Dusan Sabol

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

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.

Modeling the photochemical effects present during holographic grating formation in photopolymer materials

The development of a theoretical model of the processes present during the formation of a holographic grating in photopolymer materials is crucial in enabling further development of holographic applications. To achieve this, it is necessary to understand the photochemical and photophysical processes involved and to isolate their effects, enabling each to be modeled accurately. While photopolymer materials are practical materials for use as holographic recording media, understanding the recording mechanisms will allow their limitations for certain processes to be overcome. In this paper we report generalizations of the nonlocal polymer driven diffusion (NPDD) model to include the effects of photosensitive dye absorption and the inhibition effects.

Photoinitiation study of Irgacure 784 in an epoxy resin photopolymer

A deeper understanding of the processes, which occur during free radical photopolymerization, is necessary in order to develop a fully comprehensive model, which represents their behavior during exposure. One of these processes is photoinitiation, whereby a photon is absorbed by a photosensitizer producing free radicals, which can initiate polymerization. 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 photosensitizer, in an epoxy resin matrix, is presented. We report our experimental results and present a theoretical model to predict the physically observed behavior.

Extended model of the photoinitiation mechanisms in 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 the temporal variation in the photosensitizer (dye) concentration as a function of exposure have been presented and applied to two different types of photosensitizer, i.e., Methylene Blue and Erythrosine B, in a polyvinyl alcohol/acrylamide based photopolymer. These models include the effects of photosensitizer recovery and bleaching under certain limiting conditions. In this paper, based on a detailed study of the photochemical reactions, the previous models are further developed to more physically represent these effects. This enables a more accurate description of the time varying dye absorption, recovery, and bleaching, and therefore of the generation of primary radicals in photopolymers containing such dyes.

Extracting parameters from slanted non-uniform gratings recorded in photopolymer

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.

Low spatial frequency grating recorded in photopolymer material

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.

Photokinetic study of Irgacure 784 dye in an epoxy resin photopolymer

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.

Parameter Extraction from Gratings Recorded in Photopolymer

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.

Improved model of the photo-initiation mechanisms in 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.

Spectral study of irgacure 784 dye in an epoxy resin photopolymer

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.