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1.3 PHYSICAL SCIENCES
The development and characterisation of a photopolymerisable photopolymer holographic recording material, for red sensitisation is described. The material comprises dye (methylene blue), an electron donor (triethanolamine) and monomer (acrylamide and methylene-bis-acrylamide). A binder is also included (poly(vinylalcohol) to allow recording in dry layer format. One of the main advantages of the material is its ability to self-develop, which makes it highly suitable for a range of applications which includes the fabrication of Holographic Optical Elements (HOEs). The optimisation of the material formula for red-sensitisation using holographic techniques is described. Diffraction efficiencies of over 70% were obtained in 25 seconds with the optimised material. Sensitivity was improved considerably from approx. 650mJ/cm² at the start of the optimisation study to approx. 130mJ/cm². The chemical characterisation of the material was conducted using spectroscopic techniques. Results have shown that no chemical interaction occurs between the material components prior to exposure at the desired wavelength. The holographic characterisation was also described in terms of optimum layer thickness, beam intensity and ration and spatial frequency response, for efficient recording. Some of the physical processes that occur during holographic recording were investigated. Through the study of amplitude gratings recorded in the material and the application of a mathematical model, it has been proven that the diffusion of dye molecules is the main contributing factor leading to the decay of this type of grating. The dependence of the rate of diffusion of dye on the molecular weight of the binder has also been demonstrated. This has lead to ideas on how to improve the spatial frequency response of the material. Aspects of the material’s physical and holographic recording characteristics, which are relevant to the recording of HOEs as well as practical limitations of elements recorded in this material, are discussed. It is shown that the material can tolerate a slant angle of up to 40° without significant loss of diffraction efficiency. The results obtained for on and off-axis focusing elements are presented as well as some work on stacked elements.
Feely, Clodagh. The development and characterisation of a red-sensitised photopolymer holographic recording material. Dublin : Dublin Institute of Technology, 1998