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The shift of Bragg angular selectivity curve in darkness in glass-like photopolymer for holographic recording
Optical Materials 32 (2009) 261–265
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
The shift of Bragg angular selectivity curve in darkness in glass-like photopolymer for holographic recording Jian Wang, Xiudong Sun *, Suhua Luo, Yongyuan Jiang Department of Physics, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2009 Received in revised form 27 July 2009 Accepted 27 July 2009 Available online 26 August 2009 PACS: 42.40.Lx 42.40.My 42.70.Gi 42.70.Jk
a b s t r a c t The shift of Bragg angular selectivity curve after exposure was studied by recording single hologram and multiplexed holograms, respectively. The evolution of shift with probe time after recording, as well as the saturated shift with exposure energy, was investigated in detail. The mechanism of Bragg mismatch was analyzed. By optimizing parameters of material synthesis and holographic recording, the shift of Bragg matched angle was alleviated in PQ/PMMA photopolymer after hologram recording. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Bragg angular selectivity curve PQ/PMMA photopolymer
1. Introduction Photopolymer is a promising candidate as recording medium for holography application because of the advantages of high sensitivity, large dynamic range, durable access, low cost, etc. [1,2]. The Aprilis HMD Company and the Inphase Group have demonstrated its potential application in write-once read-many (WORM) holographic data storage systems [3,4]. Unfortunately, most photopolymers exhibit shrinkage during exposure because of the radical photopolymerization, which leads to the distortion of the retrieved patterns and limits its commercial applications [5–7]. In order to reduce volume shrinkage, several photopolymer systems have been proposed and investigated, including the doped system [8,9], organic–inorganic hybrid system [10–12], cationic ring-opening polymerization system [13,14], and the use of dendritic macromonomers [15]. However, these approaches include some demerits, such as the complex synthesis procedure in the cationic ring-opening polymerization system and severe light scattering in the nanoparticle-dispersed nanocomposite-based photopolymer. Phenanthrenequinone (PQ)-doped poly-methylmethacrylate (PMMA) photopolymer is a kind of attractive recording medium [16–19]. By separating monomer polymerization from photochemical reaction, the shrinkage coefficient of 10 5 * Corresponding author. Tel./fax: +86 451 86414129. E-mail address: [email protected] (X. Sun). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.07.023
has been achieved, which is much smaller than that of many commercial photopolymers (for example, Dupont HRF-150: 2.5–3%; Lucent: 0.35%; Polaroid CROP: 0.1%) [20]. Furthermore, the thickness of this polymer can be increased to several millimeters with good holographic performance. These characteristics are very beneficial for the volume holographic storage with mass capacity and high fidelity. Chemical analysis has presented the mechanism of photochemistry reaction [21]: the photoinitiator PQ is excited by the illumination light, resulting in the formation of radicals. These highly reactive PQ radicals react with nMMA (i.e. short PMMA chain, n P 1) oligomers and form PQ-nMMA molecules in illuminated zones. Thus, the spatial modulation of the refractive index is the result of the spatially non-uniform chemical reaction of PQ molecules. In addition, PQ molecules act as photoinitiator and initiate the polymerization of residual MMA to form nMMA oligomers. After exposure, the enhancement of grating strength with time has been studied. Shiuan-Huei Lin and Hsu [22] attributed the enhancement to the post diffusion of PQ and MMA molecules from the un-illuminated zones to the neighboring illuminated zones. Jose Mumbru et al. [23] analyzed the diffusion process of PQ molecules, and believed that the enhancement was due to the attachment of PQ radicals to either MMA or PMMA after recording. However, so far the variety of Bragg condition accompanying with the enhancement has seldom been studied after exposure, although it occurs in most photopolymers [24–26].
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J. Wang et al. / Optical Materials 32 (2009) 261–265
In this study, the method synthesizing PQ/PMMA photopolymer in our laboratory was presented. After recording patterns with various exposure energies, we measured the Bragg angular selectivity curves in different time intervals and analyzed the influences of Bragg angular selectivity curves shift on patterns retrieval, as well as the mechanism of the Bragg mismatch. Moreover, holograms were multiplexed by rotating material. The relation between Bragg angular selectivity curve shift and accumulative exposure energy was investigated. The approaches to alleviate Bragg mismatch were proposed by optimizing polymer synthesis and experimental parameters. 2. Experiments PQ/PMMA polymer is composed of PQ as sensitizer, MMA as polymerizable monomer and PMMA as matrix, respectively. The PMMA matrix is formed through polymerization reaction of MMA monomers by using thermal initiator azo-di-iso-butyro-nitrile (AIBN). Because some holographic characteristics are dependent on the chemical property of the polymer, the environmental parameters for the polymer synthesis are very important. The polymer is prepared by a typical radical polymerization reaction, which includes the steps of initiation, propagation and termination. For a typical process, 0.05 g AIBN and 0.12 g PQ are dried and dissolved in 10 g MMA solvent at the temperature of 55 °C, at which AIBN molecules can be thermal decomposed to radicals and nitrogen. The mixed solution is laid for about 5 h, including a stirring process about 30 min, in order to eliminate the nitrogen from the system. After this process, the solution is poured into a mold with various geometries and thickness and, then, the temperature is elevated to 85 °C for MMA monomers polymerization initiated by AIBN radicals. In order to avoid imploding, 15 min later the temperature is reduced to 55 °C and held for more than 48 h. In this case, the solution turns to solid accompanying with serious dimensional shrinkage. After polishing the polymer is ready to use in experiment. In order to characterize the Bragg condition in darkness, the mechanisms of photo-induced chemical reaction and gratings formation, as well as the experimental parameters during hologram recording, must be considered. Single holograms and multiplexed patterns were recorded in the prepared photopolymer with the thickness of 1.5 mm in order to investigate the Bragg condition, respectively. Fig. 1 showed the transmission geometry holographic recording setup. The 532 nm beam from a DPSS laser was split by polarized beam splitter (PBS) into two beams – signal beam and reference beam. In the signal beam path, a spatial light modulator (SLM) with the resolution of 800 600 pixels and a CCD with 1008 1018 pixels were used as input and output for the holograms. A hologram was recorded in PQ/PMMA sample at an intensity of 0.4 W/cm2 and exposure time of 30 s. After the hologram
SF
recorded, the shift of Bragg angular selectivity curve with time and the shift effect on the retrieved pattern were investigated. Moreover, the shift period of Bragg angular selectivity curve and the corresponding shift angle at different exposure energy were also investigated. In order to study the shift of Bragg angular selectivity curve in multiplexing storage, the sample was mounted on a rotation stage with rotation resolution of 0:0025 . By using the angular multiplexing of rotating material, 50, 100 and 150 holograms were multiplexed in a spot, respectively, with the same exposure energy of 0.25 J/cm2 and recording time interval 1.0 s. The angular separation between the adjacent holograms was 0:2 , which was larger than the angular selectivity in the experimental conditions. The evolutions of Bragg matched angle as the function of accumulative exposure energy in these spots were measured. Considered the damage effect of the probe beam on the recorded patterns, the power of reference beam was weakened during retrieving. The diffraction efficiency is defined as the ratio between the diffracted light intensity and the incident light intensity. 3. Results and discussion 3.1. Bragg condition of single hologram recorded Fig. 2 showed the experimental results of Bragg angular selectivity curve shift, in which the y-axis was the normalized diffraction efficiency and x-axis was the corresponding readout angles. The angle 0° corresponded to the reference angle during recording. From the results, we can see that the Bragg matched angle shifts to 0.02° after recording 30 min and to 0.035° after 60 min. About 90 min later, the shift reaches 0.0375° and it will be fixed after about the next 2 h. What is worthy to be emphasized that the Bragg angular width increases with time. While at the exposure energy below 3 J/cm2, there is no increase observed at the resolution of 0.0025° in 2 h. When a pattern with 800 600 pixels was retrieved in different probe time, the gray values of pixels in each column of the patterns were summed. The gray distribution in x direction from 200 to 800 pixels was shown in Fig. 3. It can be seen from Fig. 3 that the translation of the pattern in x direction is about 15 pixels after 60 min. Fig. 4 showed the time period of the Bragg shift to saturation at different exposure intensity, and Fig. 5 showed the corresponding saturated shift of Bragg matched angle. It can be seen from the results that the time period is in the range from 40 to 130 min. The exposure energy for saturation in our sample is approximately 20 J/cm2.
S1 PBS Laser (532nm)
SLM
Computer
S2
L1 Sample and RS L2
CCD Fig. 1. The transmission geometry holographic recording setup: PBS, polarized beam splitter; S1, S2, shutters; SF, space-filtering; SLM, spatial light modulator; L1, L2, Fourier lens; RS, rotation stage.
Fig. 2. Bragg angular selectivity curve at different time interval after exposure.
J. Wang et al. / Optical Materials 32 (2009) 261–265
263
The saturated shift of Bragg matched angle increases with exposure energy, and the maximal shift angle is approximately 0.0675°.
3.2. Bragg condition of multiplexed patterns recorded
Fig. 3. Gray distribution of retrieved pattern in x direction at different time interval after exposure.
Fig. 4. Time period of Bragg angular selectivity curve shift as the function of exposure energy.
Fig. 5. Saturated shift of Bragg matched angle as the function of exposure energy.
The Bragg angular selectivity curve shift in the spot multiplexed 50 holograms was shown in Fig. 6, in which the solid curve corresponded to the measurement immediately after exposure and the dotted curve was obtained 2 h later. From this results it can be seen that almost each pattern has a nearly uniform angular shift of 0.0850°, accompanying with the increase of pattern strength for each hologram. For the spots of 100 and 150 holograms multiplexed, the uniform shift was about 0.1375° and 0.1625°, respectively. These results indicate that the shift of Bragg matched angle increases with accumulative exposure energy in one location. The shift of Bragg angular selectivity curve with time can be explained by dark reaction (i.e. the attachment of PQ radicals to either MMA or PMMA in darkness) [23] and stress relaxation in polymer [27]. During pattern recording, PQ molecules are excited by illumination and turned into PQ radicals, which react with initiated nMMA (n P 1) radicals and form PQ-nMMA oligomers. These reactions produce shrinkage as the disappearance of intermolecular distance (for example, the size of PQ-MMA molecule is smaller than that of a PQ and a MMA molecule because of the substitution of intermolecular distances by the carbon–carbon bonds). Moreover, the structural changes of molecules in the polymer may result in the relaxation of the internal stress caused in the polymer synthesis process through the movement of polymer chains [27,28]. When exposure stops, the concentration of free radicals in the system does not decrease quickly, then, the chemical reaction of PQ and nMMA radicals will last for a time period, resulting in the shrinkage last until these radicals vanish. Furthermore, the internal stress is released with the dark reaction proceeding until a new stress balance is established a time period later [29,30]. Therefore, the Bragg mismatch takes place and the probe angle must be adjusted to satisfy the Bragg condition. At the irradiation wavelength of 532 nm, the absorbance is approximately 0.9 cm 1 before exposure. After exposure it increases to 3.8 cm 1 under the condition of recording single hologram. Considered the different light absorption along the direction of thickness according to the Lamber–Beer’s law, it is important to point out that Bragg mismatch may be different in the thickness direction. Fig. 7 shows the
Fig. 6. Shift of Bragg angular selectivity curve for the retrieved 30th, 31st, 32nd pattern.
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d
ΔL
Δd
Δθ
Fig. 8. Saturated shift of Bragg matched angle versus exposure energy in the novel sample.
Fig. 7. Schematic diagram for the hologram readout in the case of shrinkage and stress relaxation after recording transmission hologram.
schematic diagram for hologram readout in the case of shrinkage and stress relaxation. The variety of Bragg angular width can be used as an important parameter in order to analyze the mechanism of volume shrinkage. According to the coupled-wave theory of recording transmission hologram, the shrinkage of material thickness results in the Bragg angular width increase, on the other hand, the shrinkage of grating spacing leads to the width decrease. In this sense, it can be deduced from the experimental results that the thickness shrinkage is more serious than the grating spacing shrinkage in high exposure energy, while in low exposure energy below 3 J/cm2, the shrinkages of thickness and grating spacing get a balance to steady the Bragg angular width. In addition, higher exposure energy induces more PQ molecules in the exited state and more serious shrinkage as well as stress relaxation, thus the saturated shift of Bragg matched angle increases with exposure energy. When the reactive components in photopolymer are exhausted, the shift reaches the maximum. For multiplexed recording, the accumulation of exposure energy induces the internal stress cumulated in one spot. As a result, a uniform shift for the patterns is produced with the relaxation of stress. Moreover, when the exposure time increases, the Bragg mismatch occurs during exposure, as a result, the formed polymerization grating may not absolutely match the light intensity distribution. Furthermore, the increase of exposure time makes the difference between the formed polymerization grating and the light intensity distribution increase. Hence, unreacted PQ molecules, still present in the system, when initiated, can give origin to further free radicals polymerization on acrylate, that can induce the formation of nonsensical noise gratings by the subsequent exposure. This reduces the content of PQ component for the dark reaction. Therefore, the shift time period decreases when exposure energy is above 20 J/cm2, as seen in Fig. 4. Based on the above experiments and analysis, an approaches to decrease the Bragg mismatch can be proposed by optimizing the chemical composition of material and the holographic recording parameters. One approach is increasing molecular weight of polymer chains. Because the stress relaxation arises from the movement of polymer chains, increasing molecular weight may alleviate the molecule movement. According to Arrhenius theory
[31], the average molecular weight as well as the average molecular chain length of nMMA reactant and PMMA matrix can be increased by changing the MMA polymerization parameters. Based on this consideration, we modified the parameters of polymer synthesis. In the initiation process, the temperature was set to 80 °C, 25 min later the temperature was reduced to 60 °C. After about 30 min, the previous process was repeated to make a sufficient initiation. Using the new PQ/PMMA sample with the thickness of 1.5 mm, the relation of saturated shift of Bragg matched angle and exposure energy was obtained, as shown in Fig. 8. From which we can see that the maximal angle shift is 0.025° much smaller than 0.0675° as shown in Fig. 5. Another approach is recording weak grating, which will be beneficial to alleviate the shift of Bragg matched angle and the increase of Bragg angular width. As is well known, storage capacity of holographic storage is dependent on the Bragg angular width of holograms. The Bragg angular width increase will result in the decrease of storage capacity. Moreover, exposure energy accumulation as well as readdressing during patterns retrieving should be well considered especially in multiplexing holograms. A multiplexing technique which can provide equally stress relaxation for each recorded pattern is suitable for this photopolymer, such as peristrophic multiplexing combined with shift multiplexing technique. 4. Conclusions By recording transmission holograms in PQ/PMMA photopolymer, we studied the evolution of Bragg angular selectivity curve shift with time after exposure at different exposure energies. After a single hologram was recorded, the Bragg angular selectivity curve shifted with time. The saturated shift of Bragg matched angle increases with the increasing of exposure energy and reaches the maximal value at the exposure energy of 20 J/cm2 in our samples. When patterns were multiplexed, each hologram had a shift and the total shift of the multiplexed holograms in one location was a function of the accumulative exposure energy. The dark reaction as well as stress relaxation in polymer was responsible for the shift of the Bragg angular selectivity curve. Based on the mechanism of the Bragg mismatch, by increasing the average molecular weight of reactant and polymer matrix, the shift of Bragg angular selectivity curve decreased remarkably in PQ/PMMA photopolymer. Moreover, optimizing holographic recording parameters, involving recording weak grating, reducing cumulated exposure energy and
J. Wang et al. / Optical Materials 32 (2009) 261–265
readdressing during patterns retrieving, was also proposed to alleviate the Bragg mismatch. The results provide the approaches to alleviate Bragg mismatch and will be beneficial to the application of glass-like photopolymer in volume holographic storage.
Acknowledgements The research has been financially supported by the Fundamental Research Foundation of COSTIND (Grant No. 2320060089), the MOST of China (973 Project No. 2007CB307001) and Program of Excellent Team in Harbin Institute of Technology.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
The shift of Bragg angular selectivity curve in darkness in glass-like photopolymer for holographic recording Jian Wang, Xiudong Sun *, Suhua Luo, Yongyuan Jiang Department of Physics, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2009 Received in revised form 27 July 2009 Accepted 27 July 2009 Available online 26 August 2009 PACS: 42.40.Lx 42.40.My 42.70.Gi 42.70.Jk
a b s t r a c t The shift of Bragg angular selectivity curve after exposure was studied by recording single hologram and multiplexed holograms, respectively. The evolution of shift with probe time after recording, as well as the saturated shift with exposure energy, was investigated in detail. The mechanism of Bragg mismatch was analyzed. By optimizing parameters of material synthesis and holographic recording, the shift of Bragg matched angle was alleviated in PQ/PMMA photopolymer after hologram recording. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Bragg angular selectivity curve PQ/PMMA photopolymer
1. Introduction Photopolymer is a promising candidate as recording medium for holography application because of the advantages of high sensitivity, large dynamic range, durable access, low cost, etc. [1,2]. The Aprilis HMD Company and the Inphase Group have demonstrated its potential application in write-once read-many (WORM) holographic data storage systems [3,4]. Unfortunately, most photopolymers exhibit shrinkage during exposure because of the radical photopolymerization, which leads to the distortion of the retrieved patterns and limits its commercial applications [5–7]. In order to reduce volume shrinkage, several photopolymer systems have been proposed and investigated, including the doped system [8,9], organic–inorganic hybrid system [10–12], cationic ring-opening polymerization system [13,14], and the use of dendritic macromonomers [15]. However, these approaches include some demerits, such as the complex synthesis procedure in the cationic ring-opening polymerization system and severe light scattering in the nanoparticle-dispersed nanocomposite-based photopolymer. Phenanthrenequinone (PQ)-doped poly-methylmethacrylate (PMMA) photopolymer is a kind of attractive recording medium [16–19]. By separating monomer polymerization from photochemical reaction, the shrinkage coefficient of 10 5 * Corresponding author. Tel./fax: +86 451 86414129. E-mail address: [email protected] (X. Sun). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.07.023
has been achieved, which is much smaller than that of many commercial photopolymers (for example, Dupont HRF-150: 2.5–3%; Lucent: 0.35%; Polaroid CROP: 0.1%) [20]. Furthermore, the thickness of this polymer can be increased to several millimeters with good holographic performance. These characteristics are very beneficial for the volume holographic storage with mass capacity and high fidelity. Chemical analysis has presented the mechanism of photochemistry reaction [21]: the photoinitiator PQ is excited by the illumination light, resulting in the formation of radicals. These highly reactive PQ radicals react with nMMA (i.e. short PMMA chain, n P 1) oligomers and form PQ-nMMA molecules in illuminated zones. Thus, the spatial modulation of the refractive index is the result of the spatially non-uniform chemical reaction of PQ molecules. In addition, PQ molecules act as photoinitiator and initiate the polymerization of residual MMA to form nMMA oligomers. After exposure, the enhancement of grating strength with time has been studied. Shiuan-Huei Lin and Hsu [22] attributed the enhancement to the post diffusion of PQ and MMA molecules from the un-illuminated zones to the neighboring illuminated zones. Jose Mumbru et al. [23] analyzed the diffusion process of PQ molecules, and believed that the enhancement was due to the attachment of PQ radicals to either MMA or PMMA after recording. However, so far the variety of Bragg condition accompanying with the enhancement has seldom been studied after exposure, although it occurs in most photopolymers [24–26].
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J. Wang et al. / Optical Materials 32 (2009) 261–265
In this study, the method synthesizing PQ/PMMA photopolymer in our laboratory was presented. After recording patterns with various exposure energies, we measured the Bragg angular selectivity curves in different time intervals and analyzed the influences of Bragg angular selectivity curves shift on patterns retrieval, as well as the mechanism of the Bragg mismatch. Moreover, holograms were multiplexed by rotating material. The relation between Bragg angular selectivity curve shift and accumulative exposure energy was investigated. The approaches to alleviate Bragg mismatch were proposed by optimizing polymer synthesis and experimental parameters. 2. Experiments PQ/PMMA polymer is composed of PQ as sensitizer, MMA as polymerizable monomer and PMMA as matrix, respectively. The PMMA matrix is formed through polymerization reaction of MMA monomers by using thermal initiator azo-di-iso-butyro-nitrile (AIBN). Because some holographic characteristics are dependent on the chemical property of the polymer, the environmental parameters for the polymer synthesis are very important. The polymer is prepared by a typical radical polymerization reaction, which includes the steps of initiation, propagation and termination. For a typical process, 0.05 g AIBN and 0.12 g PQ are dried and dissolved in 10 g MMA solvent at the temperature of 55 °C, at which AIBN molecules can be thermal decomposed to radicals and nitrogen. The mixed solution is laid for about 5 h, including a stirring process about 30 min, in order to eliminate the nitrogen from the system. After this process, the solution is poured into a mold with various geometries and thickness and, then, the temperature is elevated to 85 °C for MMA monomers polymerization initiated by AIBN radicals. In order to avoid imploding, 15 min later the temperature is reduced to 55 °C and held for more than 48 h. In this case, the solution turns to solid accompanying with serious dimensional shrinkage. After polishing the polymer is ready to use in experiment. In order to characterize the Bragg condition in darkness, the mechanisms of photo-induced chemical reaction and gratings formation, as well as the experimental parameters during hologram recording, must be considered. Single holograms and multiplexed patterns were recorded in the prepared photopolymer with the thickness of 1.5 mm in order to investigate the Bragg condition, respectively. Fig. 1 showed the transmission geometry holographic recording setup. The 532 nm beam from a DPSS laser was split by polarized beam splitter (PBS) into two beams – signal beam and reference beam. In the signal beam path, a spatial light modulator (SLM) with the resolution of 800 600 pixels and a CCD with 1008 1018 pixels were used as input and output for the holograms. A hologram was recorded in PQ/PMMA sample at an intensity of 0.4 W/cm2 and exposure time of 30 s. After the hologram
SF
recorded, the shift of Bragg angular selectivity curve with time and the shift effect on the retrieved pattern were investigated. Moreover, the shift period of Bragg angular selectivity curve and the corresponding shift angle at different exposure energy were also investigated. In order to study the shift of Bragg angular selectivity curve in multiplexing storage, the sample was mounted on a rotation stage with rotation resolution of 0:0025 . By using the angular multiplexing of rotating material, 50, 100 and 150 holograms were multiplexed in a spot, respectively, with the same exposure energy of 0.25 J/cm2 and recording time interval 1.0 s. The angular separation between the adjacent holograms was 0:2 , which was larger than the angular selectivity in the experimental conditions. The evolutions of Bragg matched angle as the function of accumulative exposure energy in these spots were measured. Considered the damage effect of the probe beam on the recorded patterns, the power of reference beam was weakened during retrieving. The diffraction efficiency is defined as the ratio between the diffracted light intensity and the incident light intensity. 3. Results and discussion 3.1. Bragg condition of single hologram recorded Fig. 2 showed the experimental results of Bragg angular selectivity curve shift, in which the y-axis was the normalized diffraction efficiency and x-axis was the corresponding readout angles. The angle 0° corresponded to the reference angle during recording. From the results, we can see that the Bragg matched angle shifts to 0.02° after recording 30 min and to 0.035° after 60 min. About 90 min later, the shift reaches 0.0375° and it will be fixed after about the next 2 h. What is worthy to be emphasized that the Bragg angular width increases with time. While at the exposure energy below 3 J/cm2, there is no increase observed at the resolution of 0.0025° in 2 h. When a pattern with 800 600 pixels was retrieved in different probe time, the gray values of pixels in each column of the patterns were summed. The gray distribution in x direction from 200 to 800 pixels was shown in Fig. 3. It can be seen from Fig. 3 that the translation of the pattern in x direction is about 15 pixels after 60 min. Fig. 4 showed the time period of the Bragg shift to saturation at different exposure intensity, and Fig. 5 showed the corresponding saturated shift of Bragg matched angle. It can be seen from the results that the time period is in the range from 40 to 130 min. The exposure energy for saturation in our sample is approximately 20 J/cm2.
S1 PBS Laser (532nm)
SLM
Computer
S2
L1 Sample and RS L2
CCD Fig. 1. The transmission geometry holographic recording setup: PBS, polarized beam splitter; S1, S2, shutters; SF, space-filtering; SLM, spatial light modulator; L1, L2, Fourier lens; RS, rotation stage.
Fig. 2. Bragg angular selectivity curve at different time interval after exposure.
J. Wang et al. / Optical Materials 32 (2009) 261–265
263
The saturated shift of Bragg matched angle increases with exposure energy, and the maximal shift angle is approximately 0.0675°.
3.2. Bragg condition of multiplexed patterns recorded
Fig. 3. Gray distribution of retrieved pattern in x direction at different time interval after exposure.
Fig. 4. Time period of Bragg angular selectivity curve shift as the function of exposure energy.
Fig. 5. Saturated shift of Bragg matched angle as the function of exposure energy.
The Bragg angular selectivity curve shift in the spot multiplexed 50 holograms was shown in Fig. 6, in which the solid curve corresponded to the measurement immediately after exposure and the dotted curve was obtained 2 h later. From this results it can be seen that almost each pattern has a nearly uniform angular shift of 0.0850°, accompanying with the increase of pattern strength for each hologram. For the spots of 100 and 150 holograms multiplexed, the uniform shift was about 0.1375° and 0.1625°, respectively. These results indicate that the shift of Bragg matched angle increases with accumulative exposure energy in one location. The shift of Bragg angular selectivity curve with time can be explained by dark reaction (i.e. the attachment of PQ radicals to either MMA or PMMA in darkness) [23] and stress relaxation in polymer [27]. During pattern recording, PQ molecules are excited by illumination and turned into PQ radicals, which react with initiated nMMA (n P 1) radicals and form PQ-nMMA oligomers. These reactions produce shrinkage as the disappearance of intermolecular distance (for example, the size of PQ-MMA molecule is smaller than that of a PQ and a MMA molecule because of the substitution of intermolecular distances by the carbon–carbon bonds). Moreover, the structural changes of molecules in the polymer may result in the relaxation of the internal stress caused in the polymer synthesis process through the movement of polymer chains [27,28]. When exposure stops, the concentration of free radicals in the system does not decrease quickly, then, the chemical reaction of PQ and nMMA radicals will last for a time period, resulting in the shrinkage last until these radicals vanish. Furthermore, the internal stress is released with the dark reaction proceeding until a new stress balance is established a time period later [29,30]. Therefore, the Bragg mismatch takes place and the probe angle must be adjusted to satisfy the Bragg condition. At the irradiation wavelength of 532 nm, the absorbance is approximately 0.9 cm 1 before exposure. After exposure it increases to 3.8 cm 1 under the condition of recording single hologram. Considered the different light absorption along the direction of thickness according to the Lamber–Beer’s law, it is important to point out that Bragg mismatch may be different in the thickness direction. Fig. 7 shows the
Fig. 6. Shift of Bragg angular selectivity curve for the retrieved 30th, 31st, 32nd pattern.
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d
ΔL
Δd
Δθ
Fig. 8. Saturated shift of Bragg matched angle versus exposure energy in the novel sample.
Fig. 7. Schematic diagram for the hologram readout in the case of shrinkage and stress relaxation after recording transmission hologram.
schematic diagram for hologram readout in the case of shrinkage and stress relaxation. The variety of Bragg angular width can be used as an important parameter in order to analyze the mechanism of volume shrinkage. According to the coupled-wave theory of recording transmission hologram, the shrinkage of material thickness results in the Bragg angular width increase, on the other hand, the shrinkage of grating spacing leads to the width decrease. In this sense, it can be deduced from the experimental results that the thickness shrinkage is more serious than the grating spacing shrinkage in high exposure energy, while in low exposure energy below 3 J/cm2, the shrinkages of thickness and grating spacing get a balance to steady the Bragg angular width. In addition, higher exposure energy induces more PQ molecules in the exited state and more serious shrinkage as well as stress relaxation, thus the saturated shift of Bragg matched angle increases with exposure energy. When the reactive components in photopolymer are exhausted, the shift reaches the maximum. For multiplexed recording, the accumulation of exposure energy induces the internal stress cumulated in one spot. As a result, a uniform shift for the patterns is produced with the relaxation of stress. Moreover, when the exposure time increases, the Bragg mismatch occurs during exposure, as a result, the formed polymerization grating may not absolutely match the light intensity distribution. Furthermore, the increase of exposure time makes the difference between the formed polymerization grating and the light intensity distribution increase. Hence, unreacted PQ molecules, still present in the system, when initiated, can give origin to further free radicals polymerization on acrylate, that can induce the formation of nonsensical noise gratings by the subsequent exposure. This reduces the content of PQ component for the dark reaction. Therefore, the shift time period decreases when exposure energy is above 20 J/cm2, as seen in Fig. 4. Based on the above experiments and analysis, an approaches to decrease the Bragg mismatch can be proposed by optimizing the chemical composition of material and the holographic recording parameters. One approach is increasing molecular weight of polymer chains. Because the stress relaxation arises from the movement of polymer chains, increasing molecular weight may alleviate the molecule movement. According to Arrhenius theory
[31], the average molecular weight as well as the average molecular chain length of nMMA reactant and PMMA matrix can be increased by changing the MMA polymerization parameters. Based on this consideration, we modified the parameters of polymer synthesis. In the initiation process, the temperature was set to 80 °C, 25 min later the temperature was reduced to 60 °C. After about 30 min, the previous process was repeated to make a sufficient initiation. Using the new PQ/PMMA sample with the thickness of 1.5 mm, the relation of saturated shift of Bragg matched angle and exposure energy was obtained, as shown in Fig. 8. From which we can see that the maximal angle shift is 0.025° much smaller than 0.0675° as shown in Fig. 5. Another approach is recording weak grating, which will be beneficial to alleviate the shift of Bragg matched angle and the increase of Bragg angular width. As is well known, storage capacity of holographic storage is dependent on the Bragg angular width of holograms. The Bragg angular width increase will result in the decrease of storage capacity. Moreover, exposure energy accumulation as well as readdressing during patterns retrieving should be well considered especially in multiplexing holograms. A multiplexing technique which can provide equally stress relaxation for each recorded pattern is suitable for this photopolymer, such as peristrophic multiplexing combined with shift multiplexing technique. 4. Conclusions By recording transmission holograms in PQ/PMMA photopolymer, we studied the evolution of Bragg angular selectivity curve shift with time after exposure at different exposure energies. After a single hologram was recorded, the Bragg angular selectivity curve shifted with time. The saturated shift of Bragg matched angle increases with the increasing of exposure energy and reaches the maximal value at the exposure energy of 20 J/cm2 in our samples. When patterns were multiplexed, each hologram had a shift and the total shift of the multiplexed holograms in one location was a function of the accumulative exposure energy. The dark reaction as well as stress relaxation in polymer was responsible for the shift of the Bragg angular selectivity curve. Based on the mechanism of the Bragg mismatch, by increasing the average molecular weight of reactant and polymer matrix, the shift of Bragg angular selectivity curve decreased remarkably in PQ/PMMA photopolymer. Moreover, optimizing holographic recording parameters, involving recording weak grating, reducing cumulated exposure energy and
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readdressing during patterns retrieving, was also proposed to alleviate the Bragg mismatch. The results provide the approaches to alleviate Bragg mismatch and will be beneficial to the application of glass-like photopolymer in volume holographic storage.
Acknowledgements The research has been financially supported by the Fundamental Research Foundation of COSTIND (Grant No. 2320060089), the MOST of China (973 Project No. 2007CB307001) and Program of Excellent Team in Harbin Institute of Technology.
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