Photorefractive systems

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. 

An organic photorefractive system has to contain different functional groups providing for the generation, transport, and trapping of charge carriers. Moreover, a plasticizing function is required for certain formulations. Apart from the... 

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. ... 

There are three polymeric systems appropriate for holography photopolymeriz-able systems photochromic systems and photorefractive systems (Table 3.34). Detailed information on the topic of polymers in holography is available in various reviews [313-320]. 

The dominant methodology is that based on covalent bonding of the chromophore to the polymer, as this shows much improved long term stability over simple guest-host systems. Guest-host polymers using NLO guests have been extensively studied in the area of photorefractive polymers (see section 5.6.3). 

Figure 9 Diffraction efficiency of photorefractive grating in the composite systems. Note that high diffraction efficiency for the composites containing the intramolecular charge transfer dopants 3 and 4 occurs at lower applied voltages than those for the intermolecular charge transfer dopants.

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... 

Based on the BOA theory, Barzoukas and Blanchard-Desce developed a two-state two-form model for the Pq of push-pull compounds [48]. The y o was expressed as a function of the newly defined parameter MIX, which is related to the degree of mixing between the neutral and zwitterionic resonance forms in the ground and excited state. This treatment has the advantage of conceptual simplicity and that it relies on a structural parameter that can be more easily used for other push pull molecular systems than polyenes. The evolution of y o observed with MIX is identical to that with BOA. The BOA theory was later also used to optimize the figure-of-merit of dyes for photorefractive applications [49, 50]. 

Petrov, M. P., Stepanov, S. I., Khomenko, A. V. Photorefractive Crystals in Coherent Opucal Systems. Berlin Springer, (1991). 

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