The use of theoretical models to represent the photochemical effects present during the formation of spatially and
temporally varying index structures in photopolymers, is critical in order to maximise a material's potential. One such
model is the Non-local Photo-Polymerization Driven Diffusion (NPDD) model. Upon application of appropriate
physical constraints for a given photopolymer material, this model can accurately quantify all major photochemical
processes. These include i) non-steady state kinetics, (ii) non-linearity iii) spatially non-local polymer chain growth, iv)
time varying primary radical production, v) diffusion controlled effects, vi) multiple termination mechanisms, vii)
inhibition, (viii) polymer diffusion and ix) post-exposure effects. In this paper, we examine a number of predictions
made by the NPDD model. The model is then applied to an acrylamide/polyvinylalcohol based photopolymer under
various recording conditions. The experimentally obtained results are then fit using the NPDD model and key material
parameters describing the material's performance are estimated. The ability to obtain such parameters facilitates
material optimisation for a given application.
The Non-local Photo-Polymerization Driven Diffusion (NPDD) model indicates how a material's performance might be
improved, and provides a tool for quantitive comparison of different material compositions and to predict their
fundamental limits. In order to reduce the non-locality of polymer chain growth (i.e the non-local response parameter, σ)
and to improve the spatial frequency response of a photopolymer material, we introduce the chain transfer agent (CTA).
In the literature, extensive studies have been carried out on the improvements of the non-local response modifying by the
CTA, sodium formate, in the polyvinyl alcohol-acrylamide (PVA/AA) material. In this article, i) based on the chemical
reactions of CTA, we extended the CTA model in the literature; ii) we compare two different CTA materials, sodium
formate and 1-mercapto-2-propanol without cross-linker in order to obtain the experimental confirmation of the
reduction in the average polymer molecular weight is provided using a diffusion-based holographic technique; iii) we
examine the non-local responses of several spatial frequencies with the two CTAs. Using the extended CTA model it is
demonstrated that the CTA has the effect of decreasing the average length of the polyacrylamide (PA) chains formed,
thus reducing the non-local response parameter, especially, in the high spatial frequency case.
Photopolymer materials exhibit good characteristics when used as holographic recording media. Extensive studies have been carried out on the behavior of the various chemical components in such materials, with photosensitizers in particular receiving much attention. In all previous analysis of photopolymer kinetics, the effects of photosensitiser diffusion have been neglected. For rapid sequential holographic recordings in photopolymers, for example, in an application such as holographic data storage, dye diffusion effects may become more pronounced. Therefore, we examine the dye diffusion effects of erythrosine B in an acrylamide/polyvinyl alcohol material. This is achieved using simple experimental techniques and a proposed theoretical model.
KEYWORDS: Molecules, Absorption, Photopolymers, Holography, Oxygen, Data modeling, Modulation, Molecular energy transfer, Optoelectronics, Refractive index
In the literature, several studies of the time varying photon absorption effects, which occur during the photo-initiation
process in photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) The
photon absorption, (ii) the regeneration or recovery of the photosensitizer, and (iii) the photosensitizer bleaching. Based
on the analysis of these mechanisms, the production of primary radicals can be physically described and modelled. In
free radical photo-polymerization systems, the excited dye molecules induce the production of the primary radical, R•, which is a key factor that in determining how much monomer is polymerized. This in turn is closely related to the
refractive index modulation, Δn, formed during holographic recording. In this article, by modifying the composition of a Polyvinylalcohol/Acrylamide (PVA/AA) based photopolymer material, i.e., excluding any co-initiator, the photo-kinetic
behaviour of the material is greatly simplified, an experimental study is performed, which makes possible development
and verification of a new model capable of accurately predicting the time varying concentration of primary radicals.
Photopolymer materials are practical materials for use as holographic recording media due to the fact that they are
inexpensive, self-processing materials with the ability to record low loss, highly diffraction efficient volume holographic
gratings. Extensive studies have been carried out on the behaviour of the various chemical components in such materials,
with photosensitizers in particular receiving much attention, as they are an important component in initialising the
photopolymerisation reaction. However in all such analyses dye diffusion is neglected. To further develop such
materials, a deeper understanding of behaviours the photosensitizer present during the formation of holographic gratings
in these materials has become ever more crucial. We report on experimental results and theoretical analysis of the
diffusion rate of Erythrosine B, in a Polyvinylalcohol/Acrylamide layer.
The one-dimensional Non-local Photo-Polymerization Driven Diffusion (NPDD) model, which governs the temporal
evolution of holographic grating formation in photopolymers, has been further developed to include all major
photochemical processes. These effects include: i) non-steady state kinetics, ii) spatially non-local polymer chain
growth, iii) time varying photon absorption, iv) diffusion controlled effects, v) multiple termination mechanisms, vi)
inhibition, and vii) post-exposure or dark-reaction effects. The resulting analytic expressions for the monomer and
polymer concentrations are then derived and their validity tested against experimental data using a 4-harmonic,
numerical fitting regime. The temporal variation in the refractive index modulation is accounted for using the Lorentz-
Lorenz relation, and the effects of dark reactions for short holographic exposures are examined for a range of
photopolymer materials.
In order to further improve photopolymer materials for applications such as data storage, a deeper understanding of
the photochemical mechanisms which are present during the formation of holographic gratings has become ever
more crucial. This is especially true of the photoinitiation processes, since holographic data storage requires
multiple sequential short exposures. Previously, models describing for the temporal variation of the photosensitizer
concentration as a function of exposure have been presented and applied to two different types of photosensitizer,
which includes the effects of photosensitizer recovery and bleaching under certain conditions. In this paper, based
on a detailed study of the photochemical reactions, the previous model is improved to more closely represent these
physical effects in a more general fashion, enabling a more accurate description of the time varying absorption and
thus of the generation of primary radicals.
Photopolymers are promising as holographic recording media as they are inexpensive, versatile materials, which can be
made sensitive to a broad range of wavelengths. A deeper understanding of the processes, which occur during
holographic grating formation in photopolymers, is necessary in order to develop a fully comprehensive model, which
represents their behaviour. One of these processes is photo-initiation, whereby a photon is absorbed by a photosensitiser
producing free radicals, which can initiate free radical polymerisation. These free radicals can also participate in
polymer chain termination (primary termination) and it is therefore necessary to understand their generation in order to
predict the temporally varying kinetic effects present during holographic grating formation. In this paper, a study of the
photoinitiation mechanisms of Irgacure 784 dye, in an epoxy resin matrix, is carried out. This is achieved by analysing
the temporal evolution of a series of simultaneously captured experimental transmittance curves, captured at different
wavelengths, but at the same location, to enable the change in photon absorption during exposure to be estimated. We
report on the experimental results and present a theoretical model to predict the physically observed behaviour.
Photopolymer materials are practical materials for use as holographic recording media. In order to further develop such
materials, a deeper understanding of the photochemical mechanisms present during the formation of holographic
gratings in these materials has become ever more crucial. This is especially true of the photoinitiation process, which
has already received much attention in the literature. Typically the absorption mechanism varies with exposure time.
This has previously been investigated in association with several effects taking place during recording. Since
holographic data storage requires multiple short exposures, it is necessary to verify the temporal change in
photosensitizer concentration. Post exposure effects have also been discussed in the literature; however, they do not
include post exposure effects such as the photosensitizer recovery. In this paper we report experimental results and
theoretical analysis to examine the effects of the recovery and bleaching mechanisms which arise during exposure.
The one-dimensional diffusion equation, which governs the temporal evolution of holographic grating formation in
photopolymers, which includes the non-local material response, the generalized dependence of the rate of
polymerization on the absorbed illuminating intensity and the inclusion of our material's response to initiation and
inhibition effects has been previously studied and presented. The resulting analytic expressions for the monomer and
polymer concentrations have been derived and their validity tested against experimental data using a four-harmonic,
numerical fitting regime. In this paper we examine the spatial frequency response of our photopolymer material and
using our improved NPDD model we fit experimentally obtained data and extract estimates for material parameters.
We attempt to improve our material's spatial frequency response with the addition of chain transfer agents to reduce the
polymer chain length formed and the non-local chain-length variance. Achieving this should increase the locality of the
polymer chains and hence cause an improvement in the spatial frequency response of the material. It is a material's
response to high spatial frequencies, which determines a material's resolution and data storage density.
Photopolymer materials are practical materials for use as holographic recording media, as they are inexpensive and selfprocessing. By understanding the mechanisms present during recording in these materials their limitations for certain processes can be improved and a more efficient, environmentally stable material can be produced. Understanding the photochemical and photo-physical processes present during the formation of holographic gratings in photopolymer materials is crucial in enabling further development of holographic applications such as data storage, metrology, free space optical components etc. In order to achieve this, it is necessary to develop material electromagnetic theory, which models these applications. In this paper we begin by experimentally estimating parameters associated with absorption due to dye in the photopolymer. This information is needed when using Non-local Photo-Polymerization-Driven
Diffusion model (NPDD) to characterise such materials. Absorption also leads to the formation of non-uniform tapered grating structures. While the NPDD has been used to characterise materials recording slanted gratings problems have arisen in determining diffusion constants accurately. In order to deal with electromagnetic diffraction by the resulting non-uniform slanted grating structures we develop first order analytic expressions governing the replay of such gratings.
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