The invention of holography by Denis Gabor2 in 1948 initiated a new direction in information recording.However, because of limitations such as unavailability of coherent light sources, holography lay dormant for almost a decade.The discovery of the off-axis holographic technique by Leith and Upatneiks2in 1962 and the development of highly coherent laser sources created waves of excitement and a resurgence in the field of holography.Simultaneously, Denisyuk’s3and Van Heerdeen’s4proposal of Lippman holography and deep (volume) holograms at the same period gave a new life to the domain of holography.The reconstructed three-dimensional images seen through a hologram struck the imagination of scientists all over the world and there was a burst of research activity to find new recording materials and techniques capable of recording the complete optical information carried by object wavefronts5-13The demand for better materials with stringent characteristics such as high resolution, better energy sensitivity, broad wavelength sensitivity, simpler processing procedure, and erasability was severely felt and still exists today.

Holographic Recording Materials

One of the key predictions of the nonlocal photopolymerization driven diffusion (NPDD) model is that a reduction in the extent of the nonlocal effects within a material will improve the high spatial frequency response. The NPDD model is generalized to more accurately model material absorbtivity. By eliminating the necessity for the steady-state approximation to describe the rate of change of monomer radical concentration, a more accurate physical representation of the initial transient behavior, at the start of grating growth, is achieved, which includes the effects of oxygen-based inhibition. The spatial frequency response of an acrylamide/polyvinylalcohol-based photopolymer is then improved through the addition of a chain transfer agent (CTA), sodium formate. Using the NPDD model demonstrates that the CTA has the effect of decreasing the average length of the polyacrylamide (PA) chains formed, thus reducing the nonlocal response parameter, σ. Further independent confirmation of the resulting reduction in the PA average molecular weight is provided using a diffusion-based holographic technique.

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Improvement of the spatial frequency response of photopolymer materials by modifying polymer chain length

Photopolymer materials are practical materials for use as holographic recording media, as they are inexpensive and self-processing (dry processed). Understanding the photochemical mechanisms present during recording in these materials is crucial to enable further development. One such mechanism is the existence of an inhibition period at the start of grating growth during which the formation of polymer chains is suppressed. Some previous studies have indicated possible explanations for this effect and approximate models have been proposed to explain the observed behavior. We examine in detail the kinetic behavior involved within the photopolymer material during recording to obtain a clearer picture of the photochemical processes present. Experiments are reported and carried out with the specific aim of understanding these processes. The results support our description of the inhibition process in an acrylamide-based photopolymer and can be used to predict behavior under certain conditions.

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Effects of absorption and inhibition during grating formation in photopolymer materials

The one-dimensional diffusion equation governing holographic grating formation in photopolymers, which includes both nonlocal material response and generalized dependence of the rate of polymerization on the illuminating intensity, has been previously solved under the two-harmonic expansion assumption. The resulting analytic expressions for the monomer and polymer concentrations have been derived and their ranges of validity tested in comparison with the more accurate numerical four-harmonic case. We used these analytic expressions to carry out a study of experimental results presented in the literature over a 30-year period. Automatic fitting of the data with these formulas allows material parameters, including the nonlocal chain-length variance σ, to be estimated. In this way, (i) a quantitative comparison of different materials can be made, and (ii) a standard form of experimental result presentation is proposed to facilitate such a procedure.

/pdf/comparison-of-holographic-photopolymer-materials-by-use-of-3k00klpd9w.pdf

Comparison of holographic photopolymer materials by use of analytic nonlocal diffusion models

Hologram formation and diffusion reactions in photopolymer films are examined at different stages of exposure and at different spatial frequencies. Different properties of the grating formation process are evaluated from efficiency data, including the relative rates of diffusion and photoinitiated polymerization, dye absorption, and residual efficiency enhancement after UV curing. It was also found that gratings with larger periods (approximately 1.4 microm) are susceptible to erasure effects with postexposure laser illumination. In addition, crossed gratings were found to have an extended dynamic range. This effect can have a significant impact on the number of holograms formed with rotational or peristrophic multiplexing.

Dynamic hologram recording characteristics in DuPont photopolymers

Photopolymers have been the subject of special attention during recent years. Since these materials can be used for recording optical information, many studies were devoted to the evaluation of their characteristics in this respect. This paper is aimed at investigating new self-developing formulations requiring only short exposure and which are, therefore, specially suited for real-time interferometry. Well contrasted interferograms can be obtained with a recording system which does not need any post-treatment. The holographic exposure requires a few seconds with a power density of 5 mW cm-2. A practical application in which the dissolution (and the concentration gradient thereof) of a crystal is studied as a function of the time, reveals a spatial resolution better than 30 mm-1.

http://iopscience.iop.org/article/10.1088/0963-9659/1/5/002/pdf

D J Lougnot

Papers

Photopolymers for holographic recording. II. Self-developing materials for real-time interferometry