We demonstrate theoretically and experimentally that the light can be self-focused and self-trapped in a self-written optical waveguide in a bulk acrylamide/polyvinyl alcohol (AA/PVA) solid photopolymer material volume. The manufacture method, i.e., how to prepare the AA/PVA photopolymer material is detailed. In our experimental observation the refractive index changes induced are permanent. The resulting optical waveguide channel has good physical stability and can be integrated with optoelectronic devices as part of integrated optical systems. The theoretical model developed predicts the formation/evolution of the observed self-written waveguides inside the bulk material. The model involves appropriately discretizing and then numerically solving the paraxial wave equation in Fourier space and the material equation in time space.
When the large thickness is used as the holographic storage materials, a non-ignorable problem is the light intensity attenuation in depth due to high absorptive of the dye. For this reason more completely modeling the evolutions inside the material is necessary to consider into the developed standard kinetic model. In this paper the photo-polymerization processes during the large thickness holographic grating formation are analyzed. A 3-dimensional algorithm is present by deriving the system partial differential rate equations governing each associated chemical species, and using the finite difference approximation, these equations can be solved numerically. This extended model describes the time varying behaviors of the non-uniform photo-physical and the photochemical evolutions in photopolymer materials. In this model both dye molecules consumption and light energy absorption are calculated time varyingly, and then the polymer and monomer concentrations distributions are obtained. Applying the Lorenz-Lorenz relationship, the non-uniform grating formatted in material depth, and its refractive index, which is distorted from ideal sinusoidal spatial distribution, can be more accurately predicted.
Dyes often act as the photoinitiator PI/photosensitizer PS in photopolymer materials and are therefore of significant interest. The properties of the PI/PS used strongly influences grating formation when the material layer is exposed holographically. In this paper, the ability of a recently synthesized dye, D_1, to sensitise an acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer is examined and the material performance is characterised using an extended Non-local Photopolymerization Driven Diffusion (NPDD) model. Electron Spin Resonance Spin Trapping experiments (ESR-ST) are also carried out to characterize the generation of the initiator/primary radical, R•, during exposure. The results obtained are then compared to those for the corresponding situation when using a Xanthene dye, i.e., Erythrosin B (EB), under the same experiment conditions. The results indicate that the non-local effect is greater when this new photosensitizer is used in the material. Analysis indicates that this is the case because of the dyes (D_1) weak absorptivity and the resulting slow rate of primary radical production.
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.
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.
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.
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.
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.