Photorefractive sensitizer

We have reported further improvements in the photorefractive sensitivity of liquid crystals by doping with chromophores that undergo efficient photoinduced intramolecular charge transfer [24], The magnitude of the observed photorefrac-... 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

The relatively simple study of fluorescence and phosphorescence (based on the action of colour centres) has nowadays extended to nonlinear optical crystals, in which the refractive index is sensitive to the light intensity or (in the photorefractive variety (Agullo-Lopez 1994) also to its spatial variation) a range of crystals, the stereotype of which is lithium niobate, is now used. 

Mecher, E., Gallego-Gomez, F., Tillmann, H., Horhold, H.H., Hummelen, J.C., and Meerholz, K. (2002) Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination. 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

A general review of photorefractive materials was presented in 1988. 150) Also, two monographs in were published which detail theory, physical characterization and practice of the use of known photorefractives.(151) Three classes of inorganic materials dominate. Ferroelectric oxides, such as LiNbC>3 and BaTiC>3 mentioned above compound semiconductors such as GaAs and InP, and the sillenite family of oxides, exemplified by Bii2SiC>20 and Bii2TiC>20 The semiconductors are sensitive only in the infrared, while the other materials operate in the... 

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... 

The initial sensitizer anion presence makes recombination of mobile holes possible in the dark regions. Which are the compensator sites Here, there exist different explanations. One possibility is that some of the electro-optic dye molecules present in photorefractive composites to provide refractive index change may become charged positively. An alternative theory in the case of amorphous materials is that the amorphous disorder leads to defect sites forming local potential minima at which positive charge may be immobilized (Figure 5). 

Figure 12. Charge sensitizers used in organic photorefractive materials.

Various materials such as photopolymers, photorefractive crystals, photochromic materials, and polarization sensitive materials can store data. Among these materials, photorefractive recording media in which data are stored as refraaive-index change are the most promising in 3D memory because they have little absorption hence, the light penetrates into the deep layers in the media. 

To tune the sensitivity of a photorefractive polymer to a particular spectral region, the composite is doped with a sensitizer. Examples of widely used sensitizers are shown in Fig. 21. TNF was used in numerous polymers based on the photoconducting polymer PVK because mixtures of PVK/TNF were used in the first... 

If bifimctional molecules do not form glasses, they can be incorporated into polymer binders. Examples of bifunctional molecules are shown in Fig. 24. DTNBl (molecule (b) in Fig. 24) was doped in PMMA and Cgg was used as a sensitizer. Diffraction efficiencies of 7%, sub-second grating growth times, and net two-beam coupling gain coefficients of 34 cm" were observed in such samples [81]. DPANST (molecule (c) in Fig. 24) was doped into PBMA [84]. Recently, ms response times were reported in photorefractive glasses based on the bifunctional chromophore DRDCTA (molecule (d) in Fig. 24) doped with the plasticizer DOP (molecule (b) in Fig. 23) and Cgg as a sensitizer [89, 90]. 

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