pyromellitic dianhydride. This polyimide retains its nonlinear optical properties even after... 

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