Polymers photorefractive

Kishore V. Chellapan, Rani Joseph, and Dhanya Ramachandran 

Handbook of Stimuli-Responsive Materials. Edited by Marek W. Urban Copyright 2011 WILEY-VCH VerlagGmbH Co. KGaA, Weinheim ISBN 978-3-527-32700-3 

As the name implies, PR effect is light-induced modulation of the refractive index of a medium. Both photoconductivity, which refers to increase in electrical conductivity when the material is exposed to electromagnetic radiation of appropriate energy, and EO response, which refers to a change in therefractive index with electric field, are required for a medium to be PR. 

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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). 

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. 

Note 3 A polymer that exhibits a nonlinear optical effect due to anisotropic electric susceptibilities when subjected to electric field together with light irradiation is called an electro-optical polymer. A polymer that exhibits electro-optical behavior combined with photoconductivity is called a photorefractive polymer. 

Photorefractive polymers are also being developed for use in both permanent and real-time holography (see section 5.6.3). 

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). 

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

Photorefractive PDLCs are designed to combine the high efficiency at low fields of LC materials with the high resolution of photorefractive polymers. The polymer... 

In the pursuit of improved photorefractive materials, seminal research by a group at IBM led by W. E. Moemer discovered the photorefractive effect in polymers in 1990 [4,40], Photorefractive polymers are generally composite materials... 

Photorefractive Polymers Polymers that are used to write and read information through their characteristics of charge, motion and nonlinear optical response. 

Hyperbranched polyester crosslinked photorefractive polymers were prepared by Nishikata [4] consisting of Dispersant Red-19, (VII), and trimesic acid or trimesic acid containing isophthalic acid. Perfluorobenzoate Dispersant Red-19 derivatives, (VIII), were also prepared by the author [5]. 

Figure 10. Top liquid-phase absorbance spectrum of each component of a typical photorefractive polymer composite. Each component, A -vinylcarbazole (PVK), 2,4,7-trinitro-9-fluorenone (TNF), and typical chromophore (EHDNPB), is diluted in dichloromethane separately. (Absorption due to the solvent has been subtracted.) Bottom the absorbance of light in a solid sample due to the charge transfer complexation between PVK and TNF. The sample was prepared from a 9 1 ratio of PVK/ TNF. The extension of absorption to longer wavelengths is clear.

Figure 11. Possible relative positions of the energy levels in a typical guest-host photorefractive polymer composite containing PVK, TNF, and an azo-chromophore. A possible sequence of events leading to mobile charge generation is also included.

Table 1. Rate of response of selected photorefractive polymer composites a comparison of the experimental fast time constant of growth of contrast in refractive index in response to a nonuniform intensity pattern and the theoretical rate limit based on charge photogeneration rate.

Fully functional photorefractive polymer with a Ruthenium complex... 

Figure 14. Examples of fully functional photorefractive polymers.

Figure 16. Transient photocurrent signal from a time-of-flight measurement of a photorefractive polymer composite containing 47.5 % electro-optic dye (EHDNPB), 1 % TNF with PVK polymer making up the remainder 21 V was applied across the 100 nm polymer film and a 10 nm thick rhodamine 6G charge generation layer was used. The hole mobility in this material is thus...

Figure 19. A typical HTOF signal from the photorefractive polymer composite PVK/0.1 wt.% TNF and 40 wt.% 4-(hexyloxy)nitrobenzene electro-optic chromophore, reproduced with permission from [42]. A clear peak is observed which represents the transit time of holes across half a grating period. Eventually a steady state is reached due to the spatial hole distribution being smeared out by dispersive transport and hole recombination.

A major application of photorefractive polymers as holographic data storage media is a real possibility. In this application two interfering laser beams write a hologram... 

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