Inorganic photorefractive materials

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... 

An expression for the internal space-charge field can be obtained through the Kukhtarev model [38] that was developed to describe photorefractivity in most inorganic materials. In this model, the photorefractive material is described by a band model. As for a traditional semiconductor, the material consists of a conduction and a valence bands separated by a band gap as shown in Fig. 13. The model describes the transport of single carrier species and the band gap of the material contains localized energy levels that can be excited optically promoting either holes in the valence band (VB) or electrons in the conduction band (CB). In the model that we adopt here, we assume that the dopant is a donor with an energy level located in the band gap with concentration N. Furthermore, the crystal contains Nj acceptors with that are all ionized and that have accepted a... 

For several decades, the fields of photoconducting (75) and purely electrooptic polymers (74) have been very active but had almost no direct overlap. With the development of photorefractive polymers in the early nineties, the knowledge of these two research areas could be combined and has led to a rapid improvement of the performance of existing photorefractive polymers. The photorefractive polymer composite DMNPAA PVK ECZ TNF (DMNPAA 2,5 -dimethyl-4-(p-nitrophenyl-azo)anisole PVK poly(N-vinylcarbazole) ECZ N-ethylcarbazole TNF 2,4,7-trinitrofluorenone) we developed recently (P) has reached a level of performance that competes with that of the best inorganic photorefractive crystals (77,72). With the recent progress achieved in the development of new chromophores for electro-optic applications (75), the efficiency of these new materials is expected to be significantly further improved. 

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). 

Organic-inorganic hybrid materials have attracted extensive fundamental and practical interest/ Molecular design of hybrid materials could produce materials with tunable properties for optoelectronic devices, including optical waveguides,electroluminescent devices,nonlinear optical materials, photorefractive materials, and semiconductors/ ... 

Inorganic photorefractive crystals have typically been the choice for data storage recording media [32]. Their typically high thicknesses allow for storage of large amounts of information, and because their operation depends on generation of electric fields within the substrate, these materials can be erased and rewritten. Their cost and complexity of use, however, have limited their practical application. 

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. 

Photorefractive inorganic materials have been studied for some time as holographic media, but advances in photorefractive organic materials are relatively... 

The photorefractive effect in polymeric systems is different from that in inorganic crystals because the charge photogeneration quantum yield, the carrier mobility and (in case of low-Tg materials) the electro-optic activity are clearly field-dependent parameters. Polymeric materials optimized for photorefractive response have to fulfill a number of important requirements in addition to those known for conventional applications. These include spectrally optimized charge... 

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