Diffusion in the Formation of Photopolymer Holograms

The research presented in this dissertation shows that the formation of holographic gratings in the Polaroid DMP-128 photopolymer medium is largely a result of the diffusion of lithium acrylate monomer within the medium. DMP -128 and other similar holographic photopolymers are capable of recording volume phase gratings by means of the physical and chemical changes that occur in the media when they are exposed to an optical interference pattern. In the presence of light, the absorption of photons by a sensitizing dye leads to the formation of free radicals. The free radicals, in turn, catalyze the linkage of simple monomer molecules into long-chain polymer molecules. The chemical reaction alters the molar refraction, and thus the index of refraction of the medium. If the illumination is spatially periodic, as when due to the interference of two plane waves, the chemical reaction also induces periodic variations in the concentration of monomer. The result is the diffusion of monomer from dark fringes, where little polymerization occurred, to bright fringes, where monomer was depleted by the photochemical reaction. This diffusion causes a periodic modulation of the mass density, which also modulates the index of refraction of the medium, according to the Lorentz-Lorenz relationship. A mathematical model developed in this research predicts that the amplitude of the periodic concentration decays exponentially with time; the time constant varies quadratically with the period of the grating. A detectable grating is formed in DMP-128 immediately upon holographic exposure. Experiment revealed that when the grating period was 25 microns or more, the effect of diffusion was insignificant, and the diffraction efficiency continued to increase long after exposure ended, due to residual free radical in the system. Shorter grating periods led to progressively more rapid decay of diffraction efficiency after exposure. The decay can be explained by assuming that the index modulation of the medium was varying as the sum of two decaying exponentials, indicating that both monomer and the sensitizing dye were diffusing within the medium. The total amplitude of the decay indicates that less than 10% of available monomer is free to diffuse within the photopolymer medium.