Photorefractive

P. Gbnter and J. P. Huignard, Photorefractive Materials and their Applications I, II, Springer-Vedag, New York, 1988. 

Phase Materials. Phase holograms can be recorded in a large variety of materials, the most popular of which are dichromated gelatin, photopolymers, thermoplastic materials, and photorefractive crystals. Dichromated gelatin and some photopolymers require wet processing, and thermoplastic materials require heat processing. Photorefractive crystals are unique in that they are considered to be real-time materials and require no after-exposure processing. 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

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). 

P. Guntei and J.-P. Huignaid, eds., Photorefractive Materials and PheirApplications I andll Spiiugei-Vedag, Berlin 1988,1989. 

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. 

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

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. 

Nolte, D. D. Melloch, M. R., Photorefractive quantum wells and thin films, In Photorefractive Effects and Materials Nolte, D. D., Ed. Kluwer Academic Publishers, Dordrecht, 1995... 

Following the discovery of the unique electronic properties of polypyrrole, numerous polymers of pyrrole have been crafted. A copolymer of pyrrole and pyrrole-3-carboxylic acid is used in a glucose biosensor, and a copolymer of pyrrole and A-methylpyrrole operates as a redox switching device. Self-doping, low-band gap, and photorefractive pyrrole polymers have been synthesized, and some examples are illustrated [1,5]. 

Nonlinear ion traps, 15 662 Nonlinear materials, 14 680 Nonlinear optical materials, 17 442-460 advantage of, 17 448 classification of, 17 443—444 economic aspects of, 17 457-458 photorefractive materials, 17 457 second-order, 17 444r-453 third-order, 17 453—457 Nonlinear optical phenomena, 17 443 Nonlinear optics... 

VIII. Further Applications Photochromic and Photorefractive Materials... 

VIII. FURTHER APPLICATIONS PHOTOCHROMIC AND PHOTOREFRACTIVE MATERIALS... 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

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