Phenanthrenequinone-doped copolymers for
holographic data storage
Shiuan Huei Lin
National Chiao Tung University Department of Electrophysics 1001 Ta Hsueh Road Hsinchu, Taiwan 300 Taiwan E-mail: [email protected] Po-Lin Chen June-Hwa Lin
National Chiao Tung University Department of Photonics & Institute of
Electro-Optical Engineering 1001 Ta Hsueh Road
Hsinchu, Taiwan 300 Taiwan
Abstract. Two new types of phenanthrenequinone-doped poly 共methyl-methacrylate兲 共PQ/PMMA兲 are proposed and fabricated for volume ho-lographic recording. In these materials, the matrix of PQ/PMMA is re-placed by the copolymers, which are composed of either poly共trimethylolpropane triacrylate-co-methyl methacrylate兲 or poly共2-phenoxyethyl acrylate-co-methyl methacrylate兲, respectively. With the chemical analyses of the response of these materials with respect to light exposure, the physical mechanism of the holographic recording in those copolymer samples is investigated. In addition, the holographic characteristics, including the dynamic range and sensitivity, have been measured. The experimental results demonstrate that the modification of the monomer components can enhance holographic characteristics of PQ/PMMA.
© 2009 Society of Photo-Optical Instrumentation Engineers. 关DOI: 10.1117/1.3099713兴
Subject terms: holographic data storage; holographic materials; photopolymer;
PMMA; copolymer.
Paper 080679R received Aug. 30, 2008; revised manuscript received Jan. 7, 2009; accepted for publication Jan. 8, 2009; published online Mar. 31, 2009.
1 Introduction
Holographic data storage has been considered a promising information storage technology because of its large storage capacity and inherent property of parallel data recording and retrieval.1 It is known that in order to achieve high storage density, thousands of data pages should be super-imposed in the same spatial location of a storage volume by a multiplexing scheme using Bragg selectivity of thick holograms.2 To accomplish this type of volume hologram recording, a recording material of several-millimeters thickness that can produce high photoinduced refractive in-dex change is required. Furthermore, dimensional stability of the volume hologram is necessary. Otherwise, the mate-rial shrinkage during holographic recording will induce a mismatch of Bragg condition for volume holograms such that the recorded information cannot be retrieved completely.3 Several photopolymer materials with low photoinduced shrinkage have been developed for write-once materials.4–8 In our laboratory, we have developed a technique for fabricating phenanthrenequinone-doped poly共methyl-methacrylate兲 共PQ/PMMA兲 photopolymer that is 1-cm thick and has negligible photoinduced shrinkage.9,10 Holographic experiments using these PQ/ PMMA samples have showed that the coefficient of photo-induced shrinkage in this material is less than 10−5, so our
samples are especially attractive for volume hologram applications.11
However, because of the solubility of the doped element, in this case PQ, in the polymer matrix is low, the concen-trations of chemical elements for holographic recording are limited. Thus, when compared with other materials, the drawback in our PQ/PMMA is lower sensitivity and smaller
dynamic range of recording. To further improve the mate-rial, investigations on the physical mechanism of holo-graphic recording in PQ/PMMA have been performed.12–15 The investigations reveal that under illumination, photons excite the quinone double bond on the carbonyl functional group of PQ molecules, and PQ molecules become radicals. Then, the radical reacts with the carbonic double bond on the vinyl functional group on the MMA molecule to form a new compound. In this chemical reaction, the double bond of the vinyl group has been converted to a single bond such that the change in bond order contributes to a significant change in molar refraction and then the refractive index of material is changed, as indicated by Tomlinson and Chandross.16 They estimated the refractive change for the vinyl monomers to be an order of magnitude of 10−2. Thus,
a refractive-index pattern of the material follows that of the light intensity, and a phase hologram is then formed.
In this paper, we apply this knowledge to improve the PQ/PMMA photopolymer. Our strategy is to enhance the combination probability of monomer molecules with PQ radicals. This may be achieved by increasing the number of vinyl functional groups on the monomer, and/or by chang-ing the side functional group of the vinyl group on the monomer. To accomplish this goal, we chose two different monomers, trimethylolpropane triacrylate共TMPTA兲, which has three vinyl groups, and 2-phenoxyethyl acrylate共PEA兲, which has an additional benzene side functional group. Fig-ure 1 shows their chemical formulas. However, these two monomers do not easily form a hard polymer matrix.17 Thus, a copolymer technique is used here. Each of the two monomers forms a copolymer matrix system with MMA at a weight ratio of 4:1. Thus, two novel PQ-doped copolymer samples have been fabricated, both with 2-mm thickness.
In Sec. 2, we first describe the fabrication of these two copolymer materials. Then, chemical analyses of the
samples before and after light exposure are investigated, and a physical mechanism of holographic recording is pre-sented. Section 3 gives the holographic recording charac-teristics, including dynamic range and sensitivity, of differ-ent samples. In Sec. 4, we discuss the experimdiffer-ental results and propose a modified fabrication procedure of the sample to further enhance holographic characteristics of the mate-rials. The results show that an improvement of holographic characteristics of PQ/PMMA can be achieved through the copolymer technique. Section 5 gives conclusions.
2 Material Fabrication and Analyses
2.1 Sample Fabrications
First, saturated concentration 共⬃0.7 wt%兲 of photosensi-tizer, PQ, and 1 wt% of thermal initiator, 2,2-azo-bis-isobutyrolnitrile 共AIBN兲 molecule, were dissolved in a mixture solution of monomer liquid, which is a mixture of monomer components for the copolymer matrix, MMA plus PEA, or MMA plus TMPTA. These monomer compo-nents were mixed at a weight ratio of 4 to 1: MMA to PEA= 4 : 1 and MMA to TMPTA= 4 : 1. The resulting solu-tion was placed in an ultrasonic water bath at 30 ° C for 1 h, until all the components were dissolved completely. The solution was purified to remove the undissolved par-ticles so as to reduce light scattering centers. The purified solution was stirred共using a magneto stirrer兲 in an open-end glass bottle at 30 ° C for an appropriate period of time. At this fabrication step, one could observe that the liquid solution became thicker and thicker as a result of the poly-merization reaction. Thus, the viscosity of the solution is a good indicator of the degree of polymerization. Viscosity measurements with a typical capillary instrument provide useful information for the decision of when to proceed to the step for molding. Table1lists the stirring time for dif-ferent mixtures, respectively, to reach a similar viscosity. As illustrated in Table1, the solution mixture of MMA and TMPTA needs the shortest stirring time. It implies that more vinyl groups in TMPTA molecules can accelerate the
polymerization rate. On the other hand, we observe that the solution mixture of MMA and PEA needs the longest stir-ring time. This indicates that the benzene side functional group on the PEA monomer may not be easily infixed into the copolymer matrix.
After the stirring, the viscous solution was poured into a glass container with a 2-mm thick spacer. The thickness and geometrical shape of the sample was determined by the geometry of this container. The sample was then baked at 45 ° C for 72 h, until most of the monomers were polymer-ized, and then the liquid sample had been transferred into a self-sustained solid block. The sample could be removed from the mold for optical and holographic characteriza-tions. Hereafter, the samples of PQ doped in the polymer system of PMMA, the copolymer system of TMPTA and MMA, and the copolymer system of PEA and MMA are named as PQ/PMMA, PQ/poly-共TMPTA-co-MMA兲, and PQ/poly-共PEA-co-MMA兲 photopolymers, respectively.
2.2 Analyses on Photochemical Reaction
To understand the photoinduced chemical reactions in these samples, we performed mass spectrum measurement of the samples before and after an optical exposure. These spectra provide information about the molecular weight distribu-tion of the compounds in material and will reveal whether there is any new compound produced by the light exposure. Because our photopolymers were sensitive to green light, the samples were illuminated with a 514-nm argon laser beam until they were optically saturated共which means be-coming transparent兲. Figure 2 shows mass spectra of the three samples. As can be seen in Fig.2共a兲, a new charac-teristic peak signal at 308 by molecular weight appears af-ter exposure. Because the molecular weights of PQ and MMA are 208 and 100, respectively, the appearance of peak signal at 308 gives evidence that the photoproduct was formed by attachment of one MMA molecule to one PQ molecule in PQ/PMMA, as was pointed out in our pre-vious work.14
Figure2共b兲shows the measurement result for PQ/poly-共TMPTA-co-MMA兲 sample. In the mass spectrum of the unexposed sample, we cannot find a peak signal at around 296, which is the molecular weight of TMPTA molecule. This indicates that in PQ/poly-共TMPTA-co-MMA兲 only one species of residual monomer, MMA is left in this poly-mer matrix. This can be attributed to the high activity pro-vided by the three vinyl groups, such that almost all TMPTA monomers participated to form the polymer matrix during sample fabrication. Thus, there will be only residual MMA monomers for holographic recording. On the other hand, for the exposed sample in the large molecular weight
O C H2 H2 C O C O CH CH2
2-Phenoxyethyl Acrylate (PEA)
H2C CH C O O H2 C C CH2 CH2 H2 C O C C H O CH2 CH3 O C O H C H2C
Trimethylolpropane Triacrylate (TMPTA)
Fig. 1 Chemical structures of doped monomers for our
PMMA-based photopolymer: TMPTA and PEA.
Table 1 The stirring time for different mixtures to reach a similar
viscosity.
Name Stirring time共h兲
PQ/PMMA 24
PQ/poly-共PEA-co-MMA兲 48
regime 共⬎300兲, only one new peak at 308 appears. This indicates that the photochemical reaction that occurred in
the PQ/poly-共TMPTA-co-MMA兲 sample is only the attach-ment between one MMA molecule to one PQ molecule, which is the same as that in the PQ/PMMA sample.
In Fig.2共c兲, two peaks at 99 and 192 appear in the mass spectrum of the PQ/poly-共PEA-co-MMA兲 sample before exposure. This indicates that two kinds of residual mono-mer, MMA and PEA, are left in the fabricated PQ/poly-共PEA-co-MMA兲 sample. Furthermore, it can also be ob-served that several new peaks appear after exposure. Among them, signals at 308 and 400 are interesting for identifying the photoproduct. Because the molecular weight of PEA is 192, this result indicates that two kinds of the photoinduced attachments occur during light exposure: one PQ molecule to one residual MMA monomer and one PQ molecule to one PEA monomer.
Accordingly, Fig.2reveals an important message about light-induced chemical reaction in these three samples: a new compound has been formed, of which the molecular weight equals to the summation of one PQ plus one re-sidual monomer. This photoproduct can be attributed to the attachment of one PQ molecule to one residual monomer molecule. On the other hand, there is no indication about the participation of the polymer matrix on the photochemi-cal reaction. This implies that the physiphotochemi-cal mechanism of holographic recording in all these samples follows our de-sign strategy:10light exposure will not cause any change to the polymer matrix. Thus, samples could retain an impor-tant characteristic for volume holographic recording: mini-mum photochemical-shrinkage effect.
3 Holographic Characterizations
3.1 Holographic Recording in the Samples
To test the hologram recording capability of our photopoly-mer samples, we have recorded a plane-wave hologram in each sample. A s-polarized beam from an argon laser with wavelength 514 nm was collimated and split into two beams. Each beam had the intensity of 30 mW/cm2, and
the two beams were incident into the sample symmetrically with an intersection angle of 30 deg in the air. During re-cording, the diffraction efficiency, which is defined as the ratio of the diffracted intensity to the summation of dif-fracted and transmitted intensities, was measured in real time by use of a weak helium-neon laser illuminated at the Bragg-matched angle. Diffraction efficiency for each sample was plotted as a function of exposure energy, as shown in Fig.3. As indicated by the curve with symbol䊏, it is seen that the diffraction efficiency of PQ/PMMA first reaches 100% at the exposure energy of 2.5 J/cm2and then
it begins to drop with further exposure. The drop-off in diffraction efficiency is attributed to two effects: the back-ward coupling of the diffracted beam to the transmitted beam in the beginning and the fanning of noise gratings further. These noise gratings were formed by scattering in the material and then took out the power of the incident beam such that the diffraction efficiency of the hologram is reduced.13 In this case, the refractive index change ⌬n of grating calculated from diffraction efficiency is meaningful in the beginning regime of the hologram recording. It is estimated to be 1.27⫻10−4when the diffraction efficiency reaches to 100%.
Copolymer samples PQ/poly-共TMPTA-co-MMA兲 and
m/z 40 80 120 160 200 240 280 320 360 400 440 0 % 43 41 69 59 55 100 99 85 82 179 151 163 222 208 181 308 PQ/PMMA After exposure m/z 0 % 69 43 41 39 31 59 54 8599 180 75 100 153 111 126 157 208210 40 80 120 160 200 240 280 320 360 400 440 PQ/PMMA Before exposure PQ MMA PQ-MMA 100 100 PQ/Poly-(TMPTA-co-MMA) Before exposure PQ/Poly-(TMPTA-co-MMA) After exposure m/z 40 80 120 160 200 240 280 320 360 400 440 0 100 % 69 41 54 59 100 99 85 7697 180 152 151 208210 m/z 40 80 120 160 200 240 280 320 360 400 440 0 100 % 69 41 55 43 4459 180 100 99 85 102 127 152 209 308 MMA PQ PQ-MMA m/z 40 80 120 160 200 240 280 320 360 400 440 0 100 % 99 41 39 37 69 55 5159 70 94 7785 100 299 222 163 120 113 138149 180199209 234 253271284291308328 401 392 400 m/z 40 80 120 160 200 240 280 320 360 400 440 99 55 41 39 51 69 68 65 77 94 91 192 180 152 107120 150 208 194 211 299 266 285 100 0 % PQ/Poly-(PEA-co-MMA) Before exposure PQ/Poly-(PEA-co-MMA) After exposure PEA PQ PQ-PEA PQ-MMA MMA (a) (b) (c)
Fig. 2 The mass spectra of unexposed and exposed states of共a兲
PQ/PMMA sample,共b兲 PQ/poly-共TMTPA-co-MMA兲 sample, and 共c兲
PQ/poly-共PEA-co-MMA兲 sample. Fabricated using the original
PQ/poly-共PEA-co-MMA兲 both exhibit similar temporal be-haviors of diffraction efficiency, as indicated in Fig. 3 by curves with symbols䉭 and 䊊, respectively. It is seen that both the copolymers possess a larger rising slope during the first regime of hologram recording 共exposure ⬍2 J/cm2兲.
Thus, addition of either TMPTA or PEA monomer mol-ecules into the PQ/PMMA photopolymer could help to im-prove the recording sensitivity of the material. Among them, the PQ/poly-共PEA-co-MMA兲 sample has faster holo-graphic recording speed.
3.2 Characterizing Material for Holographic Data Storage
The M-number共M#兲 and sensitivity 共S兲 are the two typical parameters that have been used to characterize a holo-graphic recording media for data storage.18,19The two pa-rameters can be obtained experimentally by multiplexing a series of plane-wave holograms at a single spot in the sample until the material is chemically exhausted. After recording, the diffraction efficiency of each hologram was measured. The summation of the square roots of the dif-fraction efficiency of each hologram, named as the cumu-lative grating strength, forms a running curve of cumucumu-lative refractive-index change function, C共E兲=兺i=1
n
冑
i, where n is the total number of holograms that have been recorded at the cumulative exposure energy E. The saturation value of the cumulative grating strength gives M#. This running curve also indicates the build-up dynamics of the multiple hologram recording. According to the definition, the mate-rial sensitivity can be represented as change of the cumu-lative refractive index change divided by the corresponding exposure energy of that hologram, S
=兩⌬C共E兲/⌬E兩one hologram.19
We have recorded 175 holographic exposures at one spot of each material with a peristrophic multiplexing system,20 in which each exposure had equal exposure energy 0.23 J/cm2. The running curves of cumulative
refractive-index change for different samples are shown in Fig. 4. If we perform a curve fitting by the function: C共E兲 = Csatexp关1−共E/E兲兴, then the M# 共i.e., Csat= M#兲 and an
exposure energy constant E of the material are obtained. From these results, sensitivity of the fresh samples can be found to be given by:
S =
冏
dC共E兲 dE冏
E→0= M#
E .
Using the above formulas, the material M# and Eof the three samples are obtained and listed in Table2. First, it can be seen in the table that the M# of PQ/poly-共PEA-co-MMA兲 is 4.05, which is 1.5 times larger than that of PQ/ PMMA 共M#⬃2.85兲. On the other hand, the M# of PQ/ poly-共TMPTA-co-MMA兲 is slightly less than that of PQ/ PMMA. Then, as shown in the last column in Table2, S of the copolymers are 0.39 and 0.61 cm2/J, which have been
improved from that of PQ/PMMA by about 1.3 and 2 times, respectively. The results indicate that by adding additional monomer components to form the copolymer structure, ho-lographic recording characteristics of a PQ-doped photo-polymer can be improved.
4 Discussion
As was presented in the previous paragraph, both M# and S have been improved in PQ/poly-共PEA-co-MMA兲, whereas in PQ/poly-共TMPTA-co-MMA兲, only S had been improved slightly and M# was even reduced on the contrary. The experimental results seem to be not completely consistent
0 1 2 3 4 5 6 7 8 9 0 10 20 30 40 50 60 70 80 90 100 Di ff ra ct io n E ff ic ie n cy (% ) Exposure Energy (J/cm2) PQ/PMMA PQ/Poly-(TMPTA-co-MMA) PQ/Poly-(PEA-co-MMA)
Fig. 3 The dynamics of holographic recording in the three samples.
0 10 20 30 40 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 PQ/PMMA PQ/Poly-(PEA-co-MMA) PQ/Poly-(TMPTA-co-MMA)
Cumulative Exposure Energy (J/cm2)
Cumulat iv e g ra ti n g st re n g th 2
Fig. 4 The dynamics of multiple-hologram recording in the three
samples.
Table 2 The fitting parameters of the running curve in the three
samples.
Name PQ/PMMA PQ/Poly-共PEA-co-MMA兲 PQ/Poly-共TMPTA-co-MMA兲
M# 2.85 4.05 2.75
E共J/cm2兲 9.17 6.61 7.02
with our design strategy. However, this behavior can be understood by inspecting the mass spectrum of the copoly-mer samples, shown in Fig.2.
In Fig.2共c兲, mass spectra peaks appear at 99, 192, and 208 before optical exposure, indicating the availability of MMA, PEA, and PQ molecules for holographic recording. In the spectrum of sample after exposure, presence of peak signals at 308 and 400 indicates the occurrence of photoin-duced chemical reactions of one PQ plus one MMA, and one PQ plus one PEA, respectively. This behavior is con-sistent with our design strategy of providing an additional functional group PEA for enhancing the photochemical re-actions. This could explain the improvements of both M# and S in PQ/poly-共PEA-co-MMA兲.
On the other hand, in Fig.2共b兲, the characteristic peak of TMPTA monomer at 293 was not observed before sure, whereas a peak signal at 308 appears only after expo-sure. This implies that only PQ and MMA and no TMPTA are available in the sample before exposure, and indeed one PQ plus one MMA was the only photochemical reaction that occurred during the optical exposure. Thus, there were no residual TMPTA monomers in the copolymer sample, so that the holographic recording of this material was not im-proved.
These results indicate that to realize our design strategy of enhancing the photochemical reaction via providing more active functional groups, we have to improve the fab-rication technique. Further, if a certain portion of TMPTA monomers can be retained in the copolymer matrix, then stronger photochemical reactions between PQ, MMA, and TMPTA can be obtained, and thus M# and S might be improved accordingly.
4.1 Copolymer Sample Fabricated With a Modified Procedure
We proposed to modify the fabrication procedure so that a certain portion of additional highly active monomers such TMPTA and PEA can be retained unpolymerized. These residual monomers could be used for the photochemical reaction during holographic recording. Our method is to control timing for adding PEA or TMTPA molecules to the solution during sample preparation.
With the modified fabrication procedure, we first mixed up the saturated concentration of photosensitizer, PQ, 1 wt% of thermal initiator, AIBN molecule and liquid MMA monomers. The resulting solution was stirred at 30 ° C for 1 h until all components were dissolved com-pletely. The solution continued to be stirred at 30 ° C for another 24 h until it become viscous. At the same time, we prepared the PEA共or TMPTA兲 monomer solution with a saturated concentration of PQ molecules. The two solutions were mixed and stirred. The weight ratio of MMA and PEA 共or TMTPA兲 is controlled to be 4:1. The mixed solution was poured into a glass container with a 2-mm thick spacer. It was baked at 45 ° C for 72 h to form a bulk sample. For comparison, we also prepared a PQ/PMMA sample with the same procedure.
4.2 Mass Spectra Measurement
To analyze the species of the residual monomers and the photoproducts, we measured the mass spectra of the new samples before and after light exposure. Figure 5共a兲
illus-trates experimental results for PQ/poly-共PEA-co-MMA兲. We observe that characteristic peaks of PQ, PEA, and MMA molecules appear at 208, 192, and 100, respectively, in the unexposed sample. It indicates that these three com-ponents are available in the unexposed sample. The bottom plot in Fig. 5共a兲 shows that characteristic peaks appear at 308 and 400 in the exposed sample, which correspond to the molecular weights of one PQ plus one MMA, and one PQ plus one PEA, respectively. Thus, the photoproducts in this sample are formed by the attachment of one PQ to either one MMA or one PEA molecule, which is the same as that in the old sample.
Similarly, in Fig.5共b兲, for PQ/poly-共TMPTA-co-MMA兲 it can be seen that three characteristic peaks appear at 100, 208, and 293 in the unexposed sample. The molecular weights of 100 and 208 correspond to MMA and PQ mol-ecules, respectively. Because the molecular weight of TMPTA is 296, the peak at 293 should correspond to the TMTPA by losing three hydrogen molecules. Thus, PQ
40 80 120 160 200 240 280 320 400 0 100 40 80 120 160 200 240 280 320 360 400 440 99 69 55 41 39 51 65 77 94 79 100 120 107126138151 180192 208 % PQ/Poly-(PEA-co-MMA) Before exposure m/z PQ/Poly-(PEA-co-MMA) After exposure 360 440 0 100 % 39 68 55 54 42 59 77 94 85 100 180 152 120 107 138 163 192 209222 299308 223 41 99 69 391 400 m/z PQ PEA MMA PQ-MMA PQ-PEA 40 80 120 160 200 240 280 320 360 400 440 480 520 m/z 0 100 % 69 41 39 55 100 70 85 82 97 149 148 127 102 293 180 167 279 181 418 347 308 344 334 361375 389 40 80 120 160 200 240 280 320 360 400 440 480 m/z 0 100 % 69 41 32 44 5557 180 149 100 85 75 98101126 152167 208 181 210 293 PQ/Poly-(TMPTA-co-MMA) After exposure PQ/Poly-(TMPTA-co-MMA) Before exposure TMPTA PQ MMA PQ-MMA PQ-TMPTA (a) (b)
Fig. 5 The mass spectra of unexposed and exposed states of共a兲
PQ/poly-共PEA-co-MMA兲 samples, and 共b兲
molecules and MMA and TMPTA monomers all exist in the unexposed sample. For the exposed sample, shown in the bottom plot of Fig.5共b兲, it can be seen that a peak signal at 308 appears, which indicates the photochemical attachment of one PQ and one MMA molecule. However, there is no signal at 501 to directly show the attachment of PQ and TMPTA molecules. Instead, we see lots of new peaks ap-pear from 320 to 418. The peak located at 418 can be attributed to be the photoproducts of PQ and TMPTA by losing one acrylate group, and other peaks might belong to the fragment of them. This indicates that, compared with the samples fabricated with our conventional two-step pro-cedure shown in Sec. 2.1, the new fabrication propro-cedure produces different chemical components in the PQ/poly-共TMPTA-co-MMA兲 sample. One more point to be noticed is that the peak of PQ disappears in the exposed PQ/poly-共TMPTA-co-MMA兲 sample. This phenomenon has not been observed in Fig.2, showing that all the PQ molecules are totally chemically exhausted by optical exposure in this new sample.
Therefore, mass spectra measurement reveals two im-portant things about the samples fabricated with the modi-fied procedure. First, PQ molecules and both MMA and the additional highly active monomers are retained as residual monomers in the fabricated samples. They will be able to take part in the photochemical reaction during optical ex-posure. Second, light exposure induces new characteristic peaks in the regime of large molecular weights, which in-dicate the photoproducts resulting from the attachment of PQ molecules and new monomers or MMA. With these photochemical analyses results, we anticipate that all new samples can have better holographic characteristics through the help of higher active residual monomers.
4.3 Holographic Data Storage Characterization
We have performed a multiplexing holographic recording on the new samples. Sixty-five plane-wave holograms have been recorded in each sample, in which each hologram was recorded with equal exposure energy of 0.32 J/cm2. The
running curves for the build-up dynamics of multiple-hologram recording in these samples are shown in Figure6. The material M# and E of the materials are obtained by
curve fitting, and the sensitivities are calculated. The results are listed in Table3. Comparing this table with Table2, it is seen that holographic characteristics of these new samples have been improved. The sensitivity of new PQ/PMMA is around 2 times that of the original PQ/PMMA. The sensi-tivity of new PQ/poly-共PEA-co-MMA兲 is about 1.3 times that of the original PQ/poly-共PEA-co-MMA兲. The improve-ments in these two samples are due to the increase of re-tained monomers by using the modified fabrication tech-nique. The most striking result is that the M# and the sensitivity of new PQ/poly-共TMPTA-co-MMA兲 are 7.01 and 0.97. Compared with the original PQ/PMMA, the M# and the sensitivity of the newly fabricated PQ/poly-共TMPTA-co-MMA兲 have been enhanced by a factor of 2.5 and 3.13 improvement, respectively. Holographic experi-mental results together with the mass spectra analyses show the modified fabrication technique is an effective way to retain TMPTA monomers in PQ/poly-共TMPTA-co-MMA兲 samples. These residual monomers are helpful to provide considerable improvements of dynamic range and sensitiv-ity for holographic recording.
5 Conclusions
We have fabricated and characterized PQ/PMMA and two kinds of PQ-doped copolymer samples, PQ/poly- 共PEA-co-MMA兲 and PQ/poly-共TMPTA-co-共PEA-co-MMA兲. We have per-formed mass spectra analyses of the samples before and after light exposure. The results indicate that for all of the PQ-doped samples light exposure induces photoproducts made of one PQ molecule plus one residual monomer. The results indicate that the physical mechanism of holographic recording in these samples follows our design strategy to minimize the photochemical-shrinkage effect. We have measured the holographic recording characteristics of these samples. The results show that both the M# and the expo-sure sensitivity S have been improved by about 1.5 to 2 times by introducing additional monomer into PQ/PMMA to form copolymer structures. The mass spectra analyses show that our fabrication method would not able to keep all TMPTA from participating the polymerization reaction in the PMMA matrix during fabrication, and so the holo-graphic improvement was restricted. We found that the tim-ing for introductim-ing reactive components into the monomer solution during fabrication provides one flexibility to im-prove the situation. For this, we have developed a new fabrication method, in that by controlling the timing for adding the reactive monomers into the solution matrix, a certain portion of TMPTA monomers can be preserved as residual monomers in a polymer matrix. The holographic
0 5 10 15 20 0 1 2 3 4 5 6 7
Cumulative Exposure Energy (J/cm2)
PQ/PMMA
PQ/Poly-(PEA -co- MMA) PQ/Poly-(TMPTA -co- MMA)
Cumulati ve grati n g streng th
Fig. 6 The dynamics of multiple-hologram recording in the three
samples fabricated with modified fabricated technique.
Table 3 The fitting parameters of the running curve in the three
samples fabricated using the new preparation method.
Name PQ/PMMA PQ/Poly- 共PEA-co-MMA兲 PQ/Poly- 共TMPTA-co-MMA兲 M# 3.35 4.98 7.01 E共J/cm2兲 5.63 6.3 7.27 S共cm2/ J兲 0.6 0.79 0.97
characteristics have been further improved another 1.5 to 2 times. The results confirm that our design strategy fabri-cates large dynamic, highly sensitive, thick recording pho-topolymer materials with low shrinkage effect for holo-graphic data storage.
Acknowledgments
This research was supported in part by the National Science Council, Taiwan, under contract NSC96-2112-M-009-001 and, in part from the Ministry of Education under the Min-istry of Education 共MOE兲 Aiming for Top University 共ATU兲 Program. The authors are grateful for technical dis-cussions with Prof. Hwa-Tsung Whang, Dr. Yi-Nan Hsiao, and Prof. Ken Y. Hsu.
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Shiuan Huei Lin received his BSc in electrophysics in 1990, and his
MS and the PhD in electro-optical engineering in 1992 and 1996, respectively, all from the National Chiao Tung University in Taiwan, Republic of China. He is currently an associate professor in the department of Electrophysics at the National Chiao Tung University. His research interests are in holographic storage, optical computing, optical devices, holographic materials, and holography for optical information processing.
Po-Lin Chen received his BS in chemistry in 2003 from the National
Taiwan Normal University, and his MS in material science and engi-neering in 2005 from the National Chiao Tung University in Taiwan, Republic of China. He is currently a PhD candidate at the Institute of Electro-Optical Engineering at the National Chiao Tung University. His research interests are in fabrications of photopolymers for holo-graphic storage and organic materials for optical applications.
June-Hwa Lin received his BS in electrophysics in 2001 from the
National Chiao Tung University, and his MS in electro-optical engi-neering in 2003 from the National Chiao Tung University in Taiwan, Republic of China. He is currently a PhD candidate at the Institute of Electro-Optical Engineering at the National Chiao Tung University. His research interests are in computer-generated holograms, holo-graphic storage, and liquid crystal optics.
