Full length articleStretching the sensitivity and stability of an acrylamide based photopolymer material with mixture of dyes
Introduction
Recently, photopolymers have grabbed much attention due to their application potential against other recording materials. Dry processing, ease of storing information, high diffraction efficiency, good spatial frequency response etc. make them fit for different holographic applications like data storage, information processing and display purposes. High refractive index modulation [1], [2], [3], good light sensitivity, real-time image development [3], [4], [5], high optical quality and low cost are the important properties of photopolymers enabling them to occupy an outstanding position as holographic recording media. One of the remarkable advantages of these photopolymers is the tunability of its spectral sensitivity or energy sensitivity by modifying the composition of different chemical constituents in the photopolymer solution [3], [6].
The development of new photopolymer materials for a technologically developing world is an inevitable necessity. Photopolymer usually consists of a polymeric binder, monomer, photoinitiator system, crosslinker, and sensitizing dye [7], [8]. The major difficulty in polymerization process is to design a highly efficient initiation system to improve the holographic properties of the material. The polymer binder acts as the supporting matrix encompassing the additive components. The monomer serves as the governing species to control the index modulation, induced by optical interference [7]. Initiation of polymerization starts when the dye absorbs incident light (here, interfering light) producing free radicals which in turn results in a spatially nonuniform monomer polymerization. The combination of nonuniform polymerization and free-monomer diffusion from destructive to constructive interference region creates a spatial modulation of the refractive index, which acts as the grating [9].
Along with other parameters, stability of the recorded information (storage life) and steadiness of the developed material (shelf life) are two important parameters in holography [10]. Many reports are available regarding the improvement of these parameters with modifications in the photopolymer compositions, like adding crosslinkers or metal ions like - Copper, Chromium, Silver, Nickel, Iron etc. by sacrificing or negotiating with energy sensitivity or other holographic properties [11], [12], [13], [14], [15], [16], [17], [18], [19].
Apart from the resistance to degradation on aging, response of an imaging media towards the entire visible region was always a question of interest from the emergence of such technologies. In photopolymers, extension of spectral response towards entire visible spectrum was usually done with a group of sensitizer additives such as dyes or metal ions with an inactive binder matrix [20], [21], [22], [23], [12], [13]. The selection of sensitizers was done without compromising holographic performance of the material such as high spectral sensitivity, good diffraction efficiency, good spatial frequency response, storage life of the recorded data and shelf life of the material, etc. [10], [24]. Quality of the developed films was ensured by optimizing the concentration of the sensitizers in the photopolymer solution. Incorporation of Silver and Nickel ions into Methylene blue (MB) sensitized poly (vinyl alcohol) films could improve the storage life and sensitivity of the developed films but longevityof the material was not much enhanced. The presence of metal ions could extend the absorption range of photopolymer samples and it was able to record gratings with blue, green and red wavelengths in these materials [12], [13], [23].
Tsuchida et al. [25] studied the stability of different sensitizing dyes and it was observed that among all the dyes studied, eosin (EY) was found to have better stability. Our study on eosin sensitized poly (vinyl alcohol)/acrylamide films also confirmed the same result by storing gratings for more than a year with good stability [10]. But shelf life of the samples got deteriorated after 5 months of storage. The main reasons may be the unwanted polymerization by stray light radiation, interaction with oxygen, inhibiting proper polymerization and also direct contact with environment [20], [21], [26], [27]. Fimia et al. studied the effect of mixture of dyes (MB and Rose Bengal) in the photopolymerization kinetics of Poly (vinyl alcohol)/acrylamide system and they could span the absorption spectra towards the entire visible range. Thus, in addition to the elimination of inhibition by oxygen, they could establish panchromaticity in photopolymer samples [20]. In 2010, Meka et al. developed PVA based panchromatic photopolymer for multicolour reflection holography using three dyes (MB, Erythrosin B, Acriflavine) to absorb in the entire visible region [28]. Huang et al. reported the EY and MB dye combination and used 632.8, 514.5, 496.5 and 488 nm lasers for holographic characterization [29]. They could achieve only a maximum DE of 55% even with a layer thickness of 250 µm using 632.8 nm wavelength and all other wavelengths showed DE below this value.
The present work portraits the behavior of eosin doped Poly (vinyl alcohol)/ Acrylamide (EPVA) material when another dye, methylene blue (MB) was incorporated into this photopolymer system. By the inclusion of MB to the EPVA system, it was possible to extend the absorption range towards red region with a sustained holographic performance on aging. Holographic characterization was done using 632.8, 532 and 488 nm wavelength lasers and it was seen that incorporation of a second dye enhanced the overall holographic performance of EPVA system.
Section snippets
Materials and methods
For the preparation of photopolymer mixture, a 10% poly (vinyl alcohol) (Mw = 1, 25000), solution was used. To the PVA solution, 0.38 M acrylamide (AA-monomer), 2.8 × 10−2 M triethanolamine (TEA-Co-initiator/electron donor) was added. To this solution, eosin and methylene blue were added at a dose of 3 × 10−5 M and 1.4 × 10−5 M respectively, which were optimum values of these dyes described previously [10], [24]. Concentrations of these dyes were varied and optimized according to the
Absorption studies
Absorption studies showed that addition of methylene blue to the EPVA has extended the absorption window to red region. In the previous reported work using eosin doped PVA, a maximum DE of 85% was obtained using 488 nm laser [10] and in methylene blue doped PVA [24], a maximum DE of 75% was obtained using 632.8 nm. For the present study, initially the same dye concentrations reported in the above references were used. Samples were prepared using eosin and methylene blue mixed (EM) polymer
Conclusions
Characterization of Eosin sensitized Poly (vinyl alcohol)/Acrylamide (EPVA) photopolymer material was done using both 488 nm and 532 nm. The material responded well with these two wavelengths and could record gratings with good DE at 50 mJ/cm2. Holographic exposure showed that, writing the gratings with wavelength near to the absorption maximum was not the only criterion for getting maximum DE, but the combination of photosensitizers played an important role in improving the holographic
Acknowledgements
The authors CSK and JJP would like to acknowledge Kerala State Council for Science Technology and Environment (KSCSTE) for providing fund for this work, through Emeritus Scientist Scheme. The author CSR would like to acknowledge University Grants Commission (UGC) for supporting the work through Rajiv Gandhi National Fellowship Scheme (RGNF).
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