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Photopolymers are promising materials for use in holography. They have many advantages such as ease of use and are capable of efficiencies of up to 100%. However, the main disadvantage to these materials is their inability to record high spatial frequency gratings when compared to other materials such as dichromated gelatine and silver halide emulsion. This poor spatial frequency response is not predicted by any of the current physical modes. In this study, it is proposed that this effect is due to polymer chains growing away from their initiation point and causing a ‘smearing’ of the profile to be recorded. This is termed a non-local response. The model introduced by Zhao et al. [Zhao, 94] is extended to include this non-local effect. The predictions of the new non-local model are examined. The model predicts a fall off in spatial frequency response which is easily observable by experiment but not predicted by any other theory. The model is then manipulated to give simple analytic expression. These expressions are then applied to experimental data using a number of techniques. This allows values to be estimated for material parameters such as the diffusion coefficient of monomer, the ratio of polymerisation rate to diffusion rate and the distance that the polymer chains spread during holographic recording. The model is then adjusted to take account of a new form of intensity dependence proposed by Kwon et al [Kwon,99]. Commercially available DuPont photopolymer was used to fabricate Holographic Optical Elements. Experimental work was carried out in an attempt to improve the spatial frequency response of an acrylamide based photopolymer material. This involved changing the molecular weights of the chemical components and the addition of a retarder in order to shorten the polymer chains, thereby decreasing the non-local effect. No significant change in spatial frequency response was observed. However, with further work, the use of retarders could lead to an improvement in response for a range of photopolymers.
Lawrence, J. (2002). Diffusion based grating formation in a photopolymer material. Masters dissertation. Technological University Dublin. doi:10.21427/D7W03D