Photorefractive Polymers

Abstract

When perturbed with electromagnetic radiation of appropriate wavelength and intensity, photorefractive (PR) systems alter their local refractive index properties. To achieve PR properties, photoconductivity and electro-optical response are required. This chapter describes the physics and chemistry of PR systems to stimuli such as electric field and light. Various processes such as photogeneration of charge carriers, charge transport, electro-optic (EO) effect, and grating formation are described in the introductory sections. Material requirements, experimental techniques, and types of PR systems are discussed in the context of the recent progress in PR polymers and their applications.The PR effect is observed in materials showing both photoconductivity and an electric-field-dependent refractive index. The first observation of PR effect in a polymer system was made in the EO polymer bisphenol-A-diglycidylether 4-nitro-1,2-phenylenediamine doped with the charge-transporting molecule diethylamino-benzaldehyde diphenylhydrazone [1]. PR media can be used to store a replica of the incident nonuniform intensity pattern as a refractive index modulation. Illumination with uniform intensity distribution of appropriate wavelength will erase any grating that has already been written inside the material, making them suitable for reversible and real-time holography. Many applications such as multiplexed data storage [2], holographic filters [3], neural networks [4], and updatable 3D display [5] have been demonstrated on a laboratory scale, but commercial products are not yet available mainly because of the strict requirements on material quality and the complexity of the optics required to implement these systems. Early searches for better materials had been concentrated on inorganic crystals, but after the invention of polymer PR materials, significant research efforts have directed to this class of materials as well due to the ease of processability, which is an advantage over inorganic crystals. Diffraction efficiency approaching 100% and two-beam coupling (TBC) gain coefficient of >400 cm −1 have been achieved in PR polymers. The gain coefficients in hybrid systems combining liquid crystals or low-molecular-weight glass-forming materials and polymers have Handbook of Stimuli-Responsive Materials. Edited by Marek W. Urban