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Optimization of two-monomer-based photopolymer used for holographic recording
September 2002
Materials Letters 56 (2002) 3 – 8 www.elsevier.com/locate/matlet
Optimization of two-monomer-based photopolymer used for holographic recording Huawen Yao*, Mingju Huang, Zhongyu Chen, Lisong Hou, Fuxi Gan Shanghai Institute of Optical and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 10 September 2001; received in revised form 19 November 2001; accepted 20 November 2001
Abstract Studies of optimization and characteristics of a dry film photopolymerizable recording material are presented. The effect of variation of the concentration of each component was investigated. Diffraction efficiencies of 55%, with the energetic sensitivity of 60 mJ/cm2, have been obtained in the photosensitive films of 150-mm thickness with a spatial frequency of 2750 lines/mm. An image has been successfully stored in the material with a small distortion. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Photopolymer; Optimization; Diffraction efficiency; Sensitivity; Hologram
1. Introduction Holography is a method of storing optical information by means of photochemical and/or photophysical processes. Different recording media such as silver halide photographic emulsions, dichromated gelatins, thermoplastics, and photopolymers are normally used. Development of new holographic materials is important because many applications have been proposed in different areas, for example, optical memories [1], holographic displays, scanners, optical disk systems [2], optical computing, optical data storage [3], and holographic interferometry. Several types of photopolymers system for holographic recording have been reported in recent years [4 –8]. In
*
Corresponding author. Tel.: +86-21-59534890; fax: +86-2159528812. E-mail address: [email protected] (H. Yao).
the development of such high-quality recording materials, many aspects must be considered, such as high sensitivity, simple chemical development, good spatial– frequency response, high diffraction-efficiency level, high signal-to-noise ratio (SNR), and temporal stability of the holographic material. Photopolymeric material has advantages, such as self-development, high angular selectivity, and high resolution, that make them more suitable for applications like optical storage, holographic optical elements, optical computing, holographic interferometry, etc. [9]. A photopolymerizable system basically consists of one monomer or a mixture of monomers, a photoinitiator, and a coinitiator in a suitable polymeric film as binder. Acrylamide has been one of the most frequently used monomers because it can polymerize [10]. An improvement on the mixture that uses acrylamides was achieved when N,N V-methylenebisacrylamide (BAA) was added to speed up the polymerization reaction. Index modulation is produced when molecular space
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 0 6 - 8
4
H. Yao et al. / Materials Letters 56 (2002) 3–8
lattices of the acrylamide chains are formed, and in the lattices, the BAA builds cross-links to form a copolymer that is a transparent and rigid element. Acrylamide-based PVA films constitute a low-cost organic material, and a great deal of attention has been given to the composition of an acrylamide-based photopolymeric system initiated by triethanolamine and methylene blue in recent years [11]. It is important for us to optimize the compositions of photopolymer because each composition of the photopolymer will influence its characters greatly. In this paper, we present the optimization of an acrylamidebased photopolymer containing BAA photopolymer. We obtained holograms with a diffraction efficiency of 60% and an energy exposure of 100 mJ/cm2 in the material using Ar-ion laser for recording at a power of 4 mW/cm2, and several holograms were recorded in the material.
Fig. 1. Schematic representation of the setup for the recording of grating. Ms: Mirrors; BS: beam splitter; H: holographic plate; D: detectors; SF: spatial filter.
formation of polyacrylamide in the regions of constructive interference. The reconstruction of the hologram was made at the Bragg angle with a LD laser tuned to 650 nm at real time for monitoring the evolution of the diffracted intensity of this beam as a function of time.
2. Experimental All the materials used in this work were of the best available grade and were used without further purification, and all the samples were prepared under normal laboratory conditions [22 C; relative humidity (RH), 40 – 60%] as follows. (1) Basic PVA (MW 1750) was dissolved in distilled water and then heated to 80 C to yield a PVA aqueous solution having 10 wt.% of PVA. (2) Crystals of Erythrosine B (ErB) were dissolved in water to obtain the desired dye concentration. (3) A certain volume of triethanolamine (TEA) was dissolved in water. (4) The TEA solution was added into the PVA solution along with the ErB solution. Finally, acrylamide (AA) and BAA were added as the monomers. Mixing the solution well, we got the photopolymeric solution and the total volume was approximately 25 ml. (5) By spreading 10 ml of the photopolymeric solution on a 12.710.2 cm leveled glass plate and natural drying at 36 – 48 h, a dry layer of 150-mm thickness was obtained. The basic setup used to record holographic gratings is shown in Fig. 1. Two 514-nm beams with a power intensity of 20 mW/cm2 from an Ar-ion laser overlap at a photosensitive plate, producing appropriate interference pattern at the plane of the material through the
3. Results and discussion 3.1. The photoreaction process and absorption spectra of the photopolymer A photoreduction reaction is produced when the material is illuminated with a light beam of 514 nm. The dye is excited to the high-energy state and then reacts with the coinitiator TEA, which is the electron donor, to produce a dye radical anion and a triethanolamine radical cation. The dye radical is not usually reactive enough to initiate polymerization but the TEA radical will react with an AA molecule and polymerization may then occur [12]. A possible mechanism for this process is shown in Fig. 2. The reaction occurred in PVA matrix and we consider that the PVA matrix is inert. We use Ar-ion laser for exposure that is determined by the dye in the photopolymer because the absorption peak of Erythrosine B is around 514 nm. 3.2. The exposure time and diffraction efficiency curve of the material With the increase of laser power, we can expose those materials that are not so sensitive. In this experi-
H. Yao et al. / Materials Letters 56 (2002) 3–8
Fig. 2. Scheme of the mechanism proposed for photopolymerization. DYE: Photoinitiator; DYE*: excited photoinitiator; DYE : anion derivative of dye; LDYE: leuco-dye; Am: amine; Am*: excited amine; Am +*: cation radical of amine; M: monomer; P: polymer; hn: energy of an incident photon; k, k V: reaction constant.
5
[13]. Therefore, we can know from Fig. 3 that at the first moment of exposure, local photopolymerization is proportional to the exposure energy. When the exposure energy is attained at 100 mJ/cm2, the index modulation will be saturated; with increasing the exposure energy, the diffraction efficiency will not increase. The reason why the diffraction efficiency decreases after the saturation of exposure might be caused by the cross-talk of holograms. Some of the main sources of noise are (1) surface deformation of the recording material, (2) random scatter caused by the granularity of the recording material, and (3) nonlinear recording of the signal wave. When the exposure increased, the cross talks increase much quicker resulting in more output signal being scattered, and the diffraction efficiency decreases.
ment, the exposure intensity was 20 mW/cm2, much higher than silver halide’s. After the measurement of the material’s exposure time and diffraction efficiency, we got the relational curve as shown in Fig. 3. From this figure, we can see that at the first moment, the diffraction efficiency of the photopolymer was proportional to the exposure time, but after a while, the efficiency got to a maximum; the diffraction time decreased with increasing exposure time. When the exposure energy reach to 100 mJ/cm2, we could obtain the maximum diffraction efficiency of 55%. In a photopolymer, the index modulation is proportional to the concentration of the monomer reacted
3.3. Optimization of the dye concentration
Fig. 3. The relationship between exposure time and diffraction efficiency. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; TEA: 0.38 M; ErB: 1.1310 4; PVA: 10%. The intensity used is 20 mW/cm2.
Fig. 4. Effect of the concentration of Erythrosine B on the holographic parameters in a holographic material. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; TEA: 0.38 M; PVA: 10%. The intensity used is 20 mW/cm2.
We measured the diffraction efficiency and sensitivity of the material by changing the concentration of the dye in the photopolymer. As shown in Fig. 4, we found that the polymer’s sensitivity increased when we increased the concentration of the dye. It is easy to understand that the more the dye molecules, the more the photons being absorbed, and the more become the excited states of the dye, the easier the triethanolamine is excited and the quicker is the monomer’s polymerization rate. However, as Fig. 4 shows, the sensitivity of the material decreases with increasing concentra-
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H. Yao et al. / Materials Letters 56 (2002) 3–8
tion of Erythrosine B when it is greater than 0.05 10 3 M. The reason for this is the possible competitive polymerization mechanism that is produced when high concentrations of the dye are used [14] because the dye forms dimeric forms, which is involved in a polymerization termination reaction, resulting in a decrease in the overall rate of polymerization. After those dye molecules participating in the polymerization have been consumed, the dimeric molecules of the dye will decompose; the monomers in the material will polymerize perfectly as long as the exposure time is long enough. The decomposition process of the dimeric dye influences the polymerization rate, leading to the decrease in the sensitivity of material. However, the diffraction efficiency changes little that can be seen from Fig. 4. The evidence of the formation of dimeric compound can be seen in Fig. 5 in which the increase in the Erythrosine B produces the apparition of a band at 490 – 510 nm, which is related to the formation of dimeric forms. 3.4. Optimization of the TEA concentration As Fig. 6 shows, the diffraction efficiency keeps almost constant with changing concentration of TEA, but the sensitivity is influenced greatly by the concentration of TEA. When the concentration of TEA is about 0.30 M, the material will have the highest sensitivity; either the increase or decrease of TEA concentration will decrease the sensitivity of the
Fig. 6. Effect of the concentration of TEA on the holographic parameters in a holographic material. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; ErB: 1.1310 4; PVA: 10%. The intensity used is 20 mW/cm2.
material. The concentration of TEA used in the material is much higher than the concentration of the dye. When the material is illuminated with a wavelength of 514 nm, which corresponds to the Erythrosine B’s absorption band, the dye molecules will be excited and become radicals, which react with TEA quickly. TEA becomes radical cation and then reacts with the monomer molecules and makes them polymerize. This is the process of photopolymerization. The dye molecules become bleached molecules [15] when the excited dye molecules reacted with TEA, so the photopolymers being illuminated are transparent. The reason for using high concentration of TEA is that TEA is a plasticizer and high concentration will improve the quality of the material, reduce the precipitation of the monomer on the surface of the film, and consequently, reduce the noise of the gratings as well. Too much TEA does not improve the holographic effect of the material but makes the film shrink greatly. When the concentration is 0.6 M, the shrinkage of the film is greater than 10%. 3.5. Optimization of the AA and BAA concentrations
Fig. 5. Transmittance spectra of the material when the concentration of Erythrosine B is changed. The solid curve represents an Erythrosine B concentration of 510 4 mol/l, the dashed curve, an Erythrosine B concentration of 510 3 mol/l.
We fixed BAA concentration in the material as 0.0324 M and changed the concentration of AA. By measuring the effect of the concentration of AA on the holographic parameters in the photopolymers, we got Fig. 8. We can see that when the concentration of AA is 0.1 M, the diffraction efficiency of the photopolymer is 20%, and with increasing concentration of AA, the diffraction efficiency is improved greatly.
H. Yao et al. / Materials Letters 56 (2002) 3–8
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When the concentration of AA attains 0.30 M, maximum diffraction efficiency is achieved and remains approximately constant at higher concentrations of AA. The sensitivity of the material is improved with the increase of AAs concentration. However, it is impossible to increase the concentration of AA indefinitely because the compatibility and solubility of this monomer in the polymer film are limited. At high concentration of AA, this monomer precipitates on the surface of the film; this gives rise to the observation of noise gratings, which are produced by the interference of the scattered light of these solid particles with the incident laser beam in the process of hologram formation. Moreover, high concentration of AA makes the film shrink greatly. It is shown in Fig. 7 that when the concentration of AA is 0.30 M, the film has big diffraction efficiency. We kept the mole ratio of BAA to AA as 1:6, then changed the concentration of monomers in the material to observe the effect on the holographic parameters and got Fig. 8. At low concentrations of the monomers, the diffraction efficiency is very low; when the AA concentration is 0.05 M, the diffraction efficiency is less than 1%. However, with increasing AA concentration, the diffraction efficiency increases rapidly. When the AA concentration is 0.15 M, the diffraction efficiency will be 40%. At the same time, the sensitivity decreases continually from about 300 to 30 mJ/cm2; this fully shows that the increase of the concentration of monomers will increase the sensitiv-
ity of the photopolymer. In Fig. 7, the concentration of BAA is kept constant; when the concentration of AA is very low, the material still has relatively high diffraction efficiency. However, in Fig. 8, when the concentration of AA is low, the diffraction efficiency is also low. These testify the fact that BAA can ameliorate the characters of the material. BAA can speed-up the polymerization reaction of AA in the photopolymer, so the materials with high concentration of BAA have high diffraction efficiency and sensitivity at the same time, but too much BAA will lead to high shrinkage of the material.
Fig. 7. Effect of the concentration of AA on the holographic parameters in a holographic material. The chemical composition of the material is BAA: 0.0324 M; ErB: 1.1310 4 M; PVA: 10%; TEA: 0.38 M. The intensity used is 20 mW/cm2.
Fig. 9. The original images (left) and reconstructed images (right).
Fig. 8. Effect of the concentration of monomers on the holographic parameters in a holographic material. The chemical composition of the material is PVA: 10%; TEA: 0.38 M; ErB: 1.1310 4 M. The intensity used is 20 mW/cm2.
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H. Yao et al. / Materials Letters 56 (2002) 3–8
4. Holographic recording with the photopolymer Recording the picture in a material and then reconstructing the hologram are the primary principle of data storage in photopolymer. Using the photopolymers prepared, we successfully stored some holograms. The reconstruction was imaged onto a CCD and a computer was used to capture the images. The reconstructed and the original images are shown in Fig. 9. We can see that the SNR of the images is good and the distortion is small.
5. Conclusion The optimization and characteristics of an acrylamide-based polymeric film for holographic recording are presented. We found that each component has an optimal concentration. It is not the case that the higher the concentration, the better the holographic characters. It was also found that BAA influences the characters of acrylamide greatly and suitable use of BAA can ameliorate the characters of the photopolymer effectively. Using the photopolymers prepared in this work, we successfully stored some images and found that the distortion was small. This shows that our materials have good holographic recording properties.
References [1] S. Martin, C.A. Feely, V. Toal, Holographic recording characteristics of an acrylamide-based photopolymer, Appl. Opt. 36 (1997) 5757.
[2] W.H. Lee, Holographic optical head for compact disk application, Opt. Eng. 28 (1989) 650. [3] M.P. Bernal, et al., Holographic data storage materials, Mater. Res. Soc. Bull. 9 (1996) 50. [4] S. Blaya, et al., Holography as a technique for the study of photopolymerization kinetics in dry polymeric films with a nonlinear response, Appl. Opt. 38 (1999) 955. [5] D.J. Lougnot, C. Turck, C. Leroy-Garel, New holographic recording materials based on dual-cure photopolymer system, SPIE 3417 (1998) 165. [6] M.M. Wang, S.C. Esener, Three-dimensional optical data storage in a fluorescent dye-doped photopolymer, Appl. Opt. 39 (2000) 1826. [7] M.G. Schnoes, et al., Photopolymer-filled nanoporous glass as a dimensionally stable holographic recording medium, Opt. Lett. 24 (1999) 658. [8] G.J. Steckman, V. Shlkovnikov, I. Solomatine, et al., Holographic recording in a photopolymer by optically induced detachment of chromophores, Opt. Lett. 25 (2000) 607. [9] A. Lessard, G. Manivannan, Holographic recording materials: an overview, in: T.J. Trout (Ed.), Holographic Materials, Proc. SPIE, vol. 2405 (1995) 2. [10] S. Blaya, et al., Optimization of an acrylamide-based dry film used for holographic recording, Appl. Opt. 37 (1998) 7604. [11] S. Calixto, Dry polymer for holographic recording, Appl. Opt. 26 (1987) 3904. [12] S. Martin, P.E.L.G. Leclere, V. Toal, et al., Characterization of an acrylamide-based dry photopolymer holographic recording material, Opt. Eng. 33 (1994) 3942. [13] V.L. Colvin, R.G. Larson, A.L. Harris, et al., Quantitative model of volume hologram formation in photopolymers, J. Appl. Phys. 81 (1997) 5913. [14] C.S.H. Chen, Dye-sensitized photopolymerization: I. Polymerization of acrylamide in aqueous solution sensitized by methylene blue-triethanolamine system, J. Polym. Sci. A55 (1965) 1107. [15] G. Oster, N. Wotherspoon, Photoreduction of methylene blue by amines: I. A flash photolysis study of the reaction between triplet methylene blue and amines, Photochem. Photobiol. 24 (1976) 395.
Materials Letters 56 (2002) 3 – 8 www.elsevier.com/locate/matlet
Optimization of two-monomer-based photopolymer used for holographic recording Huawen Yao*, Mingju Huang, Zhongyu Chen, Lisong Hou, Fuxi Gan Shanghai Institute of Optical and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 10 September 2001; received in revised form 19 November 2001; accepted 20 November 2001
Abstract Studies of optimization and characteristics of a dry film photopolymerizable recording material are presented. The effect of variation of the concentration of each component was investigated. Diffraction efficiencies of 55%, with the energetic sensitivity of 60 mJ/cm2, have been obtained in the photosensitive films of 150-mm thickness with a spatial frequency of 2750 lines/mm. An image has been successfully stored in the material with a small distortion. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Photopolymer; Optimization; Diffraction efficiency; Sensitivity; Hologram
1. Introduction Holography is a method of storing optical information by means of photochemical and/or photophysical processes. Different recording media such as silver halide photographic emulsions, dichromated gelatins, thermoplastics, and photopolymers are normally used. Development of new holographic materials is important because many applications have been proposed in different areas, for example, optical memories [1], holographic displays, scanners, optical disk systems [2], optical computing, optical data storage [3], and holographic interferometry. Several types of photopolymers system for holographic recording have been reported in recent years [4 –8]. In
*
Corresponding author. Tel.: +86-21-59534890; fax: +86-2159528812. E-mail address: [email protected] (H. Yao).
the development of such high-quality recording materials, many aspects must be considered, such as high sensitivity, simple chemical development, good spatial– frequency response, high diffraction-efficiency level, high signal-to-noise ratio (SNR), and temporal stability of the holographic material. Photopolymeric material has advantages, such as self-development, high angular selectivity, and high resolution, that make them more suitable for applications like optical storage, holographic optical elements, optical computing, holographic interferometry, etc. [9]. A photopolymerizable system basically consists of one monomer or a mixture of monomers, a photoinitiator, and a coinitiator in a suitable polymeric film as binder. Acrylamide has been one of the most frequently used monomers because it can polymerize [10]. An improvement on the mixture that uses acrylamides was achieved when N,N V-methylenebisacrylamide (BAA) was added to speed up the polymerization reaction. Index modulation is produced when molecular space
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 0 6 - 8
4
H. Yao et al. / Materials Letters 56 (2002) 3–8
lattices of the acrylamide chains are formed, and in the lattices, the BAA builds cross-links to form a copolymer that is a transparent and rigid element. Acrylamide-based PVA films constitute a low-cost organic material, and a great deal of attention has been given to the composition of an acrylamide-based photopolymeric system initiated by triethanolamine and methylene blue in recent years [11]. It is important for us to optimize the compositions of photopolymer because each composition of the photopolymer will influence its characters greatly. In this paper, we present the optimization of an acrylamidebased photopolymer containing BAA photopolymer. We obtained holograms with a diffraction efficiency of 60% and an energy exposure of 100 mJ/cm2 in the material using Ar-ion laser for recording at a power of 4 mW/cm2, and several holograms were recorded in the material.
Fig. 1. Schematic representation of the setup for the recording of grating. Ms: Mirrors; BS: beam splitter; H: holographic plate; D: detectors; SF: spatial filter.
formation of polyacrylamide in the regions of constructive interference. The reconstruction of the hologram was made at the Bragg angle with a LD laser tuned to 650 nm at real time for monitoring the evolution of the diffracted intensity of this beam as a function of time.
2. Experimental All the materials used in this work were of the best available grade and were used without further purification, and all the samples were prepared under normal laboratory conditions [22 C; relative humidity (RH), 40 – 60%] as follows. (1) Basic PVA (MW 1750) was dissolved in distilled water and then heated to 80 C to yield a PVA aqueous solution having 10 wt.% of PVA. (2) Crystals of Erythrosine B (ErB) were dissolved in water to obtain the desired dye concentration. (3) A certain volume of triethanolamine (TEA) was dissolved in water. (4) The TEA solution was added into the PVA solution along with the ErB solution. Finally, acrylamide (AA) and BAA were added as the monomers. Mixing the solution well, we got the photopolymeric solution and the total volume was approximately 25 ml. (5) By spreading 10 ml of the photopolymeric solution on a 12.710.2 cm leveled glass plate and natural drying at 36 – 48 h, a dry layer of 150-mm thickness was obtained. The basic setup used to record holographic gratings is shown in Fig. 1. Two 514-nm beams with a power intensity of 20 mW/cm2 from an Ar-ion laser overlap at a photosensitive plate, producing appropriate interference pattern at the plane of the material through the
3. Results and discussion 3.1. The photoreaction process and absorption spectra of the photopolymer A photoreduction reaction is produced when the material is illuminated with a light beam of 514 nm. The dye is excited to the high-energy state and then reacts with the coinitiator TEA, which is the electron donor, to produce a dye radical anion and a triethanolamine radical cation. The dye radical is not usually reactive enough to initiate polymerization but the TEA radical will react with an AA molecule and polymerization may then occur [12]. A possible mechanism for this process is shown in Fig. 2. The reaction occurred in PVA matrix and we consider that the PVA matrix is inert. We use Ar-ion laser for exposure that is determined by the dye in the photopolymer because the absorption peak of Erythrosine B is around 514 nm. 3.2. The exposure time and diffraction efficiency curve of the material With the increase of laser power, we can expose those materials that are not so sensitive. In this experi-
H. Yao et al. / Materials Letters 56 (2002) 3–8
Fig. 2. Scheme of the mechanism proposed for photopolymerization. DYE: Photoinitiator; DYE*: excited photoinitiator; DYE : anion derivative of dye; LDYE: leuco-dye; Am: amine; Am*: excited amine; Am +*: cation radical of amine; M: monomer; P: polymer; hn: energy of an incident photon; k, k V: reaction constant.
5
[13]. Therefore, we can know from Fig. 3 that at the first moment of exposure, local photopolymerization is proportional to the exposure energy. When the exposure energy is attained at 100 mJ/cm2, the index modulation will be saturated; with increasing the exposure energy, the diffraction efficiency will not increase. The reason why the diffraction efficiency decreases after the saturation of exposure might be caused by the cross-talk of holograms. Some of the main sources of noise are (1) surface deformation of the recording material, (2) random scatter caused by the granularity of the recording material, and (3) nonlinear recording of the signal wave. When the exposure increased, the cross talks increase much quicker resulting in more output signal being scattered, and the diffraction efficiency decreases.
ment, the exposure intensity was 20 mW/cm2, much higher than silver halide’s. After the measurement of the material’s exposure time and diffraction efficiency, we got the relational curve as shown in Fig. 3. From this figure, we can see that at the first moment, the diffraction efficiency of the photopolymer was proportional to the exposure time, but after a while, the efficiency got to a maximum; the diffraction time decreased with increasing exposure time. When the exposure energy reach to 100 mJ/cm2, we could obtain the maximum diffraction efficiency of 55%. In a photopolymer, the index modulation is proportional to the concentration of the monomer reacted
3.3. Optimization of the dye concentration
Fig. 3. The relationship between exposure time and diffraction efficiency. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; TEA: 0.38 M; ErB: 1.1310 4; PVA: 10%. The intensity used is 20 mW/cm2.
Fig. 4. Effect of the concentration of Erythrosine B on the holographic parameters in a holographic material. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; TEA: 0.38 M; PVA: 10%. The intensity used is 20 mW/cm2.
We measured the diffraction efficiency and sensitivity of the material by changing the concentration of the dye in the photopolymer. As shown in Fig. 4, we found that the polymer’s sensitivity increased when we increased the concentration of the dye. It is easy to understand that the more the dye molecules, the more the photons being absorbed, and the more become the excited states of the dye, the easier the triethanolamine is excited and the quicker is the monomer’s polymerization rate. However, as Fig. 4 shows, the sensitivity of the material decreases with increasing concentra-
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H. Yao et al. / Materials Letters 56 (2002) 3–8
tion of Erythrosine B when it is greater than 0.05 10 3 M. The reason for this is the possible competitive polymerization mechanism that is produced when high concentrations of the dye are used [14] because the dye forms dimeric forms, which is involved in a polymerization termination reaction, resulting in a decrease in the overall rate of polymerization. After those dye molecules participating in the polymerization have been consumed, the dimeric molecules of the dye will decompose; the monomers in the material will polymerize perfectly as long as the exposure time is long enough. The decomposition process of the dimeric dye influences the polymerization rate, leading to the decrease in the sensitivity of material. However, the diffraction efficiency changes little that can be seen from Fig. 4. The evidence of the formation of dimeric compound can be seen in Fig. 5 in which the increase in the Erythrosine B produces the apparition of a band at 490 – 510 nm, which is related to the formation of dimeric forms. 3.4. Optimization of the TEA concentration As Fig. 6 shows, the diffraction efficiency keeps almost constant with changing concentration of TEA, but the sensitivity is influenced greatly by the concentration of TEA. When the concentration of TEA is about 0.30 M, the material will have the highest sensitivity; either the increase or decrease of TEA concentration will decrease the sensitivity of the
Fig. 6. Effect of the concentration of TEA on the holographic parameters in a holographic material. The chemical composition of the material is AA: 0.21 M; BAA: 0.0324 M; ErB: 1.1310 4; PVA: 10%. The intensity used is 20 mW/cm2.
material. The concentration of TEA used in the material is much higher than the concentration of the dye. When the material is illuminated with a wavelength of 514 nm, which corresponds to the Erythrosine B’s absorption band, the dye molecules will be excited and become radicals, which react with TEA quickly. TEA becomes radical cation and then reacts with the monomer molecules and makes them polymerize. This is the process of photopolymerization. The dye molecules become bleached molecules [15] when the excited dye molecules reacted with TEA, so the photopolymers being illuminated are transparent. The reason for using high concentration of TEA is that TEA is a plasticizer and high concentration will improve the quality of the material, reduce the precipitation of the monomer on the surface of the film, and consequently, reduce the noise of the gratings as well. Too much TEA does not improve the holographic effect of the material but makes the film shrink greatly. When the concentration is 0.6 M, the shrinkage of the film is greater than 10%. 3.5. Optimization of the AA and BAA concentrations
Fig. 5. Transmittance spectra of the material when the concentration of Erythrosine B is changed. The solid curve represents an Erythrosine B concentration of 510 4 mol/l, the dashed curve, an Erythrosine B concentration of 510 3 mol/l.
We fixed BAA concentration in the material as 0.0324 M and changed the concentration of AA. By measuring the effect of the concentration of AA on the holographic parameters in the photopolymers, we got Fig. 8. We can see that when the concentration of AA is 0.1 M, the diffraction efficiency of the photopolymer is 20%, and with increasing concentration of AA, the diffraction efficiency is improved greatly.
H. Yao et al. / Materials Letters 56 (2002) 3–8
7
When the concentration of AA attains 0.30 M, maximum diffraction efficiency is achieved and remains approximately constant at higher concentrations of AA. The sensitivity of the material is improved with the increase of AAs concentration. However, it is impossible to increase the concentration of AA indefinitely because the compatibility and solubility of this monomer in the polymer film are limited. At high concentration of AA, this monomer precipitates on the surface of the film; this gives rise to the observation of noise gratings, which are produced by the interference of the scattered light of these solid particles with the incident laser beam in the process of hologram formation. Moreover, high concentration of AA makes the film shrink greatly. It is shown in Fig. 7 that when the concentration of AA is 0.30 M, the film has big diffraction efficiency. We kept the mole ratio of BAA to AA as 1:6, then changed the concentration of monomers in the material to observe the effect on the holographic parameters and got Fig. 8. At low concentrations of the monomers, the diffraction efficiency is very low; when the AA concentration is 0.05 M, the diffraction efficiency is less than 1%. However, with increasing AA concentration, the diffraction efficiency increases rapidly. When the AA concentration is 0.15 M, the diffraction efficiency will be 40%. At the same time, the sensitivity decreases continually from about 300 to 30 mJ/cm2; this fully shows that the increase of the concentration of monomers will increase the sensitiv-
ity of the photopolymer. In Fig. 7, the concentration of BAA is kept constant; when the concentration of AA is very low, the material still has relatively high diffraction efficiency. However, in Fig. 8, when the concentration of AA is low, the diffraction efficiency is also low. These testify the fact that BAA can ameliorate the characters of the material. BAA can speed-up the polymerization reaction of AA in the photopolymer, so the materials with high concentration of BAA have high diffraction efficiency and sensitivity at the same time, but too much BAA will lead to high shrinkage of the material.
Fig. 7. Effect of the concentration of AA on the holographic parameters in a holographic material. The chemical composition of the material is BAA: 0.0324 M; ErB: 1.1310 4 M; PVA: 10%; TEA: 0.38 M. The intensity used is 20 mW/cm2.
Fig. 9. The original images (left) and reconstructed images (right).
Fig. 8. Effect of the concentration of monomers on the holographic parameters in a holographic material. The chemical composition of the material is PVA: 10%; TEA: 0.38 M; ErB: 1.1310 4 M. The intensity used is 20 mW/cm2.
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H. Yao et al. / Materials Letters 56 (2002) 3–8
4. Holographic recording with the photopolymer Recording the picture in a material and then reconstructing the hologram are the primary principle of data storage in photopolymer. Using the photopolymers prepared, we successfully stored some holograms. The reconstruction was imaged onto a CCD and a computer was used to capture the images. The reconstructed and the original images are shown in Fig. 9. We can see that the SNR of the images is good and the distortion is small.
5. Conclusion The optimization and characteristics of an acrylamide-based polymeric film for holographic recording are presented. We found that each component has an optimal concentration. It is not the case that the higher the concentration, the better the holographic characters. It was also found that BAA influences the characters of acrylamide greatly and suitable use of BAA can ameliorate the characters of the photopolymer effectively. Using the photopolymers prepared in this work, we successfully stored some images and found that the distortion was small. This shows that our materials have good holographic recording properties.
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