Nanocomposites photonic materials are being actively studied for practical applications such as touch screen, wearable devices, optical sensors, photolithography, and neutron optics. For many of these applications, it is essential to fabricate embedded phase structures into media, in order to implement various properties for its practicality. High-contrast refractive-index changes with promising flexibility are usually desired for these applications. Photopolymers as an appealing candidate are attractive because they hold several advantages, such as low cost, ease of use, shape flexibility, large-area process ability, and self-development capability. In this work, we carried out single-wall nanotube doped polymer composites, which are based on acrylate-thiol-ene photopolymer material. It is shown that a substantial increase in refractive index modulation and diffraction efficiency is realized by doping both of BzO2 and single-wall nanotubes. Moreover, the incorporation of BzO2 lowers the optimum recording intensity to 0.25 mW/cm2. These results indicate that carbon nanotube-polymer composite provides effective method to fabricate flexible films with large-area holograms for wearable devices, display, and optical sensor uses.
It is well known that in standard diffaction experiments only the amplitudes of structural Fourier components are recovered but phase information is lost. This problem is known as the ’phase problem’ in crystallography. In this contribution, we point out how the phase problem of diffraction can be solved in some particular cases by employing multi-wave interference. In the experimental situation described here, we were able to determine the form of the refractive-index profile of a 1-D nanocomposite holographic grating by using a multi-wave coupling analysis of the measured angular dependence of the diffraction efficiencies for a number of diffraction orders.
KEYWORDS: Holography, Nanoparticles, Diffraction, Diffraction gratings, Composites, Polymers, Photopolymers, 3D displays, Refractive index, Chemical elements
Dry photopolymer materials are being actively studied for practical applications such as holographic data storage, 3D display, wearable displays with diffractive optical elements and so on. Their versatility, ease of use and self-processing ability give them many advantages over more traditional recording materials such as silver halide and dichromate gelatin for holographic uses. The necessity for development and optimization of such dry photopolymer diffraction elements with higher capability and stability has been recognized and they have recently received significant attention in the areas of holographic and wearable displays. In this work, we carried out nanoparticles composites, which are based on a SiO2 nanoparticle doped acrylate-thiol-ene photopolymer material, in order to fabricate holographic diffraction elements for 3D display uses. Preliminary examination on this material and fabrication techniques shows that a flexible free-standing volume grating with thickness of 200 μm and grating period of 1 μm was fabricated by using proposed nanoparticle composite material in the recording wavelength of 532 nm. The examination on thinner material layers with varied nanoparticles, such as in the range of 10 μm, is undergoing. We believe that such holographic diffraction elements offer a significant potential in display technique.
We investigate the spatial frequency response of a volume grating recorded in a ZrO2 nanoparticle-dispersed nanocomposite. We experimentally find that there exists the optimum recording intensity to maximize the saturated refractive index modulation amplitude of a nanocomposite grating recorded at short and long grating spacing. A strong parametric relationship between grating spacing and recording intensity is seen and an increase in the saturated refractive index modulation amplitude at shorter grating spacing (< 0.5 μm)can be obtained by using higher recording intensities than those at longer grating spacing. Such a trend can be qualitatively explained by a phenomenological model used for holographic polymer-dispersed liquid crystal gratings. We also describe another method for the improvement of the high spatial frequency response by co-doping of thiol monomer that acts as a chain-transfer agent.
We introduced two continuous-wave terahertz iterative phase-contrast imaging methods. In-line digital holography has the capability to reconstruct the amplitude and phase distributions simultaneously. It is a non-destructive, high-resolution, full-field dynamic phase-contrast imaging technique. Ptychography can reconstruct the complex amplitude distribution of the transmission object from the overlapped diffraction patterns. Both methods can achieve phase-contrast imaging, and are suitable for terahertz region. In this paper, both Gabor in-line holographic and ptychographical configurations are investigated from algorithms to experiments. For in-line holography, the use of extrapolation, synthetic aperture, sub-pixel shifting and multi-plane imaging are introduced to improve the resolution and reconstruction accuracy. For ptychography, we obtained the ptychographical reconstruction results of a polypropylene alphabet sample, which provides a new imaging method for terahertz phase-contrast imaging.
The photopolymerizable nanoparticle-polymer composites (NPCs) have thus far shown their excellent performance in practical applications, such as holographic data storage, nonlinear optics and neutron optics. We have demonstrated twofold enhancement of the saturated refractive index modulation (Δnsat) of ZrO2 NPC volume gratings recorded at high spatial frequencies by doping with a single functional thiol as a chain transfer agent (CTA). This result suggested that the incorporation of a CTA in an NPC is very useful for holographic applications of volume gratings in light and neutron optics. Such chemical modification of NPC volume gratings may be more effective by doping with multifunctional thiols. This is so because polymer features such as the molecular weight and the crosslinking network density can be tailored more diversely by introducing multifunctional thiols in photopolymers. The influences of varying functionalities of thiols as chain transfer agents on the thermal stability of a volume grating recorded in a photopolymerizable ZrO2 nanoparticle-polymer composite film have been investigated.
Photopolymerizable nanoparticle-polymer composites (NPCs) have thus far shown their excellent performance in various applications, such as holographic data storage, nonlinear optics and neutron optics. Specifically, for such applications, a high spatial frequency material response is necessary, as it is the response to high spatial frequencies that determines their spatial resolution and diffraction properties. However, it is known that the spatial frequency response of a recorded hologram in multi-component photopolymers including NPCs and holographic polymer-dispersed liquid crystals exhibits a reduction in refractive index modulation at high spatial frequencies. In order to overcome this drawback, an addition of chain transfer agents (CTAs) may be useful as done for all-organic photopolymers to modify their nonlocal response and phase separation characteristics. In our work, we investigate the effect of CTAs on the spatial frequency response in NPCs. Here we employ various chain-transfer agents with three different thiol groups in a photopolymerizable ZrO2 NPC film. A range of CTA concentration is carried out, in order to explore the most effective material combination used in the examination of spatial frequency response. The significant improvement in spatial frequency response of NPCs through the addition of a CTA with the most appropriate concentration is presented.
Based on the previous study of the time varying photon absorption effects, the behavior of four different photosensitizers
in an AA/PVA photopolymer material has been further examined by using the developed 1-D Nonlocal Photopolymerization
Driven Diffusion (NPDD) model. In order to characterize the photosensitizers precisely, holographic
illuminations with different spatial frequencies are applied. Material parameters, i.e., the nonlocal response parameter,
σ, the diffusion rate of monomer, Dm, the chain initiation kinetic constant, ki, and the termination rate, kt, are extracted by
numerically fitting experimentally measure the refractive index modulation growth curve using the theoretical models.
In this paper, the four different photosensitizers under investigation are Erythrosin B; Eosin Y; Phloxine B; Rose Bengal.
Phenanthrenequinone (PQ) doped poly(methyl methacrylate) (PMMA) photopolymer material has been studied
extensively due to the growing interest in application involving photopolymers. However, to progress the development
a more physical material model has become necessary. In this article, a kinetic model is developed, which includes: (i)
the time varying photon absorption, including the absorptivity of a second absorber, i.e., the singlet excited state of PQ,
(ii) the recovery/regeneration and the bleaching of the excited state PQ, (iii) the nonlocal effect, and (iv) the diffusion
effects of both the ground and excited state PQ molecules and of the methyl methacrylate (MMA). A set of rate
equations are derived, governing the temporal and spatial variations of each chemical component concentration. The
validity of the proposed model is examined by applying it to fit experimental data for PQ-PMMA layers containing three
different initial PQ concentrations, i.e., 1 mol.%, 2 mol.% and 3 mol.%. The effect of different exposure intensities is
also examined. Material parameters are extracted by numerically fitting experimentally measure normalized
transmission curves and the refractive index modulation growth curve using the theoretical models.
Phenanthreneauinone (PQ) doped poly(methyl methacrylate) (PMMA) photopoplymer material has been actively investigated in the literature. Based on the previously developed NPDD model and the analysis of the mechanisms, the behavior of the material is being further studied. The first harmonic refractive index modulation has been examined for both long time post-exposure and under thermal treatment. Twelve and four spatial concentration harmonics in the Fourier series expansions are applied respectively for comparison. Several effects, i.e., the non-local effect, the diffusion of both the ground state and excited states PQ molecules, which occur during and post-exposure in PQ-PMMA photopolymer materials, have been studied under thermal treatment. For long time post-exposure or when the heating treatment is applied, the formation of the photoproduct, PQ/PMMA, has become very important. The effects of nonlocality, diffusion and the different exposing intensities on the distribution of PQ/PMMA over space and higher harmonic PQ/PMMA concentration have been shown. The experimental results are presented, where no thermal treatment is applied.
In recent work carried out, we introduced the developments made to the Non-local Photo-polymerization Driven Diffusion model, and illustrate some of the useful trends, which the model predicts and then analyse their implications on photopolymer improvement. The model was improved in its physicality through the inclusion of viscosity effects (changes in fractional free volume), multiple components and their photo-kinetic and photo-physical behaviour, and free space vacuoles. In this paper, we further explore this model to provide a more rigorous and informed basis for predicting the behaviours of photopolymer materials in both photo-chemical and photo-physical sides. Such improvements include a) the analysis of the effects of viscosity on the refractive index modulation, b) the effects of the introduction of free space holes, e.g. the volumetric changes, and c) an examination of the effects of local temperatures and various concentration ratios to optimise material performance.
The photopolymer materials in Holographic Data Storage (HDS) have been increasingly studied due to their growing interest in applications. In this article we make use of the time varying parameters to study the behaviors of the photopolymer materials during exposure time. The nonlocal photo-polymerization driven diffusion (NPDD) model and electromagnetic theories of Maxell equations are combined in our model development. Moreover in this model, the theories of the material molecule polarization and the excited photosensitizer conductivity production are also introduced. The numerical simulation results in both cases of transmittance and diffraction efficiency are all analyzed. Several physical parameters and photochemical rate constant values are estimated by fitting the model predictions to the experimental results of AA/PVA material.
The Non-local Photo-Polymerization Driven Diffusion (NPDD) model was introduced to describe the observed drop-off
in the material's response for higher exposing spatial frequencies. Recent work carried out on the modeling of the
mechanisms which occur in photopolymers during- and post-exposure, has led to the development of a tool, which can
be used to predict the behaviour of these materials under a wide range of conditions. In this article, based on the
chemical reactions of chain transfer agents, we explore this extended NPDD model, illustrating some of the useful trends,
which the model predicts and we analyse their implications on the improvement of photopolymer material performance.
Several studies of the time varying photon absorption effects, which occur during the photo-initiation process in
photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) the dye absorption,
(ii) recovery, and (iii) bleaching. Based on an analysis of these mechanisms, the production of primary radicals can be
physically described and modelled. In free radical photo-polymerization systems, the excited dye molecules induce the
production of the primary radicals, R•, which is a key factor in determining how much monomer is polymerized. This, in
turn, is closely related to the refractive index modulation formed during holographic recording. In this article, we
overcome the complexicy of estimating the rate constant of intersystem crossing, kst, in going from the excited singlet
state dye to the excited triplet state dye, by introducing kaS and kaT into the model, which are the rate constant of photon
absorption from ground state to singlet state and triplet state respectively.
An understanding of the photochemical and photo-physical processes, which occur during photo-polymerization, is of
extreme importance when attempting to improve a photopolymer material's performance for a given application.
Recent work carried out on the modeling of photopolymers during- and post-exposure, has led to the development of a
tool, which can be used to predict the behavior of a number of photopolymers subject to a range of physical conditions.
In this paper, we explore the most recent developments made to the Non-local Photo-polymerization Driven Diffusion
model, and illustrate some of the useful trends, which the model predicts and then analyze their implications on
photopolymer improvement.
The Non-local Photo-Polymerization Driven Diffusion (NPDD) model indicates how a material's performance might be
improved, and provides a tool for quantitive comparison of different material compositions and to predict their
fundamental limits. In order to reduce the non-locality of polymer chain growth (i.e the non-local response parameter, σ)
and to improve the spatial frequency response of a photopolymer material, we introduce the chain transfer agent (CTA).
In the literature, extensive studies have been carried out on the improvements of the non-local response modifying by the
CTA, sodium formate, in the polyvinyl alcohol-acrylamide (PVA/AA) material. In this article, i) based on the chemical
reactions of CTA, we extended the CTA model in the literature; ii) we compare two different CTA materials, sodium
formate and 1-mercapto-2-propanol without cross-linker in order to obtain the experimental confirmation of the
reduction in the average polymer molecular weight is provided using a diffusion-based holographic technique; iii) we
examine the non-local responses of several spatial frequencies with the two CTAs. Using the extended CTA model it is
demonstrated that the CTA has the effect of decreasing the average length of the polyacrylamide (PA) chains formed,
thus reducing the non-local response parameter, especially, in the high spatial frequency case.
The use of theoretical models to represent the photochemical effects present during the formation of spatially and
temporally varying index structures in photopolymers, is critical in order to maximise a material's potential. One such
model is the Non-local Photo-Polymerization Driven Diffusion (NPDD) model. Upon application of appropriate
physical constraints for a given photopolymer material, this model can accurately quantify all major photochemical
processes. These include i) non-steady state kinetics, (ii) non-linearity iii) spatially non-local polymer chain growth, iv)
time varying primary radical production, v) diffusion controlled effects, vi) multiple termination mechanisms, vii)
inhibition, (viii) polymer diffusion and ix) post-exposure effects. In this paper, we examine a number of predictions
made by the NPDD model. The model is then applied to an acrylamide/polyvinylalcohol based photopolymer under
various recording conditions. The experimentally obtained results are then fit using the NPDD model and key material
parameters describing the material's performance are estimated. The ability to obtain such parameters facilitates
material optimisation for a given application.
Photopolymer materials exhibit good characteristics when used as holographic recording media. Extensive studies have been carried out on the behavior of the various chemical components in such materials, with photosensitizers in particular receiving much attention. In all previous analysis of photopolymer kinetics, the effects of photosensitiser diffusion have been neglected. For rapid sequential holographic recordings in photopolymers, for example, in an application such as holographic data storage, dye diffusion effects may become more pronounced. Therefore, we examine the dye diffusion effects of erythrosine B in an acrylamide/polyvinyl alcohol material. This is achieved using simple experimental techniques and a proposed theoretical model.
KEYWORDS: Molecules, Absorption, Photopolymers, Holography, Oxygen, Data modeling, Modulation, Molecular energy transfer, Optoelectronics, Refractive index
In the literature, several studies of the time varying photon absorption effects, which occur during the photo-initiation
process in photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) The
photon absorption, (ii) the regeneration or recovery of the photosensitizer, and (iii) the photosensitizer bleaching. Based
on the analysis of these mechanisms, the production of primary radicals can be physically described and modelled. In
free radical photo-polymerization systems, the excited dye molecules induce the production of the primary radical, R•, which is a key factor that in determining how much monomer is polymerized. This in turn is closely related to the
refractive index modulation, Δn, formed during holographic recording. In this article, by modifying the composition of a Polyvinylalcohol/Acrylamide (PVA/AA) based photopolymer material, i.e., excluding any co-initiator, the photo-kinetic
behaviour of the material is greatly simplified, an experimental study is performed, which makes possible development
and verification of a new model capable of accurately predicting the time varying concentration of primary radicals.
Photopolymer materials are practical materials for use as holographic recording media due to the fact that they are
inexpensive, self-processing materials with the ability to record low loss, highly diffraction efficient volume holographic
gratings. Extensive studies have been carried out on the behaviour of the various chemical components in such materials,
with photosensitizers in particular receiving much attention, as they are an important component in initialising the
photopolymerisation reaction. However in all such analyses dye diffusion is neglected. To further develop such
materials, a deeper understanding of behaviours the photosensitizer present during the formation of holographic gratings
in these materials has become ever more crucial. We report on experimental results and theoretical analysis of the
diffusion rate of Erythrosine B, in a Polyvinylalcohol/Acrylamide layer.
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