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Holographic properties of doped Si-MFI nanoparticles ‘thick’ photopolymer layers
Optics Communications 309 (2013) 114–120
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Holographic properties of doped Si-MFI nanoparticles ‘thick’ photopolymer layers Mohammad S. Mahmud a,n, Qiaohuan Cheng a,b, Bepari Mohammed Sohel c, Mohammad M. Kamal d, Badsha Alam a, Elsa Leite e a
Biophotonics and Bioengineering Laboratory, Ryerson University, Toronto, M5B 2K3, Canada School of Material Science and Engineering, Henan University of Technology, Henan, China c Intel mobile communications (IMC), Munich, Germany d Department of Burn and Plastic Surgery, Dhaka Medical College, Bangladesh e INOV-Inesec Inovacao, Rua Alves Redol, 9-1000-029, Lisbon, Portugal b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2011 Received in revised form 23 April 2013 Accepted 23 June 2013 Available online 5 July 2013
Behavior of diffraction gratings recorded in thick (1-mm) acrylamide-based photopolymer layers doped with Si-MFI Zeolite nanoparticles (1.9% w/V) was studied. The gratings were recorded at a spatial frequency of 1000 lines/mm at three different values of absorbance (0.10, 0.18 and 0.37). A modified method was developed for the preparation of l-mm thick layers with uniform layer thickness and at lower surface roughness. By measuring the diffraction efficiency growth and studying the diffraction pattern, the influence of nanoparticles (Si-MFI) doped thick volume holographic gratings was analyzed. For all above absorbances, Si-MFI nanocomposite doped 1-mm thick layers showed higher diffraction efficiency (DE) than the undoped thick layers. High-contrast image and DE was observed for the absorbance of 0.18 in 1-mm thick Si-MFI nanocomposite layers. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Thick photopolymer Si-MFI nanoparticles Volume gratings Michelson image contrast Diffraction efficiency Noise gratings
1. Introduction Nanoparticles-doped photopolymer layers are promising materials for developing new holographic sensors [1,2] and for holographic data storage applications [3–6]. Most of these studies are usually performed in thin photopolymer layers for recording and displaying purposes. Thick photopolymer layers (4500 mm) are essential for applications such as high capacity holographic data storage media [7–11] and for phase coded multiplexing techniques (PCM), where a complete overlap of object and reference beams needs to occur inside the layers. The application of photopolymers as a recording material has been restricted in recording thick volume gratings mainly due to limitations of layer thickness and material shrinkage effects during and after recording [12–14]. In volume data storage applications, the material shrinkage problem is significant, as it can change the fringe spacing. This can cause the change of the reconstruction angle and as a result no light is diffracted at the expected reconstruction angle and ultimately the stored data page cannot be recovered. Layers with different compositions and thicknesses and the characteristics of recorded
n
Corresponding author. Tel.: +1 6478854428. E-mail addresses: [email protected], [email protected] (M.S. Mahmud).
holograms in thick layers have been reported elsewhere [15–18]. Recording in thick photopolymer layers showed two major drawbacks: the difference between the effective optical thickness and the physical thickness in the material increases with the increasing layer thickness [19], and the scattering and absorption losses increase due to the existence of noise gratings which is formed due to scattering from inhomogeneities in the recording material and has been explained elsewhere [17,20]. To improve the holographic performance of gratings recorded in photopolymer, one of the successful approaches is to introduce inorganic nanoparticles with different refractive index of the photopolymer. The presence of nanoparticles in photopolymer layers contributes to an overall increase in the dynamic range [3,5,21–26] with improved stability [27–29] and/or the suppression of polymerization shrinkage effects [6,30]. Studies of different types of zeolite nanoparticles (inorganic porous material) incorporated into acrylamide based thin photopolymer layers suggested that zeolite Si-MFI nanoparticles type were responsible for suppressing the shrinkage to a greater extent [6,31]. The behavior of Si-MFI nanoparticles doped acrylamide-based thin ( o200 mm) photo-polymer and their holographic recording gratings were studied elsewhere [1,2,6]. However, thick photopolymer layers offer the opportunity to be used in volume grating data storage application. For this purpose, here we studied the behavior of Si-MFI nanoparticles doped acrylamide-based thick
0030-4018/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.06.033
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photopolymer layers. The compositions of the composites were changed and we showed a modified method of preparing the thick layers. Results were compared at three different values of absorbance (0.10, 0.18 and 0.37). The expected improvement shown in thick layers (reduced shrinkage and better DE) were also tested by comparing the holographic performance namely DE and different image contrast. The diffraction efficiency (DE) is defined here as the ratio of the first order diffracted beam intensity (ID) and the incident beam intensity (I0) of the probe beam, and expressed as a percentage.
2. Theory The acrylamide based photopolymer is a self-developing dry layer, consisting of monomers, an electron donor or initiator, a photosensitizer and a polymer binder acting as a matrix in which other components are dispersed. Holographic recording in this material is based on photo-polymerization reactions in the areas illuminated by laser light of appropriate wavelength. Briefly, the dye molecules absorb photons and go to the excited states in which they react with the electron donor molecules and generate free radicals, that are responsible for initiating the polymerization process [32]. As a result of these processes, there is acrylamide (monomer) depletion in the bright fringe regions. This produces a concentration gradient which drives diffusion of unreacted monomers from dark regions into the bright fringe areas where they also participate in the polymerization reaction. A refractive index modulation (Δη) occurs due to the conversion of double to single bonds (polymerization) and to density changes driven by the diffusion. It was demonstrated that the polymerization rate depends on the concentration of monomer and on the diffusion of monomer from unpolymerized dark region to polymerized bright region [2,6]. The photopolymer materials can be sensitized to different wavelengths by using different dyes with different properties. Nanoparticles possess a wide variety of refractive indices and, in principle, doped photopolymer layers can yield much higher refractive index changes than the undoped layers [3–6,33] if there is an effective redistribution of nanoparticles during the holographic record process. One of the proposed mechanisms is that the nanoparticles are assumed to be an inactive component, uniformly dispersed in the photopolymer solution when unexposed. During the polymerization process, monomers diffuse from the dark to the bright regions, whereas, the photo insensitive inorganic nanoparticles experience counter diffusion from the bright to the dark regions, driven by the resulting concentration gradient [3–6,22]. Such a mutual diffusion process essentially continues until photopolymerization is completed and in this way, periodic assembly of nanoparticles under holographic exposure is accomplished (Fig. 1). The reason for the increased dynamic range of nanoparticles doped photopolymer layer could be spatial redistribution of nanoparticles components inside the photopolymer and hence increased the refractive index (RI) modulation [22]. Maps of Si-MFI nanoparticles concentration as a function of position in the samples after recording were described by Raman spectroscopy and AFM (atomic force microscope) [34]. It was observed that mass transfer of Si-MFI zeolite nanoparticles took place during the recording of holographic gratings and thus we find that Si-MFI is segregated during the holographic recording.
3. Experiment 3.1. Preparation of thick photopolymer layers The components of the photopolymer are acrylamide monomer (0.6 g) and N,N′-methylene-bisacrylamide cross linking monomer
Fig. 1. Mechanism of nanoparticles doped thick photopolymer layers before and during holographic exposure. When photopolymer is exposed with light, the polymerization process starts and the monomers start diffusing from dark to bright regions, whereas, nanoparticles experience counter diffusion from bright to dark regions, driven by the resulting concentration gradient. Such a mutual diffusion process essentially continues until photo-polymerization is completed and in this way, periodic assembly of nanoparticles under holographic exposure is accomplished.
(0.2 g), triethanolamine initiator (2 ml), 10 ml polyvinyl alcohol binder (30% w/V water stock solution) and Erythrosin B sensitizing dye (0.11%wt. stock solution). The amount of dye added to the layer was adjusted to maintain constant absorbance independent of layer thickness. The 30% concentration of PVA was used for faster drying of the photopolymer layer although the higher viscosity of this stock solution required more time to prepare. The conventional method of preparing thick layers is to deposit photopolymer solution in a Petri dish [17] or in molds [15,16]. Our experience with this method is that the photopolymer solution takes about two weeks to dry completely at room temperature. Once dried only the centre part of the dried layer is used for recording and the remaining solid layer are discarded. This unsuitability is due to non-uniform layer thickness near to the edge of the Petri dish. A modified approach was introduced to prepare 1-mm thick layers with accurate height and even surface roughness while reducing the waste of photopolymer solution. In this technique, 1-mm thick spacers were placed between two microscopic glass slides and boundaries were sealed with glue in order to make a cell. This empty cell was filled with photopolymer solution by using a syringe. Once filled completely it was kept at room temperature and a low humidity dark room. The upper part of the layers was dried fast but the lower was still viscous. The cell was then turned upside down and the glue was removed from all three boundaries, which allowed the layer to dry at room temperature. Once dried the upper glass plate was removed and this photopolymer was ready for recording gratings in it. This method is versatile, as different thickness of layers can be achieved by using different thickness of spacers. For Si-MFI nanoparticles doped photopolymer layers, the components of the solution and method of preparation follows what we described above for undoped acrylamide based photopolymer. However, instead of using 30% (w/V) PVA solution, we used the same quantity of PVA (30 g) powder which was dissolved into 100 ml of 1.9% (w/V) zeolite Si-MFI doped (InnosLab Ltd) solution instead of water. Nanoparticles were randomly distributed on the surface of the resultant nanocomposite [35]. However, an increase in zeolite concentration of these layers leads to an increase of surface roughness, from 1 nm (undoped photopolymer) to 4 nm for 3% MFI doped photopolymer which was measured with white light interferometer (WLI). Therefore, Si-MFI doping concentration
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was not exceeded above 1.9%wt (w/V) to prepare good optical quality nanoparticles doped photopolymer layer. 3.2. Experimental setup for holographic recording A two-beam holographic optical set-up (Fig. 2) with an angle of 151 between the beams was used to record unslanted transmission gratings by an Nd-YVO4 laser (532 nm). The gratings were recorded in both undoped and Si-MFI-doped 1 mm photopolymer thick layers at a recording intensity of 5 mW/cm2 and at a spatial frequency of 1000 lines/mm. The recording intensity was controlled by a variable neutral density filter (N). The absorption of the photopolymer at 633 nm is negligible, so a He–Ne laser (633 nm) was used as a probe beam at the Bragg angle for monitoring the diffracted efficiency (DE) growth in real-time. Both probe and recording beams were s-polarized. The intensity was read by an optical power meter (Newport 842-PE) and the data transferred to a computer with a DAQ card (NI USB-6009). The ‘Bragg condition’ was initially ensured by adjusting the angle of incidence of the probe beam to maximize the photo-detector output using a thin sample layer in which the grating was not over-modulated [7]. As described in Section 1, DE is defined as the ratio of the first order diffracted beam intensity (ID) and the incident beam intensity (I0) of the probe beam, and expressed as a percentage. For imaging recording a data page, the laser beam was split into two parts; a signal beam, which contained the data page and a reference beam for the reproduction of the signal beam (Fig. 2b). The data were digitally encoded and were sent to a spatial light modulator (HoloEye, SLM-2002). These codes translated into bright and dark pixels by blocking or transmitting light after an analyzer (A). A customized-written program in LabVIEW was used for controlling the voltage of each or collective pixels of the SLM (800 600 pixels). The telescope arrangement (4-f system) with an aperture was used in order to remove higher order diffractions from the SLM. To retrieve the data page, the recording medium is illuminated with only the reference beam. The reconstructed data page was then projected onto a CCD camera and the reconstructed images were captured.
doped photopolymer layers. For all above absorbances, one can observe that the nanoparticles doped photopolymer layers show slightly higher absorbance (o 2%) than the same thickness undoped layers. This small variation may be due to the scattering caused by the nanoparticles (Si-MFI) inside the sample that diffract light, as described in Section 3.1. 4.2. Real-time DE measurements Fig. 4 shows the dependence of DE on exposure time for recording gratings in 1 mm thick undoped and Si-MFI doped photopolymer layers at three different sets of absorbances: 0.10, 0.18 and 0.37. For each absorbance, DE increases with exposure and reaches its maximum value. Further increases of exposure (o15 s) do not improve the DE because of the photo polymerization of the monomer inside the layer. Fig. 4(a) shows for 1 mm thick undoped photopolymer layers, that at the absorbances of 0.10, 0.18 and 0.37, the maximum DE was 2%, 12% and 5%, respectively. For the above three absorbances, the Si-MFI doped 1 mm thick photopolymer layers the DE was 12%, 26% and 19%, respectively. As implied by the higher values for DE shows that the inclusion of Si-MFI did improve the holographic process. Previous studies [6,31] in thin acrylamide-based photopolymer layers suggested that Si-MFI doped layers suppress the shrinkage effect that occurs during polymerization and this could be a reason for the improvement of the DE in Si-MFI nanocomposite photopolymer thick layers. However, further investigation is necessary for this claim for thick photopolymer layers. In addition, it was observed that DE increases with absorbance, reaches maximum at absorbance 0.18 and then decreases again at the highest absorbance (0.37). This is mainly due to the formation of noise gratings [17]. In addition, none of the 1 mm thick layers reached DE above 50%. This could be explained by the high absorbance and
0.5
0.4
A
0.3
4. Results and discussion
0.2 A= 0.17
4.1. UV–vis spectra of 1 mm thick layers
0.1
A= 0.10 Reading wavelength
Samples were prepared at three different absorbances in order to find the relationship between DE with absorbances. The absorption spectra were measured using a UV–vis–NIR absorption spectrometer (Perkin-Elmer Lambda 1050). Fig. 3 shows the absorption spectra at absorbances of 0.37, 0.18 and 0.10 at laser wavelength 532 nm for 1 mm (75 mm) thick undoped and Si-MFI
0.0 400 420 440 460 480 500 520 540 560 580 600 620 640
Wavelength, (nm) Fig. 3. Spectra for 1 mm thick undoped and Si-MFI doped photopolymer layers at different absorbance, (■) 0.10, (○) 0.10 (MFI), ( ) 0.18, (●) 0.18 (MFI), (+) 0.37 and (n) 0.37 (MFI).
Fig. 2. Experimental setup. Recording gratings (a) and Recording a data page (b). Here, N: Neutral density filter, S: shutter, BE: beam expander, BS: beam splitter, M: mirror, SLM-spatial light modulator, L: Lens, I: aperture, A: analyser, D: optical detector, C: camera.
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30 Undoped layer
A: 0.10 A: 0.18 A: 0.37
Si-MFI doped layer
A: 0.10 A: 0.18 A: 0.37
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Fig. 4. DE vs. exposure time at (a) undoped and (b) Si-MFI nanoparticles doped 1mm thick photo-polymer layers at absorbance, 0.10 (n), 0.18 (○), and 0.37 ( 1000 lines/mm and an intensity of 5 mW/cm2.
) at recorded at
Fig. 5. Undoped (Row-I) and Si-MFI nanoparticles doped (Row-II) 1-mm thick photopolymer layers. Here, Original image (a), Reconstructed image after 4 s (b), 10 s (c), 14 s (d), 20 s (e). Intensity distribution profiles were observed across red lines in the same position of each of these images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
scattering losses from inhomogeneities which are the main causes of noise gratings described elsewhere [17]. A possible way to overcome this and thus increase DE for thick layers is to reduce the number of inhomogeneities by optimizing the photopolymer composition during preparation of thick samples. Namely an alteration of PVA matrix (by weight, the major component of the photopolymer) properties, by appropriate choice of the molecular weight and the percentage of hydrolysis, could lead to lower scattering photopolymer layers. These studies can be extended for future research. 4.3. Image contrast recording Fig. 5 shows the images of a reconstructed data page recorded in undoped and Si-MFI doped 1-mm thick photopolymer layers at four different recording times (4 s, 10 s, 14 s, 20 s) at 1000 lines/mm and the same exposure intensity 5 mW/cm2. Following the results described above an absorbance of 0.18 was chosen as the optimum value. In order to analyze the quality of the recorded images, a line (red) was marked at the same location in each of these images. A Matlab program was used for calculating the intensity distribution profile across these red lines. The results were then compared for undoped and Si-MFI nanoparticles doped 1 mm thick photopolymer layers for the same exposure time (Fig. 6). An
arrow points the initial position of each of these lines. The ‘0’ and ‘255’ indicate the complete ‘dark’ and ‘bright’ pixel of the data page. The smearing off the reconstructed image information could be found by the intensity distribution profiles at this point. For example, for 1 mm thick undoped layers the gray level at the starting point (just after exposure), after 10 s and 14 s exposure times was 0, 170, and 198, respectively; whereas 0, 100, and 130 values were observed in the case of Si-MFI doped photopolymer layers. Comparing both set of values, the reconstructed images of the nanoparticles doped layers show better results, i.e. lower values were obtained. Another noticeable feature of Fig. 6 is that the step change of the intensity distribution lines for the reconstructed images was more pronounced in the nanoparticles doped layers. An image will lack contrast when there are no sharp differences between the ‘dark’ and ‘white’ pixel. Michelson contrast, [M¼ (Imax Imin)/ (Imax+Imin)] is commonly used for measuring the quality of the image. Here, we calculated M value for each of the intensity distribution profile line for all above images and put these values in Table 1. Here, Imax and Imin represent the highest and lowest intensity. The contrast (M) of the images just after exposure, after 10 s, and 14 s for undoped photopolymer layers were found as 1.0, 0.24 and 0.15 for undoped layers, whereas, for doped layers, they were 0.99, 0.43 and 0.39, respectively.
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Fig. 6. Intensity distribution profiles of undoped (left column) and Si-MFI doped (right column) 1mm thick photopolymer layers. Image just after exposure (0 s) (b), reconstructed image after 10 s (d), 14 s (e). An Arrow (pink color) in right side of each the graph indicates the initial contrast value at different exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Comparing both sets of data, Si-MFI nanoparticles doped photopolymer layers shows higher contrast values than the correspondent undoped layers and therefore have improved overall image quality.
4.4. Noise grating measurement Diffraction patterns of undoped and Si-MFI doped thick layers were visualized at absorbances of 0.10, 0.18, and 0.37 (Fig. 7).
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Table 1 Michelson contrast (M) for undoped and doped layers at different exposure. Exposure time
0s
10 s
14 s
M for undoped layer M for doped layer
1.0 0.99
0.24 0.43
0.15 0.39
Fig. 7. Scattering patterns observed in a two-beam recording setup for recording wavelength 532 nm of 0.10 (Column I), 0.18 (Column II), and 0.37 (Column III) of undoped (Row I) and Si-MFI doped (Row II) 1 mm thick layers with an exposure intensity of 5 mW/cm2 and an exposure time of 50 s. The first-order diffracted beam (Left spot), are clearly visualized in Si-MFI doped layers then in undoped thick layers. For the same condition, the ‘ring structure’ which indicates the presence of noise grating increases in intensity (brightness) in doped layers rather than in undoped thick layers.
A screen was placed at a distance of approximately 10 cm from the samples to obtain diffraction pattern. The spot on the right side of each image corresponds to the transmitted beam whereas; the spot on the left side is the first-order diffracted beam recording at laser wavelength 532-nm at an exposure intensity of 5 mW/cm2 and an exposure time of 50 s. For all above absorbances, first-order diffracted beams (left spots) were clearly visualized in Si-MFI nanoparticles doped layers rather than in undoped thick layers. Section 4.2 supported this claim as improved DE was found in Si-MFI doped layers and reaches maximum up to 25% at the layer absorbance of 0.18. The ‘ring structure’ which indicates the presence of noise grating [17,20] increases in intensity (brightness) in doped layers rather than in undoped thick layers. This is due to the presence of nanoparticles inside the sample that scatters light and strengthens the noise gratings. The explanation and the formation of noise gratings are described in detailed in our previous studies [17]. Strength of noise gratings can be decreased by lowering the value of absorbance. However, decrease in absorbance requires long exposure, because of the reduced concentration of the photosensitive dye. As the number of dye molecules is low, the production of free radicals will be reduced and hence will slower the polymerization process. Based on our study to date, the presence of Si-MFI nanoparticles doped layers improves the diffraction efficiency (DE) of the holographic gratings rather than undoped thick layers and the optimized DE was achieved at an absorbance of 0.18. Reduce noise gratings can be obtained at an absorbance of
0.10 but it does not improve the value of DE. Characterization of the scattering losses and the material shrinkage of the thick layers and further improvement of the overall DE while using other nanoparticles is a part of our continuing research.
5. Conclusion An improved preparation technique has been used for fabricating 1 mm thick photopolymer layers with good optical quality and uniform layer thickness. A detailed explanation of mechanisms for recording holograms in both undoped and nanoparticles doped photopolymers was described. Holographic characteristics (realtime diffraction efficiency growth curves) for undoped layers were compared to Si-MFI nanoparticles doped acrylamide-based thick photopolymer layers at three different absorbances (0.10. 0.18 and 0.37). Optimum absorbance (0.18) and image-contrast were determined for 1-mm thick layers. Similar to thin layers, it was observed that for 1-mm thick layers the inclusion of Si-MFI nanoparticles improves the DE (25%) as well as overall image quality compared to undoped thick layers (10% DE). The potential use of Si-MFI doped thick photopolymer layers is significant as they can be used as volume data storage medium as well as phase coded multiplexing techniques (PCM) can be applied in these thick photopolymer layers.
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Holographic properties of doped Si-MFI nanoparticles ‘thick’ photopolymer layers Mohammad S. Mahmud a,n, Qiaohuan Cheng a,b, Bepari Mohammed Sohel c, Mohammad M. Kamal d, Badsha Alam a, Elsa Leite e a
Biophotonics and Bioengineering Laboratory, Ryerson University, Toronto, M5B 2K3, Canada School of Material Science and Engineering, Henan University of Technology, Henan, China c Intel mobile communications (IMC), Munich, Germany d Department of Burn and Plastic Surgery, Dhaka Medical College, Bangladesh e INOV-Inesec Inovacao, Rua Alves Redol, 9-1000-029, Lisbon, Portugal b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2011 Received in revised form 23 April 2013 Accepted 23 June 2013 Available online 5 July 2013
Behavior of diffraction gratings recorded in thick (1-mm) acrylamide-based photopolymer layers doped with Si-MFI Zeolite nanoparticles (1.9% w/V) was studied. The gratings were recorded at a spatial frequency of 1000 lines/mm at three different values of absorbance (0.10, 0.18 and 0.37). A modified method was developed for the preparation of l-mm thick layers with uniform layer thickness and at lower surface roughness. By measuring the diffraction efficiency growth and studying the diffraction pattern, the influence of nanoparticles (Si-MFI) doped thick volume holographic gratings was analyzed. For all above absorbances, Si-MFI nanocomposite doped 1-mm thick layers showed higher diffraction efficiency (DE) than the undoped thick layers. High-contrast image and DE was observed for the absorbance of 0.18 in 1-mm thick Si-MFI nanocomposite layers. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Thick photopolymer Si-MFI nanoparticles Volume gratings Michelson image contrast Diffraction efficiency Noise gratings
1. Introduction Nanoparticles-doped photopolymer layers are promising materials for developing new holographic sensors [1,2] and for holographic data storage applications [3–6]. Most of these studies are usually performed in thin photopolymer layers for recording and displaying purposes. Thick photopolymer layers (4500 mm) are essential for applications such as high capacity holographic data storage media [7–11] and for phase coded multiplexing techniques (PCM), where a complete overlap of object and reference beams needs to occur inside the layers. The application of photopolymers as a recording material has been restricted in recording thick volume gratings mainly due to limitations of layer thickness and material shrinkage effects during and after recording [12–14]. In volume data storage applications, the material shrinkage problem is significant, as it can change the fringe spacing. This can cause the change of the reconstruction angle and as a result no light is diffracted at the expected reconstruction angle and ultimately the stored data page cannot be recovered. Layers with different compositions and thicknesses and the characteristics of recorded
n
Corresponding author. Tel.: +1 6478854428. E-mail addresses: [email protected], [email protected] (M.S. Mahmud).
holograms in thick layers have been reported elsewhere [15–18]. Recording in thick photopolymer layers showed two major drawbacks: the difference between the effective optical thickness and the physical thickness in the material increases with the increasing layer thickness [19], and the scattering and absorption losses increase due to the existence of noise gratings which is formed due to scattering from inhomogeneities in the recording material and has been explained elsewhere [17,20]. To improve the holographic performance of gratings recorded in photopolymer, one of the successful approaches is to introduce inorganic nanoparticles with different refractive index of the photopolymer. The presence of nanoparticles in photopolymer layers contributes to an overall increase in the dynamic range [3,5,21–26] with improved stability [27–29] and/or the suppression of polymerization shrinkage effects [6,30]. Studies of different types of zeolite nanoparticles (inorganic porous material) incorporated into acrylamide based thin photopolymer layers suggested that zeolite Si-MFI nanoparticles type were responsible for suppressing the shrinkage to a greater extent [6,31]. The behavior of Si-MFI nanoparticles doped acrylamide-based thin ( o200 mm) photo-polymer and their holographic recording gratings were studied elsewhere [1,2,6]. However, thick photopolymer layers offer the opportunity to be used in volume grating data storage application. For this purpose, here we studied the behavior of Si-MFI nanoparticles doped acrylamide-based thick
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photopolymer layers. The compositions of the composites were changed and we showed a modified method of preparing the thick layers. Results were compared at three different values of absorbance (0.10, 0.18 and 0.37). The expected improvement shown in thick layers (reduced shrinkage and better DE) were also tested by comparing the holographic performance namely DE and different image contrast. The diffraction efficiency (DE) is defined here as the ratio of the first order diffracted beam intensity (ID) and the incident beam intensity (I0) of the probe beam, and expressed as a percentage.
2. Theory The acrylamide based photopolymer is a self-developing dry layer, consisting of monomers, an electron donor or initiator, a photosensitizer and a polymer binder acting as a matrix in which other components are dispersed. Holographic recording in this material is based on photo-polymerization reactions in the areas illuminated by laser light of appropriate wavelength. Briefly, the dye molecules absorb photons and go to the excited states in which they react with the electron donor molecules and generate free radicals, that are responsible for initiating the polymerization process [32]. As a result of these processes, there is acrylamide (monomer) depletion in the bright fringe regions. This produces a concentration gradient which drives diffusion of unreacted monomers from dark regions into the bright fringe areas where they also participate in the polymerization reaction. A refractive index modulation (Δη) occurs due to the conversion of double to single bonds (polymerization) and to density changes driven by the diffusion. It was demonstrated that the polymerization rate depends on the concentration of monomer and on the diffusion of monomer from unpolymerized dark region to polymerized bright region [2,6]. The photopolymer materials can be sensitized to different wavelengths by using different dyes with different properties. Nanoparticles possess a wide variety of refractive indices and, in principle, doped photopolymer layers can yield much higher refractive index changes than the undoped layers [3–6,33] if there is an effective redistribution of nanoparticles during the holographic record process. One of the proposed mechanisms is that the nanoparticles are assumed to be an inactive component, uniformly dispersed in the photopolymer solution when unexposed. During the polymerization process, monomers diffuse from the dark to the bright regions, whereas, the photo insensitive inorganic nanoparticles experience counter diffusion from the bright to the dark regions, driven by the resulting concentration gradient [3–6,22]. Such a mutual diffusion process essentially continues until photopolymerization is completed and in this way, periodic assembly of nanoparticles under holographic exposure is accomplished (Fig. 1). The reason for the increased dynamic range of nanoparticles doped photopolymer layer could be spatial redistribution of nanoparticles components inside the photopolymer and hence increased the refractive index (RI) modulation [22]. Maps of Si-MFI nanoparticles concentration as a function of position in the samples after recording were described by Raman spectroscopy and AFM (atomic force microscope) [34]. It was observed that mass transfer of Si-MFI zeolite nanoparticles took place during the recording of holographic gratings and thus we find that Si-MFI is segregated during the holographic recording.
3. Experiment 3.1. Preparation of thick photopolymer layers The components of the photopolymer are acrylamide monomer (0.6 g) and N,N′-methylene-bisacrylamide cross linking monomer
Fig. 1. Mechanism of nanoparticles doped thick photopolymer layers before and during holographic exposure. When photopolymer is exposed with light, the polymerization process starts and the monomers start diffusing from dark to bright regions, whereas, nanoparticles experience counter diffusion from bright to dark regions, driven by the resulting concentration gradient. Such a mutual diffusion process essentially continues until photo-polymerization is completed and in this way, periodic assembly of nanoparticles under holographic exposure is accomplished.
(0.2 g), triethanolamine initiator (2 ml), 10 ml polyvinyl alcohol binder (30% w/V water stock solution) and Erythrosin B sensitizing dye (0.11%wt. stock solution). The amount of dye added to the layer was adjusted to maintain constant absorbance independent of layer thickness. The 30% concentration of PVA was used for faster drying of the photopolymer layer although the higher viscosity of this stock solution required more time to prepare. The conventional method of preparing thick layers is to deposit photopolymer solution in a Petri dish [17] or in molds [15,16]. Our experience with this method is that the photopolymer solution takes about two weeks to dry completely at room temperature. Once dried only the centre part of the dried layer is used for recording and the remaining solid layer are discarded. This unsuitability is due to non-uniform layer thickness near to the edge of the Petri dish. A modified approach was introduced to prepare 1-mm thick layers with accurate height and even surface roughness while reducing the waste of photopolymer solution. In this technique, 1-mm thick spacers were placed between two microscopic glass slides and boundaries were sealed with glue in order to make a cell. This empty cell was filled with photopolymer solution by using a syringe. Once filled completely it was kept at room temperature and a low humidity dark room. The upper part of the layers was dried fast but the lower was still viscous. The cell was then turned upside down and the glue was removed from all three boundaries, which allowed the layer to dry at room temperature. Once dried the upper glass plate was removed and this photopolymer was ready for recording gratings in it. This method is versatile, as different thickness of layers can be achieved by using different thickness of spacers. For Si-MFI nanoparticles doped photopolymer layers, the components of the solution and method of preparation follows what we described above for undoped acrylamide based photopolymer. However, instead of using 30% (w/V) PVA solution, we used the same quantity of PVA (30 g) powder which was dissolved into 100 ml of 1.9% (w/V) zeolite Si-MFI doped (InnosLab Ltd) solution instead of water. Nanoparticles were randomly distributed on the surface of the resultant nanocomposite [35]. However, an increase in zeolite concentration of these layers leads to an increase of surface roughness, from 1 nm (undoped photopolymer) to 4 nm for 3% MFI doped photopolymer which was measured with white light interferometer (WLI). Therefore, Si-MFI doping concentration
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was not exceeded above 1.9%wt (w/V) to prepare good optical quality nanoparticles doped photopolymer layer. 3.2. Experimental setup for holographic recording A two-beam holographic optical set-up (Fig. 2) with an angle of 151 between the beams was used to record unslanted transmission gratings by an Nd-YVO4 laser (532 nm). The gratings were recorded in both undoped and Si-MFI-doped 1 mm photopolymer thick layers at a recording intensity of 5 mW/cm2 and at a spatial frequency of 1000 lines/mm. The recording intensity was controlled by a variable neutral density filter (N). The absorption of the photopolymer at 633 nm is negligible, so a He–Ne laser (633 nm) was used as a probe beam at the Bragg angle for monitoring the diffracted efficiency (DE) growth in real-time. Both probe and recording beams were s-polarized. The intensity was read by an optical power meter (Newport 842-PE) and the data transferred to a computer with a DAQ card (NI USB-6009). The ‘Bragg condition’ was initially ensured by adjusting the angle of incidence of the probe beam to maximize the photo-detector output using a thin sample layer in which the grating was not over-modulated [7]. As described in Section 1, DE is defined as the ratio of the first order diffracted beam intensity (ID) and the incident beam intensity (I0) of the probe beam, and expressed as a percentage. For imaging recording a data page, the laser beam was split into two parts; a signal beam, which contained the data page and a reference beam for the reproduction of the signal beam (Fig. 2b). The data were digitally encoded and were sent to a spatial light modulator (HoloEye, SLM-2002). These codes translated into bright and dark pixels by blocking or transmitting light after an analyzer (A). A customized-written program in LabVIEW was used for controlling the voltage of each or collective pixels of the SLM (800 600 pixels). The telescope arrangement (4-f system) with an aperture was used in order to remove higher order diffractions from the SLM. To retrieve the data page, the recording medium is illuminated with only the reference beam. The reconstructed data page was then projected onto a CCD camera and the reconstructed images were captured.
doped photopolymer layers. For all above absorbances, one can observe that the nanoparticles doped photopolymer layers show slightly higher absorbance (o 2%) than the same thickness undoped layers. This small variation may be due to the scattering caused by the nanoparticles (Si-MFI) inside the sample that diffract light, as described in Section 3.1. 4.2. Real-time DE measurements Fig. 4 shows the dependence of DE on exposure time for recording gratings in 1 mm thick undoped and Si-MFI doped photopolymer layers at three different sets of absorbances: 0.10, 0.18 and 0.37. For each absorbance, DE increases with exposure and reaches its maximum value. Further increases of exposure (o15 s) do not improve the DE because of the photo polymerization of the monomer inside the layer. Fig. 4(a) shows for 1 mm thick undoped photopolymer layers, that at the absorbances of 0.10, 0.18 and 0.37, the maximum DE was 2%, 12% and 5%, respectively. For the above three absorbances, the Si-MFI doped 1 mm thick photopolymer layers the DE was 12%, 26% and 19%, respectively. As implied by the higher values for DE shows that the inclusion of Si-MFI did improve the holographic process. Previous studies [6,31] in thin acrylamide-based photopolymer layers suggested that Si-MFI doped layers suppress the shrinkage effect that occurs during polymerization and this could be a reason for the improvement of the DE in Si-MFI nanocomposite photopolymer thick layers. However, further investigation is necessary for this claim for thick photopolymer layers. In addition, it was observed that DE increases with absorbance, reaches maximum at absorbance 0.18 and then decreases again at the highest absorbance (0.37). This is mainly due to the formation of noise gratings [17]. In addition, none of the 1 mm thick layers reached DE above 50%. This could be explained by the high absorbance and
0.5
0.4
A
0.3
4. Results and discussion
0.2 A= 0.17
4.1. UV–vis spectra of 1 mm thick layers
0.1
A= 0.10 Reading wavelength
Samples were prepared at three different absorbances in order to find the relationship between DE with absorbances. The absorption spectra were measured using a UV–vis–NIR absorption spectrometer (Perkin-Elmer Lambda 1050). Fig. 3 shows the absorption spectra at absorbances of 0.37, 0.18 and 0.10 at laser wavelength 532 nm for 1 mm (75 mm) thick undoped and Si-MFI
0.0 400 420 440 460 480 500 520 540 560 580 600 620 640
Wavelength, (nm) Fig. 3. Spectra for 1 mm thick undoped and Si-MFI doped photopolymer layers at different absorbance, (■) 0.10, (○) 0.10 (MFI), ( ) 0.18, (●) 0.18 (MFI), (+) 0.37 and (n) 0.37 (MFI).
Fig. 2. Experimental setup. Recording gratings (a) and Recording a data page (b). Here, N: Neutral density filter, S: shutter, BE: beam expander, BS: beam splitter, M: mirror, SLM-spatial light modulator, L: Lens, I: aperture, A: analyser, D: optical detector, C: camera.
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40
30 Undoped layer
A: 0.10 A: 0.18 A: 0.37
Si-MFI doped layer
A: 0.10 A: 0.18 A: 0.37
25
35 30
DE %
20
DE %
117
15
25 20 15
10 10 5
5 0
0 0
5
10
15
20
25
30
35
40
45
50
Time(s)
0
5
10
15
20
25
30
35
40
45
50
Time(s)
Fig. 4. DE vs. exposure time at (a) undoped and (b) Si-MFI nanoparticles doped 1mm thick photo-polymer layers at absorbance, 0.10 (n), 0.18 (○), and 0.37 ( 1000 lines/mm and an intensity of 5 mW/cm2.
) at recorded at
Fig. 5. Undoped (Row-I) and Si-MFI nanoparticles doped (Row-II) 1-mm thick photopolymer layers. Here, Original image (a), Reconstructed image after 4 s (b), 10 s (c), 14 s (d), 20 s (e). Intensity distribution profiles were observed across red lines in the same position of each of these images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
scattering losses from inhomogeneities which are the main causes of noise gratings described elsewhere [17]. A possible way to overcome this and thus increase DE for thick layers is to reduce the number of inhomogeneities by optimizing the photopolymer composition during preparation of thick samples. Namely an alteration of PVA matrix (by weight, the major component of the photopolymer) properties, by appropriate choice of the molecular weight and the percentage of hydrolysis, could lead to lower scattering photopolymer layers. These studies can be extended for future research. 4.3. Image contrast recording Fig. 5 shows the images of a reconstructed data page recorded in undoped and Si-MFI doped 1-mm thick photopolymer layers at four different recording times (4 s, 10 s, 14 s, 20 s) at 1000 lines/mm and the same exposure intensity 5 mW/cm2. Following the results described above an absorbance of 0.18 was chosen as the optimum value. In order to analyze the quality of the recorded images, a line (red) was marked at the same location in each of these images. A Matlab program was used for calculating the intensity distribution profile across these red lines. The results were then compared for undoped and Si-MFI nanoparticles doped 1 mm thick photopolymer layers for the same exposure time (Fig. 6). An
arrow points the initial position of each of these lines. The ‘0’ and ‘255’ indicate the complete ‘dark’ and ‘bright’ pixel of the data page. The smearing off the reconstructed image information could be found by the intensity distribution profiles at this point. For example, for 1 mm thick undoped layers the gray level at the starting point (just after exposure), after 10 s and 14 s exposure times was 0, 170, and 198, respectively; whereas 0, 100, and 130 values were observed in the case of Si-MFI doped photopolymer layers. Comparing both set of values, the reconstructed images of the nanoparticles doped layers show better results, i.e. lower values were obtained. Another noticeable feature of Fig. 6 is that the step change of the intensity distribution lines for the reconstructed images was more pronounced in the nanoparticles doped layers. An image will lack contrast when there are no sharp differences between the ‘dark’ and ‘white’ pixel. Michelson contrast, [M¼ (Imax Imin)/ (Imax+Imin)] is commonly used for measuring the quality of the image. Here, we calculated M value for each of the intensity distribution profile line for all above images and put these values in Table 1. Here, Imax and Imin represent the highest and lowest intensity. The contrast (M) of the images just after exposure, after 10 s, and 14 s for undoped photopolymer layers were found as 1.0, 0.24 and 0.15 for undoped layers, whereas, for doped layers, they were 0.99, 0.43 and 0.39, respectively.
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Fig. 6. Intensity distribution profiles of undoped (left column) and Si-MFI doped (right column) 1mm thick photopolymer layers. Image just after exposure (0 s) (b), reconstructed image after 10 s (d), 14 s (e). An Arrow (pink color) in right side of each the graph indicates the initial contrast value at different exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Comparing both sets of data, Si-MFI nanoparticles doped photopolymer layers shows higher contrast values than the correspondent undoped layers and therefore have improved overall image quality.
4.4. Noise grating measurement Diffraction patterns of undoped and Si-MFI doped thick layers were visualized at absorbances of 0.10, 0.18, and 0.37 (Fig. 7).
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Table 1 Michelson contrast (M) for undoped and doped layers at different exposure. Exposure time
0s
10 s
14 s
M for undoped layer M for doped layer
1.0 0.99
0.24 0.43
0.15 0.39
Fig. 7. Scattering patterns observed in a two-beam recording setup for recording wavelength 532 nm of 0.10 (Column I), 0.18 (Column II), and 0.37 (Column III) of undoped (Row I) and Si-MFI doped (Row II) 1 mm thick layers with an exposure intensity of 5 mW/cm2 and an exposure time of 50 s. The first-order diffracted beam (Left spot), are clearly visualized in Si-MFI doped layers then in undoped thick layers. For the same condition, the ‘ring structure’ which indicates the presence of noise grating increases in intensity (brightness) in doped layers rather than in undoped thick layers.
A screen was placed at a distance of approximately 10 cm from the samples to obtain diffraction pattern. The spot on the right side of each image corresponds to the transmitted beam whereas; the spot on the left side is the first-order diffracted beam recording at laser wavelength 532-nm at an exposure intensity of 5 mW/cm2 and an exposure time of 50 s. For all above absorbances, first-order diffracted beams (left spots) were clearly visualized in Si-MFI nanoparticles doped layers rather than in undoped thick layers. Section 4.2 supported this claim as improved DE was found in Si-MFI doped layers and reaches maximum up to 25% at the layer absorbance of 0.18. The ‘ring structure’ which indicates the presence of noise grating [17,20] increases in intensity (brightness) in doped layers rather than in undoped thick layers. This is due to the presence of nanoparticles inside the sample that scatters light and strengthens the noise gratings. The explanation and the formation of noise gratings are described in detailed in our previous studies [17]. Strength of noise gratings can be decreased by lowering the value of absorbance. However, decrease in absorbance requires long exposure, because of the reduced concentration of the photosensitive dye. As the number of dye molecules is low, the production of free radicals will be reduced and hence will slower the polymerization process. Based on our study to date, the presence of Si-MFI nanoparticles doped layers improves the diffraction efficiency (DE) of the holographic gratings rather than undoped thick layers and the optimized DE was achieved at an absorbance of 0.18. Reduce noise gratings can be obtained at an absorbance of
0.10 but it does not improve the value of DE. Characterization of the scattering losses and the material shrinkage of the thick layers and further improvement of the overall DE while using other nanoparticles is a part of our continuing research.
5. Conclusion An improved preparation technique has been used for fabricating 1 mm thick photopolymer layers with good optical quality and uniform layer thickness. A detailed explanation of mechanisms for recording holograms in both undoped and nanoparticles doped photopolymers was described. Holographic characteristics (realtime diffraction efficiency growth curves) for undoped layers were compared to Si-MFI nanoparticles doped acrylamide-based thick photopolymer layers at three different absorbances (0.10. 0.18 and 0.37). Optimum absorbance (0.18) and image-contrast were determined for 1-mm thick layers. Similar to thin layers, it was observed that for 1-mm thick layers the inclusion of Si-MFI nanoparticles improves the DE (25%) as well as overall image quality compared to undoped thick layers (10% DE). The potential use of Si-MFI doped thick photopolymer layers is significant as they can be used as volume data storage medium as well as phase coded multiplexing techniques (PCM) can be applied in these thick photopolymer layers.
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