Photorefractive mechanism

Figure 3.86. Photorefractive mechanism discussed by using an objective. (From Ref. [247] with permission of Wiley-VCH.)...

O. Ostroverkhova and W. E. Moerner, Organic photorefractives mechanisms, materials, and applications. Chem Rev 104 (2004), 3267-314. 

The mechanism for the photoelectric effect has implications for the design of photorefractive polymers, which must demonstrate the following characteristics. 

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. 

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]... 

Several studies show that photorefractive keratectomy and laser in situ keratomileusis (LASIK) can induce or exacerbate dry eye. Refractive surgery is believed to cause aqueous-deficient dry eye by a neural-based mechanism. 

Table 5. A comparison of the photorefractive performance parameters from a range of materials exhibiting different alignment properties and/or different mechanisms for generating a refractive index contrast. 

The photorefractive effect is a physical mechanism where the change in the intensity-dependent refractive index is dependent on the spatial variations of intensity. It is a non-local process, because unlike most processes, the change in the refractive index is not dependent on the magnitude of the intensity that produces such change. 

Poly(N-vinyl carbazole), PVK, is a glassy polymer which easily fails by a cracking fracture. Plasticizers are needed to improve mechanical properties, reduce viscosity, and lower processing temperatnre. PVK is employed in photorefractive systems and as such it must possess photoconductivity and electro-optic effects. These ate related to the crystallization and the glass transition temperature. ... 

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