Holographic recording at shorter wavelengths enables to capture holograms with a greater resolution. Photopolymer
material sensitisation to a blue or violet wavelength might require replacement of photosensitive dye or whole
photosensitiser system which leads to different photoinitiation kinetics. There are known photoinitiator systems which
have high values of key photoinitiation parameters, e.g., molar absorption coefficient at a broad range of wavelengths,
quantum yield etc. An example of such photosensitiser is an organometallic titanocene, Irgacure 784. However Irgacure
784 in an epoxy resin photopolymer undergoes a complex photo-kinetics which is neither fully understood nor
quantified. This complex photo-kinetics results in different bleaching evolution when using green and blue exposing
light. The aim of this paper is to identify relevant photo-kinetic reactions taking place during exposure and driving the
bleaching process. For this purpose photopolymer layers of four material compositions containing Irgacure 784 were
prepared and exposed for nine exposure times. Absorbance spectrum was measured before after each exposure. We
report on our experimental results and draw conclusions identifying relevant reactions of the Irgacure 784 photo-kinetics
in epoxy resin photopolymers.
Despite the physical significance of the slanted holographic gratings, most materials research presented in literature
involves the use of the unslanted recording geometry. A physically accurate electromagnetic model of the slanted
holographic non-uniform gratings recorded in photopolymers is necessary in order to extract key material parameters. In
this paper we present derivation of a model based on a set of two coupled differential equations, which include the
effects of: (i) An exponential decay of refractive index modulation in the direction of the beam propagation due to the
variation of absorption with depth; (ii) Gaussian profile of refractive index modulation due to recording by finite
Gaussian beams profile, and (iii) A quadratic variation in the spatial period of the grating (i.e. chirp). The model is
applied to fit experimental data, i.e. angular scans, of unslanted gratings recorded in Polyvinylalcohol/Acrylamide
material for different slant angles in order to extract key volume grating parameters.
Photopolymer materials are practical materials for use as holographic recording media as they are cheap and maintain
high diffraction efficiencies at low noise level. Applications such as holographic data storage require large thickness in
order to enable outstanding performance and store many pages of information in small angular steps recorded within the
same volume. Such holographic gratings can be recorded by rotating the material sample peristrophically with respect to
the recording beams or by altering one or both the incident angles of the recording beams. This results in gratings, which
in general having a slanted geometry. Despite the physical significance of the slanted holographic gratings, most of the
research presented in literature is based on the simplified unslanted recording geometry. A physically accurate electromagnetic
representation of the slanted holographic gratings recorded in photopolymers is necessary in order to extract
key material parameters. In this paper we present a model based on a set of two coupled differential equations, which
include the effects of: (i) An exponential decay of refractive index modulation in the direction of the beam propagation
due to the variation of absorption with depth; and (ii) A linear variation in the spatial period of the grating (i.e. chirp).
Numerical and approximate analytical solutions of this model are found. The model is applied to analyze the
experimental data in order to extract key volume grating and photopolymer material parameters.
Various photopolymer materials have recently found a significant number of useful applications in microelectronics and
the PC board industries. Some of these materials have also become attractive optical recording materials for the
recording of holographic devices such as diffractive optical elements and gratings, or as data storage media, for the
fabrication of optical waveguides and photonic processing structures. Ever increasing requirements, driven by
application developments, has lead to the rapid development of newer generations of such materials. As the ever
increasing number of new materials is developed and used, there is a need to characterize the material behavior pre-and
post exposure. In order to produce materials with a desired set of material properties one has to understand the
photochemical processes present during recording. Although in most case emphasis is placed on studying the high
spatial frequency response and the related limitations of such materials, the low spatial frequency response
characteristics can also supply useful information regarding the processes taking place during grating formation. In this
paper we present the experimental results obtained following a detailed examination of the low spatial frequency
response of a photopolymer material in the case of exposure at different recording intensities. The time dependence of
the diffraction efficiency of the grating must be then analyzed using the appropriate diffraction theory of phase gratings.
Furthermore the results of examining the angular scans of the resulting grating diffraction efficiency are presented in
order to specify the condition of the diffraction regime (e.g. thin, thick) for such low spatial frequency gratings.
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
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.
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 self processing.
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. In order to achieve
this, it is necessary to develop material electromagnetic theory, which models these applications. 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.
Photopolymer materials are practical materials for use as holographic recording media, as they are inexpensive
and self-processing. 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. In this paper we briefly review the application of photopolymer materials in the area of holographic
data storage. In particular we discuss the recent development of the Non-local Polymerisation Driven Diffusion
Model, (NPDD) including absorption and inhibition, and analysis of the photochemical effects present during
the evolution of holographic grating formation. The inclusion of these effects allows a more accurate
understanding of the photo-polymerisation process.
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.