23 March 1999 Development of photorefractive polymers for real-time optical information storage
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Abstract
A large number of strong nonlinear optical and electro optical molecules and crystals are identified recently. With the discovery of the photorefractive (PR) effect and early realization of its potential utility, PR materials are of considerable interest for the development of all optical devices, such as high density optical data storage and image processing techniques. Organic materials are known to show strong electro optic effects. In organic materials, the properties required for the PR effect including photosensitivity, photoconductivity and electro optic response are provided by different molecules. As a result, the properties can be optimized separately, unlike in inorganic PR crystals such as LiNbO3. This paper describes the utilization of third order non-linearity induced in Xanthene dye doped gelatin and poly (eosin acrylate) & poly (eosin acrylate-co-isobutyl acrylate) films resulting in direct storage without the need for any further processing i.e., no wet chemical or post thermal/photochemical processing are required. With required amount of solvent and monomer in the presence of benzoyl peroxide (initiator), polymerization reaction was carried out under nitrogen atmosphere. Polymer samples were characterized by NMR, IR, FT-IR & TGA. The polymers were soluble in THF, DMSO, DMF & DMAC solvents and form good optical quality films by spin as well as dip coating. Polymer thin films were prepared with different concentrations of polymer solution onto the glass slides. The UV-visible absorption spectra of the spin coated polymer films showed a maximum at 538 nm. In our simplest optical system, Q- switched, second harmonic Nd-YAG laser light at wavelength 532 nm was used for recording. Two beams split from the same laser were made to superpose with path difference less than the coherent length. One of the beam acted as information carrying beam while the other acted as reference beam. In this present study, we report the direction formation of surface relief gratings on photorefractive surfaces upon exposure to an interference pattern without any subsequent processing steps. This paper also explains the mechanisms of photorefractive effect involved in our samples and how optical data could be stored in a few square micron surface area of the film in the form of surface relief Fourier hologram. Scanning electron microscopic pictures proved the formation of surface relief. The surface relief generated on this photorefractive surface has made it possible to prepare a metallic master to achieve mass replication thereby making the commercial introduction of such technology feasible. Reconstruction was easily made with low power cw laser such as He-Ne. The diffraction efficiency of the hologram formed on these films were found to be around 40%.
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D. Mangaiyarkarasi, P. K. Palanisamy, P. Kannan, "Development of photorefractive polymers for real-time optical information storage", Proc. SPIE 3638, Holographic Materials V, (23 March 1999); doi: 10.1117/12.342815; https://doi.org/10.1117/12.342815
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