Feidhlim T. O’Neill

Adjusted intensity nonlocal diffusion model of photopolymer grating formation

Diffusion-based models of grating formation in photopolymers have been proposed in which the rate of monomer polymerization (removal) is directly proportional to the illuminating intensity inside the medium. However, based on photochemical considerations, the rate of polymerization is proportional in the steady state to the square root of the interference intensity. Recently it was shown that, by introducing a nonlocal response function into the one-dimensional diffusion equation that governs holographic grating formation in photopolymers, one can deduce both high-frequency and low-frequency cutoffs in the spatial-frequency response of photopolymer materials. Here the first-order nonlocal coupled diffusion equations are derived for the case of a general relationship between the rate of polymerization and the exposing intensity. Assuming a two-harmonic monomer expansion, the resultant analytic solutions are then used to fit experimental growth curves for gratings fabricated with different spatial frequencies. Various material parameters, including monomer diffusion constant D and nonlocal variance σ, are estimated.

Photopolymer holographic recording material parameter estimation using a nonlocal diffusion based model

It has recently been shown [Sheridan and Lawrence, J. Opt. Soc. Am. A 17, 1108 (2000)] that by introducing a nonlocal response function into the one-dimensional diffusion equation governing holographic grating formation in photopolymers, both high frequency and low frequency cutoffs in the spatial frequency response of photopolymer materials can be deduced. Starting with this result, analytic solutions to this equation are derived assuming a two harmonic expansion of the monomer concentration. These expressions are first compared to the more rigorous four harmonic numerical results, indicating the range of physical parameters for which they are accurate. They are then compared to the experimental diffraction intensity growth curves produced during the formation of sinusoidal gratings recorded with different spatial frequencies. Fitting these curves using the analytic expressions, various material parameters, including the monomer diffusion constant, D, and the nonlocal variance, σ, can be estimated. Trend...

Cellular and transcriptomic analysis of human mesenchymal stem cell response to plasma-activated hydroxyapatite coating

Atmospheric pressure plasma has recently emerged as a technique with a promising future in the medical field. In this work we used the technique as a post-deposition modification process as a means to activate hydroxyapatite (HA) coatings. Contact angle goniometry, optical profilometry, scanning electron microscopy morphology imaging and X-ray photoelectron spectroscopy analysis demonstrate that surface wettability is improved after treatment, without inducing any concomitant damage to the coating. The protein adsorption pattern has been found to be preferable for MSC, and this may result in greater cell attachment and adhesion to plasma-activated HA than to untreated samples. Cell cycle distribution analysis using flow cytometry reveals a faster transition from G(1) to S phase, thus leading to a faster cell proliferation rate on plasma-activated HA. This indicates that the improvement in surface wettability independently enhances cell attachment and cell proliferation, which is possibly mediated by FAK phosphorylation. Pathway-specific polymerase chain reaction arrays revealed that wettability has a substantial influence on gene expression during osteogenic differentiation of human MSC. Plasma-activated HA tends to enhance this process by systemically deregulating multiple genes. In addition, the majority of these deregulated genes had been appropriately translated, as confirmed by ELISA protein quantification. Lastly, alizarin red staining showed that plasma-activated HA is capable of improving mineralization for up to 3 weeks of in vitro culture. It was concluded from this study that atmospheric pressure plasma is a potent tool for modifying the biological function of a material without causing thermal damage, such that adhesion molecules and drugs might be deposited on the original coating to improve performance.

Optimized holographic data storage: diffusion and randomization

The control of the exposure patterns, which maximizes the capacity of a holographic recording medium, is of critical importance in any holographic data storage system. In this paper, we develop a method to theoretically explore linear photopolymer storage media in which a polymerization driven monomer diffusion process takes place. Using this method we examine a technique, involving randomly shifting the exposure pattern between exposures, to increase material capacity. The randomization acts to reduce the effects of past exposures by reducing the monomer concentration spatial distribution which arises due to those exposures. Following a detailed description of a series of assumptions and approximations, we derive an analytic formula, which allows us to explore the results of applying the technique. It is shown that, in the particular situation discussed, randomization can provide a useful tool in reducing constraints on the relaxation times necessary between exposures.

Examination of the temporal and kinetic effects in acrylamide based photopolymer using the nonlocal polymer driven diffusion model (NPDD)

The Nonlocal Polymer Driven Diffusion (NPDD) model successfully predicts high spatial frequency cut-off and higher harmonic generation, experimentally evident in holographic gratings recorded in free radical chain photopolymer materials. In this paper the NPDD model is extended to include a nonlocal material temporal response. Previously it was assumed that following a brief transient period, the spatial effect of chain growth was instantaneous. However, where the use of short exposures is necessary, as in optical data storage, temporal effects become more significant. Assuming that the effect of past chain initiations will have less effect on monomer concentration at a later point in time than current initiations, a normalized exponential function is proposed to describe the process. The extended diffusion model is then solved using a Finite-Difference Time-Domain technique to predict the evolution of the monomer and polymer concentrations during and after grating recording. The Lorentz-Lorenz relation is used to determine the corresponding refractive index modulation and The Rigorous Coupled Wave Method applied to determine and/or process diffraction efficiencies. A fitting technique is then used which first solves the diffusion model as described and determines a set of parameters which give best fits to the experimental data. Results show that the inclusion of the nonlocal temporal response is necessary to accurately describe grating evolution for short exposures i.e. continued polymer chain growth for some period after recording resulting in an increase in the refractive index modulation. Monomer diffusion is also shown to influence refractive index modulation post-exposure. Monomer diffusion rates determined to be of the order of D ~ 10-11 cm2/s and the time constant of the nonlocal material temporal response function being of the order of τn ~ 10-2s.

Automatic Computation of Crossing Point Numbers Within Orthogonal Interpolation Line-Graphs

The recording of atmospheric pressure plasmas (APP) electro-acoustic emission data has been developed as a plasma metrology tool over the last couple of years. In this work low moment analysis of acoustic time-series data is examined for structure complexity (in terms crossing points) within 2- and 3-dimensional time-series line graph datasets for the purpose of plasma control. A theoretical analysis of the structural complexity analysis is given, and the embedding algorithms obtained are implemented using LabVIEW™ for the purpose of real-time plasma control. The software uses a threshold segmentation process to map high contrast images of the line-graphs into binary (red and black) images that maps particles (red) directly to the number of crossing points within the cluster. It is found that single and bimodal cluster systems can be analyzed within the pixelation error limit. The approach taken here is generic and may be transferred to other (non-acoustic) datasets.