Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm^?2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm^?2.     

Multilayer holographic recording using a two-color-absorption photopolymer

We developed a sensitive two-color-absorption photopolymer in which holograms are recorded by simultaneous irradiation with a 660 nm interference light and a 410 nm gate light. Doped with bis(silyl)pentathiophene as a two-color-photosensitive dye and 2,2-dimethoxy-2-phenylacetophenone as a radical photopolymerization initiator; its matrix contains low-refractive index binding polymers and high-refractive index monomers. The sensitivity and diffraction efficiency of 25 mum thick layers are from 1.2 x 10(-9) to 3.7 x 10(-9) cm2/mJ and from 1% to 4%. We made a three-photopolymer-layer waveguide structure, where each photopolymer layer and high-refractive index adhesive layer serves as a core layer and is sandwiched between two low-refractive index glass substrates that serve as clad layers. Gate light propagated through the adhesive layers, reference and object beams intersected the photopolymer layers, and different diffraction patterns could be written in each layer.     

Analysis and simulations of two-wave mixing in photopolymer holographic recording media

Abstract

The process of two-wave mixing in photopolymer recording materials was investigated theoretically. The diffraction grating already forms during exposition and it may influence the original interference field distribution through diffraction of waves on the refractive index modulation. In order to show this, Kogelnik's coupled wave theory was extended to demonstrate the possibility of energy transfer from one recording wave to the other. The energy transfer and the intensity distribution during the recording process were systematically analysed depending on the boundary conditions. As a next step, the first harmonic model of the transmission grating recording, based on a simple material model, was implemented and solved. The ratio of the input intensities was found to be a crucial parameter and thus extensive simulations for various ratios of intensities were carried out. Modelling implies that the interference field and the refractive index grating just coincide for equal intensities. For intensities differing from unity they do not overlap themselves during the recording process. It has also turned out that the diffraction efficiency of the recorded grating drops against the case where the effects of two-wave mixing are not considered. The results of our analysis and simulation help give a better understanding of the physics of the recording process and proper adjustment of recording parameters in such applications as optical holography and holographic memories.     

High efficiency panchromatic photopolymer recording material for holographic data storage systems

Studies carried out to gauge the potential of a metal-ion doped panchromatic photopolymer recording material for application in real-time holographic data storage is presented. The photopolymer films are spin coated on glass plates to ensure better surface uniformity. Volume holographic transmission gratings with peak diffraction efficiency of 80% could be stored in the photopolymer films of 100 μm thickness. An efficiency of 70% is achievable even for gratings recorded with exposure energy as low as 10 mJ/cm². A checkerboard pattern data page recorded in the photopolymer film using a defocused 4-f recording geometry could be reconstructed with good image quality. The experimental results illustrate the competency of the developed photopolymer for holographic data storage applications.     

Comparison of holographic photopolymer materials by use of analytic nonlocal diffusion models

The one-dimensional diffusion equation governing holographic grating formation in photopolymers, which includes both nonlocal material response and generalized dependence of the rate of polymerization on the illuminating intensity, has been previously solved under the two-harmonic expansion assumption. The resulting analytic expressions for the monomer and polymer concentrations have been derived and their ranges of validity tested in comparison with the more accurate numerical four-harmonic case. We used these analytic expressions to carry out a study of experimental results presented in the literature over a 30-year period. Automatic fitting of the data with these formulas allows material parameters, including the nonlocal chain-length variance sigma, to be estimated. In this way, (i) a quantitative comparison of different materials can be made, and (ii) a standard form of experimental result presentation is proposed to facilitate such a procedure.     

The shift of Bragg angular selectivity curve in darkness in glass-like photopolymer for holographic recording

The shift of Bragg angular selectivity curve after exposure was studied by recording single hologram and multiplexed holograms, respectively. The evolution of shift with probe time after recording, as well as the saturated shift with exposure energy, was investigated in detail. The mechanism of Bragg mismatch was analyzed. By optimizing parameters of material synthesis and holographic recording, the shift of Bragg matched angle was alleviated in PQ/PMMA photopolymer after hologram recording.     

Optimization of holographic storage with modulated recording beams in a thick polyvinyl alcohol/acrylamide photopolymer

In a holographic photopolymer system, the storage properties were often limited due to the attenuation in depth of light during the recording step. To obtain smaller values of the depth attenuation profiles in 1 mm thick polyvinyl alcohol/acrylamide (PVA/acrylamide) photopolymers, we used a triangle prism, sitting one face tilted at 13.7 degrees to the axis within the focus of a lens, to modulate the distribution of recording beams. Doing this permitted larger refractive index modulation depth to be achieved, and larger dynamic range (M#=9.2) was obtained in the PVA/acrylamide photopolymers.     

Dynamics of hologram recording in DuPont photopolymer

Several dynamical aspects of the DuPont photopolymer film HRF-150-38 for holographic storage are described. We study temporal aspects of exposure, exposure time, processing situations, and storage effect. The quantities studied are diffraction efficiencies, thickness changes, and Bragg angle. The experimental results are performed with an argon-ion laser at 514.5 nm.     

Narrow-bandwidth holographic reflection filters with photopolymer films

Narrow-bandwidth holographic reflection filters are demonstrated that use volume gratings in 100-mum-thick photopolymer films. A full width at half-maximum of 0.09 nm can be achieved with ~35% peak reflectance near the 900-nm region. Detailed fabrication procedures and filter performances are described.     

Improvement of photopolymer materials for holographic data storage

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. In general these materials absorb light of an appropriate wavelength, causing photo-polymerization of the local monomer, inducing a change in the material’s refractive index. These small changes in refractive index enable the storage of large quantities of data using holographic techniques. In an attempt to further develop the data storage capacity and quality of the information stored, i.e., resolution, in such materials, a deeper understanding of the photochemical mechanisms present during the formation of holographic gratings has become ever more crucial. From this understanding the response of an acrylamide/polyvinylalcohol based photopolymer to high spatial frequency information is improved through the addition of a chain transfer agent, i.e., sodium formate, HCOONa.     

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.     

Characterization of dupont photopolymer: determination of kinetic parameters in a diffusion model

We investigate the recording dynamics of Omnidex photopolymer film from DuPont. We use a reviewed version of the diffusion model proposed by Zhao and Mouroulis [J. Mod. Opt. 41, 1929 (1994)] in order to describe the recording response that combined photopolymerization and free-monomer diffusion process. Two different experiments are detailed that lead to the determination of material kinetic parameters. These values are introduced in the numerical model to provide quantitative simulations of a grating formation under various holographic exposures. Theoretical results are experimentally checked as a validation of the model. We extend its applications to several secondary investigations, such as volume-shrinkage influence on refractive-index distribution and spectral selectivity of reflection gratings. This study improves the understanding of the recording process and consequently allows to build more accurate holographic components in this material to be built.     

Characterization of the DuPont photopolymer for three-dimensional holographic storage

DuPont's HRF-150 photopolymer film is investigated for use in three-dimensional holographic memories. Measurements of sensitivity, hologram persistence, the lateral spread of the photoinitiated reaction, and the variation of diffraction efficiency with modulation depth, spatial frequency and tilt angle, and intensity are reported. We observed that the diffraction efficiency of the HRF-150 photopolymer for a given exposure decreases with increases in intensity and grating tilt angle. The holograms were nondestructively reconstructed for long periods of time at room temperature. The photoinitiated reaction spread less than 100 μm over a period of 16 h.     

Optical response of photopolymer materials for holographic data storage applications

We briefly review the application of photopolymer recording materials in the area of holographic data storage. In particular we discuss the recent development of the Non-local Polymerisation Driven Diffusion model. Applying this model we develop simple first-order analytic expressions describing the spatial frequency response of photopolymer materials. The assumptions made in the derivation of these formulae are described and their ranges of validity are examined. The effects of particular physical parameters of a photopolymer on the material response are discussed.     

Model of holographic recording in thermoplastic materials

A method for the evaluation of images reconstructed from holograms recorded in thermoplastic materials is reported. The method is based on the use of the experimental modulation transfer function and nonlinear holographic characteristics of the recording material. Calculations have been carried out for high-numerical-aperture holograms of a five-element Ronchi ruling. The quality of the reconstructed image as a function of the recording parameters has been computed. The model predicts that it is possible to optimize holographic recording in these materials.     

Electronic speckle pattern shearing interferometer with a photopolymer holographic grating

A photopolymer holographic grating is used to produce the two sheared images in an electronic speckle pattern shearing interferometer. A ground glass screen following the grating eliminates unwanted diffraction orders and removes the requirement for the CCD camera to resolve the diffraction grating's pitch. The sheared images on the ground glass are further imaged onto the CCD camera. The fringe pattern contrast was estimated to be above 90%. A validation of the system was done by comparing the theoretical phase difference distribution with the experimental data from the three-point bending test.     

Efficient holographic recording in novel azo-containing polymer

We report on periodic structures inscribed through holographic recording by both intensity light patterns and polarization patterns in novel azo-containing polymer poly[oxyethylene({6-[4-(biphenyl-4-azo)phenoxy]hexyl}imino) ethylene-carbonylimino(2-methyl-1,3-phenylene)iminocarbonyl] abbreviated as A-TDI. The kinetic of grating recording and diffraction efficiency of recorded gratings using polarized cw Ar⁺ laser light have been measured for s–s, p–p and s–p polarization configurations. The translation grating technique applied during the light self-diffraction process allowed for investigations of phase and amplitude gratings contributions to light diffraction in the studied polymer. Full reversibility of polarization (s–p) grating recording and observation of a half-period structure development during optical erasure process makes this material a good candidate for further studies as a potential material for dynamic holography purposes.     

High-density recording in photopolymer-based holographic three-dimensional disks

The performance specifications of a holographic three-dimensional disk system are experimentally characterized. A surface density of 10 bits/μm(2) is experimentally demonstrated with a 100-μm-thick photopolymer as the recording medium.     

Holographic characteristics of a 1-mm-thick photopolymer to be used in holographic memories

Poly(vinyl alcohol-acrylamide) photopolymers are materials of interest in the field of digital information storage (holographic memories). We analyzed the behavior of a 1-mm-thick photopolymer. Using a standard holographic setup, we recorded unslanted diffraction gratings. The material has high angular selectivity (0.4 degrees), good sensitivity (88 mJ/cm2), and small losses caused by absorption and scattering of light. It also has a high maximum diffraction efficiency (70%). A significant induction period was seen in the material. The authors hypothesize that, during most of this induction period, polymerization does in fact take place but is not reflected in the appearance of the diffracted light until a certain threshold value of exposure is reached.     

Application of a photopolymer to a holographic reflector for reflective liquid-crystal displays

A photopolymer for fabrication of volume-type transmission holograms, which are used primarily on the holographic reflector for reflective LCD's, is reported. This photopolymer consists of bisphenol-type epoxy resin and an acrylic monomer with diaryliodonium salt and 3-ketocoumarin as a complex initiator. The chemistry of the imaging formation is based on the radical polymerization of the monomer initiated by a holographic exposure, followed by the cationic polymerization of epoxy resin by UV exposure after postexposure baking. The yellowish color of the hologram derivatives that resulted from the sensitizing dye and from the photospeed were improved to satisfy the specifications for the holographic reflector. The holographic reflector now gives brighter images.