Diffusion in the Formation of Photopolymer Holograms
Final technical rept. Sep 1987-Nov 1991,
ARMY MISSILE COMMAND REDSTONE ARSENAL AL WEAPONS SCIENCE DIRECTORATE
Pagination or Media Count:
The research presented in this report 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 optimal 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 illustration is spatially periodic, as when due to the interference of two plane waves,the chemical reaction also induces variations in the concentration of monomer. This diffusion causes a periodic modulation of the mass density, which also modulates the index of refraction of the medium. Detectable gratings are formed in DMP-128 upon holographic exposure. Experiment revealed that when the grating period was 25 micronm or more, the effect of diffusion was insignificant, and the diffraction efficience continued to increase long after exposure ended, due to residual free radical in the system.
- Physical Chemistry
- Polymer Chemistry