QUANTITATIVE MODEL OF VOLUME HOLOGRAM FORMATION IN PHOTOPOLYMERS

A quantitative model is presented to describe the formation of volume holograms in a polymeric medium containing photopolymerizable acrylate monomers that undergo spatially modulated gelation as a result of exp osure to a visible ''write'' beam. The model refines the simple diffus ion model of Zhao and Mouroulis [J. Mod. Opt. 41, 1929 (1994)], by inc luding cure dependence of both the photoreaction kinetics and the mono mer diffusivity. These dependences are determined by experimental meas urements, using near infrared spectroscopy to quantify the degree of c ure and the time dependence of the hologram formation to infer the cur e-dependent diffusivity. The cure-dependent diffusion coefficient can be fit by an expression from a free-volume theory, and the cure-depend ent reaction rate coefficient is found to be proportional to the diffu sivity, showing the reaction rate to be diffusion limited. With the mo del parameters determined experimentally, predictions are then made of the first, second, and third harmonics of the grating profile, and th ese are found to be in good agreement with the measured values. The re sults show the validity of the model and its usefulness in predicting the optimal exposure conditions and performance of a given holographic material. (C) 1997 American Institute of Physics.