Impact of inhibitor diffusion in holographic photopolymers

Holographic photopolymers develop permanent index change via the polymerization and subsequent diffusion of monomer. It is well-known that to achieve high-fidelity recording, the rate of polymerization must be small in comparison to the rate of diffusion, particularly for strong exposures that consume significant fractions of the available monomer. When this condition is violated, polymerization is slowed in high-intensity regions by the local depletion of monomer, resulting in broadening of recorded features. This paper shows that a diffusing inhibitor has analogous dynamics controlled by the ratio of inhibitor diffusion rate to inhibition rate. When the ratio is small, inhibitor is locally depleted in bright regions, resulting in localized acceleration of polymerization. This causes recorded index features to be narrower than the incident optical exposure. Theoretical, numerical and experimental studies are used to illustrate this fact and show that this narrowing can be used to compensate for the broadening caused by monomer dynamics. Both effects are emphasized for rapid, strong recordings, suggesting that an inhibitor may be used to increase recording fidelity in this limit.