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1.3 PHYSICAL SCIENCES
Photopolymer materials are a practical alternative to high resolution silver halide photographic emulsions for the recording of holograms and high spatial frequency holographic gratings. The new holographic recording material reported in this thesis consists of a green sensitive dye (erythrosine B), an electron donor (triethanolamine), and a monomer mixture (acrylamide and methylene bisacrylamide) in a polyvinyl alcohol binder. ON illumination with light of the appropriate wavelength the dye absorbs a photon and the excited triplet state of the dye reacts with the electron donor to produce free radicals. These free radicals may then initiate local polymerization of the monomer. The changes in density and molecular polarizability which accompany polymerization cause a change in local refractive index and the hologram is thereby recorded as a volume phase hologram. After recording no further chemical or physical processing steps are required. The chemical and physical processes leading to the refractive index modulation are investigated in detail. The relative contributions to the refractive index change, of density change through diffusion and decrease in molecular polarizability are discussed. The conclusions drawn lead to a new understanding of the processes of hologram formation in this type of material. An original non-holographic technique for the investigation of the initiation processes and comparison of sensitizing dyes is presented. The comparison of quantum yields of bleaching in the absence and presence of monomer is used to estimate the amount of initiation occurring in monomer containing layers. This, and other more conventional holographic methods are used to optimize the chemical composition of the material for sensitivity, diffraction efficiency, shelf life and layer quality. Thick layers of the optimized material require only 50mJ/cm² for an 80% diffraction efficiency or 80 mJ/cm² for a 96% diffraction efficiency. The optimized material is found to perform well in transmission mode for slanted or unslanted gratings and has a resolution of up to 3000 lines/mm. However the material, in its present formulation, will only record weak reflection gratins (>2% efficiency). A demonstration of the suitability of this self developing material for holographic interferometry is carried out with excellent results.
Martin, Suzanne. A new photopolymer recording material for holographic applications: photochemical and holographic studies towards an optimized system. Dublin : Dublin Institute of Technology, 1995