Michael R. Gleeson

Temporal analysis of grating formation in photopolymer using the nonlocal polymerization-driven diffusion model

The nonlocal polymerization-driven diffusion model (NPDD) has been shown to predict high spatial frequency cut-off in photopolymers and to accurately predict higher order grating components. We propose an extension to the NPDD model to account for the temporal response associated with polymer chain growth. An exponential response function is proposed to describe transient effects during the polymerization process. The extended model is then solved using a finite element technique and the nature of grating evolution examined in the case when illumination is stopped prior to the saturation of the grating recording process. Based on independently determined refractive index measurements we determine the temporal evolution of the refractive index modulation and the resulting diffraction efficiency using rigorous coupled wave theory. Material parameters are then extracted based on fits to experimental data for nonlinear and both ideal and non-ideal kinetic models.

Nonlocal photopolymerization kinetics including multiple termination mechanisms and dark reactions. Part I. Modeling

The photochemical processes present during free-radical-based holographic grating formation are examined. A kinetic model is presented, which includes, in a more nearly complete and physically realistic way, most of the major photochemical and nonlocal photopolymerization-driven diffusion effects. These effects include: (i) non-steady-state kinetics (ii) spatially and temporally nonlocal polymer chain growth (iii) time varying photon absorption (iv) diffusion controlled viscosity effects (v) multiple termination mechanisms, and (vi) inhibition. The convergence of the predictions of the resulting model is then examined. Comparisons with experimental results are carried out in Part II of this series of papers [J. Opt. Soc. Am. B26, 1746 (2009)].

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.

Nonlocal photopolymerization kinetics including multiple termination mechanisms and dark reactions. Part II. Experimental validation

In the first of this series of papers [J. Opt. Soc. Am. B26, 1736 (2009)], a new kinetic model, which includes most of the major photochemical and nonlocal photopolymerization driven diffusion effects, was proposed. Predictions made using the model were presented, and the numerical convergence of these simulations were examined when retaining higher-concentration harmonics. The validity and generality of the model is examined by applying it to fit experimental data for two different types of photopolymer material appearing in the literature. The first of these photopolymer materials involves an acrylamide monomer in a polyvinylalcohol matrix. The second is a more complex photopolymer in an epoxy resin matrix. Using the new model, key material parameters are extracted by numerically fitting experimentally obtained diffraction efficiency growth curves. The growth curves used include data captured both during exposure and post-exposure, allowing examination and analysis of “dark reactions.”

Monomer diffusion rates in photopolymer material. Part I. Low spatial frequency holographic gratings

For photopolymers, knowing the rate of diffusion of the active monomer is important when modeling the material evolution during recording in order to understand and optimize their performance. Unfortunately, a confusingly wide range of values have been reported in the literature. Re-examining these results, experiments are carried out for both coverplated (sealed) and uncoverplated material layers and the measurements are analyzed using appropriate models. In this way, a more detailed analysis of the diffraction processes taking place for large-period gratings is provided. These results, combined with those in Part II, provide unambiguous evidence that the monomer diffusion rate in a commonly used acrylamide polyvinyl alcohol-based material is of the order of 10 �10 cm 2 =s. This value closely agrees with the predictions of the nonlocal polymerization-driven diffusion model.

Comparison of a new self developing photopolymer with AA/PVA based photopolymer utilizing the NPDD model.

The development of suitable recording media for applications such as holographic optical elements and holographic data storage are of significant research and commercial interest. In this paper, a photopolymer material developed by Bayer MaterialScience is examined using various optical techniques and then characterised using the Non-local Photo-polymerization Driven Diffusion model. This material demonstrates the capabilities of a new class of photopolymer offering high index modulation, full colour recording, high light sensitivity and environmental stability. One key result of this study is the materials high spatial frequency resolution, indicating a very low non-local effect, thus qualifying it as a very good storage medium.

Non-local photo-polymerization kinetics including multiple termination mechanisms and dark reactions: Part III. Primary radical generation and inhibition

Photopolymers are playing an ever more important role in diverse areas of research such as holographic data storage, hybrid photonic circuits, and solitary waves. In each of these applications, the production of primary radicals is the driving force of the polymerization processes. Therefore an understanding of the production, removal, and scavenging processes of free radicals in a photopolymer system is crucial in determining a material’s response to a given exposure. One such scavenging process is inhibition. In this paper the non-local photo-polymerization driven diffusion model is extended to more accurately model the effects of (i) time varying primary radical production, (ii) the rate of removal of photosensitizer, and (iii) inhibition. The model is presented to specifically analyze the effects of inhibition, which occur most predominantly at the start of grating growth, and comparisons between theory and experiment are performed which quantify these effects.

A review of the modelling of free-radical photopolymerization in the formation of holographic gratings

In this paper a brief review of the theoretical modelling of free-radical photopolymerization, which has been discussed in the literature, is presented. Particular attention is given to identifying the key contributions of each work and the common assumptions made in the derivation of each model. The overview enables the recognition of outstanding issues, which have not yet been incorporated into a fully comprehensive theoretical model of the complex photochemical processes that occur during photopolymerization.

Examination of the photoinitiation processes in photopolymer materials

Holographic data storage requires multiple sequential short exposures. However, the complete exposure schedule may not necessarily occur over a short time interval. Therefore, knowledge of the temporally varying absorptive effects of photopolymer materials becomes an important factor. In this paper, the time varying absorptive effects of an acrylamide/polyvinylalcohol photopolymer material are examined. These effects are divided into three main photochemical processes, which following identification, are theoretically and experimentally examined. These processes are (i) photon absorption, (ii) photosensitizer recovery, and (iii) photosensitizer bleaching.

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.