Quantitative model of volume hologram formation in photopolymers

Abstract

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 exposure to a visible ''write'' beam. The model refines the simple diffusion model of Zhao and Mouroulis ͓J. Mod. Opt. 41, 1929 ͑1994͔͒, by including cure dependence of both the photoreaction kinetics and the monomer diffusivity. These dependences are determined by experimental measurements, using near infrared spectroscopy to quantify the degree of cure and the time dependence of the hologram formation to infer the cure-dependent diffusivity. The cure-dependent diffusion coefficient can be fit by an expression from a free-volume theory, and the cure-dependent reaction rate coefficient is found to be proportional to the diffusivity, showing the reaction rate to be diffusion limited. With the model parameters determined experimentally, predictions are then made of the first, second, and third harmonics of the grating profile, and these are found to be in good agreement with the measured values. The results show the validity of the model and its usefulness in predicting the optimal exposure conditions and performance of a given holographic material.

Photoinduced surface relief studies in an acrylamide-based photopolymer

“…There are different theoretical models explaining the formation of holograms in photopolymer material [19,20] due to mass transport from dark to bright regions. From the diffusion studies of acrylamide-based photopolymer [21] it was observed that the mass transport from dark to bright regions is faster than in other photopolymer systems [22,23].…”

Dynamic Bragg Angle Shift in the Course of Volume Hologram Formation

“…[6,7] A research challenge of considerable importance concerns the optical shrinkage in the course of hologram formation. [8,9] The photocuring is always linked to shrinkage of the volume, as a result of the shortened particle distance in the covalent bond, compared with the individual van der Waals radii prior to the curing.…”

Curing Dynamics of Photopolymers Measured by Single-shot Heterodyne Transient Grating Method

“…The reaction rate and degree of polymerization have been studied 4 by differential scanning calorimetry (DSC), 5 IR absorption spectroscopy, 6,7 fluorescence monitoring during curing, 8 holographic grating formation by UV irradiation and its detection by diffraction. [9][10][11][12][13] Several theoretical models to understand the reaction kinetics have been proposed [14][15][16][17] and the polymerization theory, including monomer diffusion, has been established and used for the explanation of curing. However, the theoretical models are still not enough to cover the actual reactions because of a lack of a detection of the reaction processes.…”