Vincent Toal

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 an acrylamide-based dry photopolymer holographic recording material

Recent work on an acrylamide-based photopolymer holographic recording medium is presented. The lifetime of recorded gratings is improved by the addition of cross-linking monomers. Shelf life is also improved. The effects of various constituents of the photosensitive material are studied to determine an optimum composition, and five xanthene dyes are compared as possible sensitizers for the system. With the most sensitive dye and optimum concentrations of the other constituents an improved formulation is presented with high sensitivity and very high diffraction efficiency. The new formulation also works well in reflection mode.

Investigation of the diffusion processes in a self-processing acrylamide-based photopolymer system

Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental research on optimization of the high-spatial-frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive-index modulation responsible for the diffraction grating buildup. Monomer diffusion from dark to bright fringe areas increases the refractive-index modulation. It is characterized with diffusion constant D0 = 1.6 x 10(-7) cm2/s. A second diffusion process takes place during the recording. It decreases the refractive-index modulation and we ascribe it to diffusion of short-chain polymer molecules or radicals from bright to dark fringe areas. The estimated diffusion coefficient for this process is D0 = 6.35 x 10(-10) cm2/s. The presence of the second process could be responsible for the poor high-spatial-frequency response of the investigated photopolymer system. Comparison with the diffusion in photopolymer systems known for their good response at high spatial frequencies shows that both investigated diffusion processes occur in a much faster time scale.

A visual indication of environmental humidity using a color changing hologram recorded in a self-developing photopolymer

A reflection hologram for visual indication of environmental humidity has been studied. The hologram is recorded in a self-developing photopolymer and changes color when exposed to a change in humidity and is fully reversible. The range of color change, reversibility, and the response time of the hologram have been studied in a controlled humidity environment. Fully reversible holograms with response times from few seconds to tens of minutes have been designed. Extremely sensitive bright visual humidity indicators, capable of dramatic color change within a few seconds of breathing on them are demonstrated.

Photopolymerizable nanocomposites for holographic recording and sensor application.

Novel nanocomposites consisting of a water-soluble acrylamide-based photopolymer and colloidal zeolite nanoparticles of zeolite Beta and zeolite A were prepared. The interactions between the photopolymer components and zeolite nanoparticles in the photopolymerizable nanocomposites were characterized for the first time by (13)C nuclear magnetic resonance and visible spectroscopy. It was found that the zeolite Beta nanoparticles (up to 5 wt. %) behave as a noninert additive, resulting in an effective increase in layer thickness, which causes doubling of the diffraction efficiency of the nanocomposite in comparison to that of the undoped photopolymer. On the other hand, the nanocomposite containing zeolite A nanoparticles showed no evidence of interaction with the polymer matrix, had similar values of diffraction efficiency, and--up to a small addition of nanoparticles (up to 2.5 wt. %)--showed slightly higher light-induced refractive index modulation of the grating when compared to the undoped photopolymer. The good optical compatibility between the zeolite nanoparticles and the polymer allows a versatile design of photopolymerizable nanocomposites with different properties by selecting the adequate type of zeolite. The nanocomposite containing zeolite Beta nanoparticles demonstrates selective sensing behavior toward toluene and can be coated in either glass or plastic substrates and exposed directly to the environmental conditions.

Two-way diffusion model for short-exposure holographic grating formation in acrylamide- based photopolymer

A theoretical model for formation of a short-exposure holographic grating is presented. The model accounts for both monomer and polymer diffusion and distinguishes between short polymer chains capable of diffusing and long polymer chains that are immobile. It is shown that the experimentally observed decrease of diffraction efficiency at higher spatial frequency can be predicted by assuming diffusion of short-chain polymers away from the bright fringes. The time evolution of the refractive-index modulation after a short exposure is calculated and compared with experimental results. The effects of diffusion coefficients, polymerization rates, intensity, and spatial frequency of recording on the properties of weak diffraction gratings are investigated by numerical simulations.

Characterization of an acrylamide-based photopolymer for data storage utilizing holographic angular multiplexing

An acrylamide-based photopolymer formulated in the Centre for Industrial and Engineering Optics has been investigated with a view to further optimization for holographic data storage. Series of 18–30 gratings were angularly multiplexed in a volume of photopolymer layer at a spatial frequency of 1500 lines mm−1. Since the photopolymer is a saturable material, an exposure scheduling method was used to exploit the entire dynamic range of the material and allow equal strength holographic gratings to be recorded. This investigation yielded the photopolymer M/# for moderately thin layers. Photopolymer temporal stability was also studied by measuring variations of material shrinkage, Bragg selectivity curve, and diffraction efficiency.

White light interferometric surface profiler

We describe an optical system for 3-D profilometry based on the white light interferometer. Recently many different methods have been used to analyze the data obtained from white light interferometric profilers. Many commercially available white light profilers are also in use today. We detail a simple way to construct a profiler that uses two simple and efficient algorithms. It deals with the data in a fast and simple man- ner, thus reducing both the acquisition and analysis time. The system has a theoretically unlimited range and can profile both optically rough and smooth surfaces.

Holographic patterning of acrylamide-based photopolymer surface

The patterning of an acrylamide-based photopolymer surface by holographic recording is studied. The patterns are induced by light alone and no post-processing is required. Periodic surface modulation is observed in addition to a volume phase grating. An investigation has been carried out using white light interferometry into the dependence of the amplitude of the photo induced surface relief modulation on the spatial frequency, intensity of recording and sample thickness. The observed dependencies indicate that the diffusion of material during the holographic recording plays a major role in surface relief formation. The possibility for inscription of surface relief patterns opens the door to at least two new applications for this photopolymer: fabrication of diffractive optical elements and biosensors.

Holographic recording in nanoparticle-doped photopolymer

A nanoparticle-doped acrylic photopolymer is characterised as a material for holographic recording. The influence of nanoparticles on the photopolymer dynamic range, dynamics of recording, temporal stability and mechanical stability in terms of shrinkage has been studied. The dynamics of recording and the temporal stability are investigated by real time monitoring of the build up of diffraction gratings of spatial frequencies of 200 to 2000 1/mm. The shrinkage has been characterised by recording slanted transmission gratings and observation of the change in the Bragg angle.