Fabrication and characterization of methylene-blue-doped polyvinyl alcohol-polyacrylic acid blend for holographic recording

A methylene-blue-sensitized polymer blend of polyvinyl alcohol and polyacrylic acid is fabricated and tested for holographic recording. It was found to have good characteristics such as high sensitivity, storage stability, ease of fabrication, and environmental stability. Optimization of the ratio of polyvinyl alcohol/polyacrylic acid, the sensitizer concentration, pH, energy, diffraction efficiency measurements, etc., have been done. pH is found to have a great influence on the recovery of the dye in this matrix. The results of experimental investigations into the properties of this new material are reported.     

Rewritable optical-disk fabrication with an optical recording material made of naphthalocyanine and polythiophene

A novel rewritable optical disk that uses an optical recording material made of naphthalocyanine and polythiophene as the recording layer is described. In this system the complex refractive index of the recording material changes reversibly, depending on the aggregation states of naphthalocyanine induced by a conformation change of the polythiophene matrix. After recording, the imaginary part of the refractive index of the recording material increases to three times that of the unrecorded part, at a wavelength of 790 nm at which a semiconductor laser emits light. The fabricated polythiophene naphthalocyanine optical disk shows a high reflectivity of 58% and a large readout modulation amplitude (I(11)/I(top)) of 0.63. It is confirmed that the polythiophene/naphthalocyanine optical disk can be played back on conventional compact-disc players even after ten cycles of rewriting.     

Photochromic diarylethene for polarization holographic optical recording

Photochromic diarylethene, 1,2-bis[2-methyl-5-(3-fluorophenyl)-3-thienyl] perfluorocylcopentene (1a), was synthesized. The compound showed good photochromic reactions both in solution and in PMMA matrix by photo-irradiation. Using the diarylethene 1b/PMMA film as recording medium and a He-Ne laser for recording and readout, four types of polarization holographic optical recording were accomplished for the first time. The results show that the orthogonal circular polarization recording is the best method for holographic optical recording when the target photochromic diarylethene is used as recording material.     

Holographic recording characteristics of an acrylamide-based photopolymer

Recent research on an acrylamide-based photopolymerizable holographic recording material is presented. The recording characteristics of the material are discussed in detail in terms of sensitivity, diffraction efficiency, recording linearity, resolution limit, and sources of noise. Although the resolution is not sufficient for reflection holography, the recording characteristics are excellent for transmission gratings. The material was found to suffer no shrinkage during recording, and high-diffraction-efficiency slanted gratings were made. Finally, the suitability of this self-developing material to both double-exposure and real-time holographic interferometry is demonstrated.     

Formation of the interphase in organic-matrix composites

This paper addresses attempts made to determine the mechanical and chemical properties of material within a micrometre of the surface of a single carbon fibre in an epoxy/amine matrix. Work reported earlier has shown that deformation data near a fibre can be interpreted as showing that the modulus of this material could be lower than that of the bulk matrix for a distance of about 0.5 μm from the fibre. Spectral evidence suggests that the material in this region is chemically different from material remote from the interface. Analysis of the variation of fibre debond forces in a composite as a function of the fibre content also suggests that the matrix is softer near the fibre. The present paper addresses studies done to determine a mechanism for the variation of mechanical and chemical properties over such a distance. Possibilities considered include modification of resin stoichiometry in the region by preferential absorption of active species, formation of a zone of incomplete polymerization caused by entropic effects related to the two-dimensional nature of the interface, and modification of the resin solidification mechanism in the region of the fibre.     

Polymer-coated metal and alloy particles for magnetic recording

Fine particles of cobalt and an iron-cobalt alloy were prepared and encapsulated in a plastic film to protect them from oxidation. The magnetic properties of the coated powders were determined and compared with the properties of coated iron powders studied previously. The thermal degradation of magnetic properties was also investigated. Polymer-coated iron and cobalt powders were incorporated in a Hypalon rubber matrix to obtain additional magnetic property data and to study recording characteristics. The experimental materials exhibited better high-frequency response in magnetic recording than a standard γ-Fe2O3material, and one material had a higher signal output as well.     

Modified holographic recording setup for photopolymerizable/flexible recording materials

An improvement in the conventional holographic recording setup has been done for liquid (photopolymerizable) or flexible recording materials. The new vertical recording setup that comprises a horizontal substrate holder permits holographic recording on such materials without positioning them between two substrates, as has been done until now. This setup can be conveniently used for the fabrication of transmission, reflection, Fourier transform, and computer-generated holograms with few simple modifications as indicated.     

Performance analysis of content-addressable search and bit-error rate characteristics of a defocused volume holographic data storage system

One of the methods for smoothing the high intensity dc peak in the Fourier spectrum for reducing the reconstruction error in a Fourier transform volume holographic data storage system is to record holograms some distance away from or in front of the Fourier plane. We present the results of our investigation on the performance of such a defocused holographic data storage system in terms of bit-error rate and content search capability. We have evaluated the relevant recording geometry through numerical simulation, by obtaining the intensity distribution at the output detector plane. This has been done by studying the bit-error rate and the content search capability as a function of the aperture size and position of the recording material away from the Fourier plane.     

Combining Maxwell’s methodology with the BEM for evaluating the two-dimensional effective properties of composite and micro-cracked materials

Maxwell’s methodology is combined with the boundary element method (BEM) for evaluating the two-dimensional effective elastic properties of composite, porous, and microcracked isotropic materials with periodic or random structure. The approach is based on the idea that the effective properties of the material can be deduced from the effects that a cluster of fibers, pores, or cracks embedded in an infinite matrix has on the far-fields. The fibers, pores, or cracks can have arbitrary shapes, sizes, and elastic properties, provided that the overall behavior is isotropic, and their effects are assumed to be the same as those of an equivalent circular inhomogeneity. The key aspect of the approach is to precisely account for the interactions between all the constituents in the cluster that represent the material in question. This is done by using the complex-variables version of the BEM to solve the problem of a finite cluster of fibers, pores or cracks embedded an infinite isotropic, linearly elastic matrix. The effective properties of the material are evaluated by comparing the far-field solutions for the cluster with that of the equivalent inhomogeneity. It is shown that the model adequately captures the influence of the micro-structure of the material on its overall properties.     

A nano-indentation study on the mechanical behaviour of the matrix material in an AA6061 - Al2O3 MMC

The nano-indentation technique is a suitable technique to measure hardness and elastic moduli profiles of AA6061 reinforced with Al₂O₃ particles, since it allows measurements of mechanical properties on a micrometer range. To investigate possible local variations in mechanical behaviour of the matrix material due to precipitation reactions being affected by the presence of ceramic reinforcements, nano-indentation tests were done on both metal matrix composite (MMC) as well as unreinforced reference material, in three different heat treatment conditions. Matrix response depends on heat treatment condition, but is approximately equal for the MMC and the base reference alloy. Due to the various imposed heat treatments, magnesium enrichment around the ceramic particles was observed, but hardness and elastic modulus of this interfacial layer could not be measured. To confirm the preferential segregation of Mg near the particle/matrix interface, linescans were made with a Scanning Electron Microscope (SEM) equipped with EDS (Energy Dispersive Spectrum) facilities. The limited width of the Mg rich zone explains the absence of typical 'interphase' indentations in this investigation. Hardly any differences in calculated elastic moduli and hardness values were found for the three heat treatment conditions investigated, when comparing results of AA6061 reference material with results of an AA6061 matrix in an MMC. This result is of great importance when modelling the mechanical behaviour of MMCs using the finite element method, since it permits the assumption that the MMC matrix material behaves similar to the same aluminium alloy without ceramic reinforcements.     

Multimaterial Extrusion-Based Additive Manufacturing of Compliant Crack Arrester: Influence of Interlayer Length,Thickness, and Applied Strain Rate

Additive manufacturing is a useful tool for fabricating complex multimaterial structures. Compliant interlayers (ILs) can easily be introduced into stiff materials to increase toughness and stop or impede crack growth, as is done in nature. The aim herein is to analyze the influence of varying IL lengths and thicknesses on crack propagation in 3D-printed polymers at different loading rates. A glycol-modified poly(ethylene terephthalate) is used as a matrix material, while a thermoplastic elastomer on a copolyester basis serves as an compliant IL. Specimen fabrication is done with a fused filament fabrication 3D printer equipped with a multimaterial unit, which allows to print a component composed of several materials within one print. Additively manufactured Charpy samples are tested in three-point bending at loading rates between 0.1 mm min⁻¹ and 3.8 m s⁻¹. The thickness of the IL almost shows no effect on energy absorption as long as remaining in the same printing orientation and loading rate. For varying IL lengths, a constant fight between crack penetration and crack deflection occurs. At low loading rates, the IL acts as a defect. As the loading rate increases, the total absorbed energy of composites increases compared with the pure matrix material.     

Model of noise-grating selectivity in volume holographic recording materials by use of Monte Carlo simulations

A model describing the angular selectivity of noise gratings in volume holographic recording materials is presented. The noise grating is treated as an ensemble of superimposed, statistically distributed planar gratings. Rigorous coupled-wave analysis is used to treat reconstruction with various polarization states. The model accounts for material properties such as thickness change, absorption, and the angular distribution of scattered light within the recording medium. Results show good agreement with noise gratings that are experimentally formed in a thick cationic ring-opening photopolymer material.     

Three-dimensional optical data storage in a fluorescent dye-doped photopolymer

We propose a new, to our knowledge, monolithic multilayer optical storage medium in which data may be stored through the diffusional redistribution of fluorescent molecules within a polymer host. The active portion of the medium consists of a photopolymer doped with a fluorescent dye that is polymerized at the focal point of a high-numerical-aperture lens. We believe that as fluorescent molecules bond to the polymer matrix they become more highly concentrated in the polymerized regions, resulting in the modulated data pattern. Since data readout is based on detection of fluorescence rather than index modulation as in other photopolymer-based memories, the problems of media shrinkage and optical scatter are of less concern. An intensity threshold observed in the recording response of this material due to the presence of inhibitor molecules in the photopolymer allows for the three-dimensional confinement of recorded bits and therefore multilayer recording. The nonlinear recording characteristics of this material were investigated through a simple model of photopolymerization and diffusion and verified experimentally. Both single-layer and multilayer recordings were demonstrated.     

光聚合诱导相分离型光学记录材料的制备

制备了一种含有特殊光聚合单体、聚醚、交联剂和光引发剂的新型光学记录材料,该材料具有光聚合诱导相分离的特性,经紫外曝光后能呈现透过率较低的浑浊态.在掩膜下对该材料进行紫外曝光、显影后可以实现对掩膜图案的光学记录,并详细探讨了影响记录性能、收缩率以及稳定性的因素.该光学记录材料的出现将为光学记录提供新的发展空间.     

Optimization of pH and direct imaging conditions of complexed methylene blue sensitized poly(vinyl chloride) films

Significant results of our experimental investigations on the dependence of pH on real time transmission characteristics on recording media fabricated by doping PVC with complexed methylene blue are presented. The optimum pH value for faster bleaching was found to be 4.5. In typical applications, the illumination from one side, normal to the surface of this material, initiates a chemical sequence that records the incident light pattern in the polymer. Thus direct imaging can be successfully done on this sample. The recorded letters were very legible with good contrast and no scattering centres. Diffraction efficiency measurements were also carried out on this material.     

Neural signal recording using microelectrode arrays fabricated on liquid crystal polymer material

Microelectrode arrays have been developed for simultaneous multi-channel recordings from nervous systems, typically using silicon substrates. However, it has been known that it is difficult to meet the biocompatibility and durability requirements using silicon and other dielectric materials (SiO₂, Si₃N₄), due to environmental moisture and ions. Additional disadvantage of silicon being the rigid material makes it hard to apply these materials in chronic recording situations.Liquid Crystal Polymer (LCP) was recently introduced as a candidate material for electronic packaging purposes. The material acts as efficient barrier against ions and moisture, a desirable feature for a substrate material of microelectrode arrays. In this paper, we report on the neural recording performed using the LCP-based microelectrode arrays.The cell adhesion on the new material was compared very favorably with that using silicon, SiO₂, or polyimide material. The microelectrode arrays were patterned with Ti (500 Å)/Au (3500 Å) on the LCP film and were employed in both stimulation and recording from rat sciatic nerve. The electrical characteristic of the recorded signal was as good as those using other substrate materials, proving this material as an excellent candidate for next generation microelectrode arrays.     

Iterative focus detection in hologram tomography

Hologram tomography is a two-step method for three-dimensional topometry of extended objects. The first step consists of the hologram recording with a single laser pulse of 35 ns duration and storage in a photosensitive material. In the second step the hologram is optically reconstructed and digitized, which leads to a set of two-dimensional projections at different axial positions. A maximization of a focus measure has to be performed to extract the surface position out of the projections. Unlike with well-established methods, where the comparison of sharpness values is done parallel to the optical axis, we propose an iterative solution to perform the maximization along the direction of image formation, which is evaluated for each surface point individually. This leads to a better reproducibility of the surface in the off-axis regions.     

High‐Density Stretchable Electrode Grids for Chronic Neural Recording

Electrical interfacing with neural tissue is key to advancing diagnosis and therapies for neurological disorders, as well as providing detailed information about neural signals. A challenge for creating long‐term stable interfaces between electronics and neural tissue is the huge mechanical mismatch between the systems. So far, materials and fabrication processes have restricted the development of soft electrode grids able to combine high performance, long‐term stability, and high electrode density, aspects all essential for neural interfacing. Here, this challenge is addressed by developing a soft, high‐density, stretchable electrode grid based on an inert, high‐performance composite material comprising gold‐coated titanium dioxide nanowires embedded in a silicone matrix. The developed grid can resolve high spatiotemporal neural signals from the surface of the cortex in freely moving rats with stable neural recording quality and preserved electrode signal coherence during 3 months of implantation. Due to its flexible and stretchable nature, it is possible to minimize the size of the craniotomy required for placement, further reducing the level of invasiveness. The material and device technology presented herein have potential for a wide range of emerging biomedical applications.     

Synthetic aperture superresolution with multiple off-axis holograms

An optical setup to achieve superresolution in microscopy using holographic recording is presented. The technique is based on off-axis illumination of the object and a simple optical image processing stage after the imaging system for the interferometric recording process. The superresolution effect can be obtained either in one step by combining a spatial multiplexing process and an incoherent addition of different holograms or it can be implemented sequentially. Each hologram holds the information of each different frequency bandpass of the object spectrum. We have optically implemented the approach for a low-numerical-aperture commercial microscope objective. The system is simple and robust because the holographic interferometric recording setup is done after the imaging lens.     

Fourier transforms — An alternative to finite elements for elastic-plastic stress-strain analyses of heterogeneous materials

Summary The intent of this paper is to apply the technique of discrete Fourier transforms (DFT) to the computation of the stress and strain fields around holes in an externally loaded two-dimensional representative volume element (RVE). This is done to show that DFT is capable to handle geometries with rather sharp corners as well as steep gradients in material properties which is of importance for modeling changes in micro-morphology. To this end DFT is first briefly reviewed. In a second step it is applied to the appropriate equations which characterize a linear-elastic as well as a time-independent elastic-plastic, heterogeneous material subjected to external loads. The equivalent inclusion technique is used to derive a functional equation which, in principle, allows to compute numerically the stresses and strains within an RVE that contains heterogeneities of arbitrary shape and arbitrary stiffness (in comparison to the surrounding matrix). This functional equation is finally specialized to the case of circular and elliptical holes of various slenderness which degenerate into Griffith cracks in the limit of a vanishing minor axis. The numerically predicted stresses and strains are compared to analytical solutions for problems of the Kirsch type (a hole in an large plate subjected to tension at infinity) as well as to finite element studies (for the case of time-independent elastic/plastic material behavior).