By employing low molecular-weight polyvinyl alcohol (PVA) as binder, the spatial resolution of a red-sensitive PVA/acrylamide based photopolymer are improved from 1000 lines/mm to 3000 lines/mm. By increasing the ambient temperature during the holographic recording, the photosensitivity of photopolymer is also increased about 5 times. The optimized photopolymer system has high capacity such as high photosensitivity (8 mJ/cm2), high spatial resolution (over 3000 lines/mm) and high diffraction efficiency (over 94%). To our knowledge, its holographic recording performance is the best of ever reported PVA/acrylamide based photopolymer systems. It has good application prospects in real-time holographic interferometry, holographic storage and holographic display.
© 2010 OSA
In recent years, the PVA/acrylamide based photopolymer system has drawn much research interests because of its good properties such as real-time dry imaging, adjustable spectral response range, high refractive index modulation, good angular selectivity, easy preparation of large-area photosensitive film, and low cost, etc [1–5]. Moreover, its water-soluble recipe is much less toxic compared with oil-soluble systems [6–8]. Some applications of holographic data storage, holographic interferometry, holographic display, etc., also have been reported by using PVA/acrylamide based photopolymers [2,4,9–13]. But its spatial resolution (usually 1000~2000 lines/mm) and photosensitivity (60-150mJ/cm2) are not high [1–5], it is difficult to obtain high diffraction efficiency at high spatial frequency over 2000 lines/mm, which will limit its wide applications in reflection holography, high-density holographic storage and real-time holographic interferometry.
In this paper, based on our previous research work [12,13], some new physical strategies and methods such as employing lower molecular weight binder and increasing the ambient recording temperature, are reported to effectively improve the spatial resolution, photosensitivity and diffraction efficiency of the PVA/Acrylamide based photopolymer system, its physical mechanism of performance enhancement are also discussed in this paper.
2. Preparation of photopolymer
The photosensitive layer of our red-sensitive photopolymer consists of acrylamide as primary monomer, methylene bisacrylamide and acrylic acid as supplementary monomers and promoters, methylene blue dye as sensitizer, triethanolamine as photoiniator and electron donor, PVA with different molecular weight as binder. The above reagents are all dissolved in deionized water to prepare a photosensitive solution, the experimentally optimized concentrations are given in Table 1 . By pouring certain amount of photosensitive solution over leveled glass plates, the photopolymer plates are then dried for two days in the dark under normal laboratory conditions (room temperature 20~25°C, relative humidity 50~70%).
3. Improvement of spatial resolution
In order to study the effects of the molecular weight of binder on the spatial resolution and diffraction efficiency, some photosensitive plates with binder of different molecular weight (9000, 15000 and 72000) was prepared. The thickness of dry photosensitive film, measured with contact-type thickness meter with a 1μm measurement precision,was found to be 60-65 μm. The holographic recording characteristics of photopolymer materials were studied by recording unslanted volume transmission holographic gratings with two collimated He-Ne laser beams, the intensity of the collimated beams is 1mW/cm2, and different spatial frequency was obtained by changing the interference angle of two recording laser beams.
Figures 1(a) (b)(c) showed the diffraction efficiencies of holographic gratings with different spatial frequency as a function of exposure for different molecular weight of binder. For a low spatial frequency of 1200 lines/mm in Fig. 1(a), one can find that the photopolymer plate with a binder of 72000 molecular weight cannot achieve a peak diffraction efficiency higher than 30%, and its photosensitivity 120 mJ/cm2 is also not high (the required exposure for achieving maximum diffraction efficiency is defined here for the evaluation of photosensitivity). But the photopolymer plate with molecular weight of 15000 or 9000 can achieve much higher peak diffraction efficiency (up to 90%) and higher photosensitivity (about 80 and 50 mJ/cm2 respectively for molecular weights of 15000 and 9000).
When the spatial frequency increased to more than 2000 lines/mm, the diffraction efficiency of holographic grating of 72000 Molecular weight was very low, so we did not give the results in Figs. 1(b) and (c). For spatial frequency of 2000 lines/mm in Fig. 1(b), the photopolymer plates with Molecular weight of 15000 and 9000 can both obtain high peak diffraction efficiency up to 90%, and they had almost the same photosensitivity (about 60 mJ/cm2). So these two photopolymer plates both had spatial resolution higher than 2000 lines/mm.
For a high spatial frequency of 3000 lines/mm in Fig. 1(c), the photopolymer plate with Molecular weight of 9000 can get much higher peak diffraction efficiency than that with molecular weight of 15000, and the photosensitivity of the former (about 60 mJ/cm2) was also much higher than the photosensitivity of the latter (about 120 mJ/cm2). So it is obvious that the photopolymer system with binder of 9000 molecular weight has higher spatial resolution, photosensitivity and diffraction efficiency than that with molecular weight of 15000 or 72000.
It can be concluded that the molecular weight of binder surely had a great effect on the spatial resolution, photosensitivity and diffraction efficiency of photopolymer system, with the decrease in molecular weight from 72000, 15000 to 9000, the spatial resolution of photopolymer was obviously improved from about 1000 lines/mm, 2000 lines/mm, till to higher than 3000 lines/mm, and the photosensitivity also steadily increased.
4. Improvement of photosensitivity
A physical method of increasing the ambient recording temperature is proposed in our laboratory to improve the photosensitivity of photopolymer. As mentioned above, the photopolymer with binder of 9000 molecular weight has the best performance, so all the following experiments will use the photopolymer based on binder with 9000 molecular weight, the thickness of dry film is 60µm. The setup used to record holographic gratings at different ambient temperature is shown in Fig. 2 . The recording wavelength is 632.8nm, the ratio of the two recording beams is 1:1, and a simple heating device is designed to control the ambient temperature during holographic recording. As shown in the bottom right side of Fig. 2, four electrothermal rods symmetrically surround the photopolymer plate to provide a uniform heating, by changing the electronic current applied to the electrothermal rods, the different ambient temperature is obtained. After the thermal equilibrium is reached, the ambient temperature around the photopolymer plate is homogeneous.
Figure 3(a) gives the diffraction efficiencies of 1000 lines/mm holographic gratings as a function of exposure for different ambient temperature. It can be seen that at 20°C one needs 40mJ/cm2 of exposure to obtain maximum diffraction efficiency 94%, but at 40°C and 60°C one only needs about 8 mJ/cm2 of exposure to obtain maximum diffraction efficiency 94%. The experimental results show that higher ambient recording temperature can increase the photosensitivity of photopolymer about 5 times.
From Fig. 3(a), one can find that the holographic grating recorded at 60°C has slightly lower photosensitivity and diffraction efficiency than that recorded at 40°C, so we don’t give results at 60°C in Figs. 3(b)and (c). Figures 3(b)(c) shows the relationship curves between the diffraction efficiencies and exposure of 2000 and 3000 lines/mm holographic gratings at different ambient recording temperature. For high spatial frequencies, we can also obtain the same conclusion that the photosensitivity at higher ambient temperature is improved about 5 times. Furthermore, from Figs. 3(a)(b)(c), one can find that almost the same maximum diffraction efficiency can be obtained at different ambient temperature and spatial frequency, which shows that the increase in ambient recording temperature had no obviously negative effect on the diffraction efficiency and spatial resolution of the photopolymer system.
5. Analysis and discussions
From above experiments, one can find that the spatial resolution, photosensitivity and diffraction efficiency of the PVA/Acrylamide based photopolymer system are effectively improved by employing lower molecular weight binder and increasing the ambient recording temperature. Its holographic recording characteristics are compared with other typical PVA/acrylamide based photopolymer systems in Table 2 , it can be seen that our system has the best performance, which is similar to Du Pont’s commercial photopolymers [7,8].
Figure 4 gives the schematic photopolymerization process of photopolymer system. When photopolymer is irradiated by light, the photon energy is transferred to the photoinitiator to produce primary free radicals, the primary free radicals bind to monomer molecules to form secondary free radicals. In the following steps, these secondary free radicals continuously react with monomers to grow a large polymer chain, so the refractive index in the exposure region (bright region) increases. With the depletion of monomer molecules in the exposure region, the monomer concentration gradients between the bright region and dark region will lead to the diffusion of monomers from dark region to the bright region to take part in the photopolymerization reaction, which will result in the further increase in the refractive index in the exposure region. Finally the stable phase grating with refractive index modulation is formed.
From the point of view of photopolymerization process, the physical mechanism of photosensitivity and spatial resolution enhancement by changing ambient recording temperature and molecular weight of binder is discussed preliminarily as follows.
(1) PVA with low molecular weight can decrease the viscosity of photosensitive film, this will make the diffusion of monomers from dark region to bright region much easier and faster. The increase in diffusion speed will improve the photosensitivity. Besides, the more monomers are transferred into bright region to take part in the photopolymerization reaction, this will help to increase the refractive index modulation and diffraction efficiency.
(2) The increase in ambient temperature will also increase the diffusion speed of monomers in photosensitive film, so higher temperature means that more monomers are transferred from dark region to bright region during a certain period of time. As a result, the photosensitivity will be improved. But much higher ambient temperature will result in the decrease in diffraction efficiency, and the value of favorable ambient temperature may vary with the ingredient of photopolymer.
The photosensitivity, spatial resolution and diffraction efficiency of PVA/acrylamide based photopolymer system were successfully improved by using new strategies of employing low molecular-weight binder and increasing the ambient recording temperature. as far as we know, its holographic performance, such as high spatial resolution (over 3000 lines/mm), high photosensitivity (8 mJ/cm2) and high diffraction efficiency (over 94%), is the best of ever reported PVA/acrylamide based photopolymer systems. it can be expected to have practical applications in real-time holographic interferometry, high-density 3-D holographic storage, and holographic display, etc.
This work was supported by the National Natural Science Foundation of China under Grant No. 60678044 and J0830308, and partially supported by the Open Foundation of Key Laboratory of Functional Crystals and Laser Technology, TIPC, Chinese Academy of Sciences.
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