Holographic Recording Characteristics and Physical
Mechanism of Zinc Methacrylate/Nitroaniline-co-doped
Poly(methyl methacrylate)/9,10-Phenanthrenequinone
Photopolymers
Cheng-Jung Ko,1Po-Lin Chen,2Yi-Nan Hsiao,2 Shiuan-Huei Lin,3Wha-Tzong Whang,1 Ken Y. Hsu,2Kuo-Jung Huang,1Chun-Chao Chen,1I-Hsiang Tseng,4Mei-Hui Tsai4 1
Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsin-Chu 300, Taiwan, Republic of China
2
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsin-Chu 300, Taiwan, Republic of China
3
Department of Electrophysics, National Chiao Tung University, Hsin-Chu 300, Taiwan, Republic of China
4
Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan, Republic of China
In this study we developed an approach to improve the characteristics of photopolymers for holographic data storage. Through codoping different compounds, N,N-dimethyl-4-nitroaniline (DMNA) and zinc methylacrylate (Zn(MA)2), into 9,10-phenanthrenequinone (PQ) doped
poly(methyl methacrylate) (PMMA), the diffraction effi-ciency and the value of dynamic range (M#) have been progressed. We enhanced the diffraction efficiency (from 36.1 to 86.2%) and the dynamic range (M#, from 2.9 to 10.7) of PQ-doped PMMA through codoping with DMNA and Zn(MA)2. Using mass spectrometry and
X-ray photoelectron spectroscopy, we investigated the mechanism behind the improvements in optical stor-age induced by the presence of Zn(MA)2and DMNA in
PMMA/PQ. POLYM. ENG. SCI., 53:1297–1305, 2013. ª2012 Society of Plastics Engineers
INTRODUCTION
Volume holographic recording is a promising data stor-age technology because it allows rapid data access with excellent storage capacity. The nature of the recording
medium plays a key role in the realization of this promis-ing technology; photopolymers appear to be the most suit-able materials [1–9]. Among the photopolymers tested, poly(methyl methacrylate) (PMMA)-based polymers have attracted the most attention because of their superior opti-cal qualities, high flexibility, and negligible shrinkage when exposed to light [8–13].
Doping 9,10-phenanthrenequinone (PQ) into PMMA provides a holographic recording material from which holographic gratings have been fabricated [8–15]. The recording mechanism of the grating involves reactions between the PQ molecules and residual MMA monomer. Doping with organometallic compounds, such as zinc methacrylate [Zn(MA)2] [16–20], can be used to improve the holographic recording characteristics of PMMA/PQ photopolymers. The addition of Zn(MA)2 is not only to provide a catalyst to enhance the photoreaction between PQ and MMA molecules via the Zn ions in the Zn(MA)2 molecules, but also increase the number of reactants for photoreaction [16, 20]. Both reactions are effective for improving the sensitivity and dynamic range for holo-graphic recording characteristics. Alternatively, doping with PQ-based derivatives can also improve holographic data storage [9]. For example, the presence of electron donor groups that accelerate the photoreaction can increase the recording sensitivity; on the other hand, the electron-accepting nitro group (NO2) of 2-nitrophenan-threnequinone results in diffusely distributed electrons
Correspondence to: Y.-N. Hsiao; e-mail: [email protected] or W.-T. Whang; e-mail: [email protected]
Contract grant sponsor: National Science Council of Taiwan, Republic of China; contract grant number: NSC100-2221-E009-023-MY3. DOI 10.1002/pen.23361
Published online in Wiley Online Library (wileyonlinelibrary.com).
V
and increases the birefringence effect of the sample, thereby improving its holographic characteristics.
Nitroaniline compounds, which are simple nonlinear optical (NLO) materials [21–23], possess both electron donor and acceptor groups within their structures. In a previous study [23], there were three different nitroaniline compounds ((4-nitroaniline; pNA), (N-Methyl-4-nitroani-line; MNA) and [N,N-Dimethyl-4-nitroani(N-Methyl-4-nitroani-line; DMNA)] codoped into the PMMA/PQ photopolymer. Investigations show that the PMMA/PQ/DMNA photopolymer had greatest holographic characteristics than pNA and MNA system. After light exposed, the pNA and MNA com-pounds predictably react with PQ to form new photoprod-ucts. These new photoproducts would be providing a larger birefringence change and superior holographic re-cording performance. On the other hand, the DMNA did not react with either MMA or PQ to form new photoprod-ucts when exposed to light. In the PMMA/PQ/DMNA system, the coexisting DMNA molecules affected the polarizability of PQ during the recording process [23, 24], which induces larger birefringence effects, thereby, improves the holographic characteristics. We suspected that doping with nitroaniline compounds possessing larger birefringence effects would improve the holographic char-acteristics.
In addition, the metal ion (Liþ, Naþ, Cu2þ, Zn2þ) was interacted with the nitroaniline to form other compounds and the dipole moment of compounds can be increased [25–27]. The compounds with a higher dipole moment led to higher birefringence by the exposed samples and increased the holographic recording characteristics. There-fore, taking advantage of previous research [9, 16–23], in this study we codoped Zn(MA)2 and nitroaniline into the PMMA/PQ photopolymer to enhance its holographic stor-age capacity and recording characteristics, as measured in terms of diffraction efficiency and dynamic range (M#). We also propose herein a recording mechanism, deduced from mass spectrometric and X-ray photoelectron spectro-scopic analyses. Our results indicate that the presence of both Zn(MA)2 and nitroaniline within PMMA/PQ photo-polymers can significantly improve the dynamic range and sensitivity.
EXPERIMENTAL
Materials
Zn(MA)2(Sigma–Aldrich) was dried in a vacuum oven at 1108C for 12 h. 2,20-Azobisisobutyronitrile (AIBN; Showa) was purified through recrystallization from etha-nol (99.5%). Methyl methacrylate (MMA; Showa) was distilled at low pressure to remove the stabilizer and then kept in a refrigerator until required for use. PQ (Tokyo Chemical Industry) was used as received. DMNA (98%) was obtained from Tokyo Chemical Industry. N-Methyl-4-nitroaniline (MNA, 97%) and N-Methyl-4-nitroaniline (pNA,
98%) were purchased from Alfa Aesar. The chemical structures of these compounds are provided in Fig. 1.
Photopolymer Fabrication
Zn(MA)2 (0.35 wt%) was dissolved in the MMA monomer completely through ultrasonication in a bath for 2 h. AIBN (2 wt%) was added to this solution, which was then stirred at room temperature for 2 h. Next, PQ (0.7 wt%) and a nitroaniline compound (PQ-to-nitroaniline molar ratio: 1:1) were added into the solution, which was then stirred for 6 h. The mixed solution was filtered (pore size: 0.2 lm) and stirred at 358C for 12 h until the solu-tion turned highly viscous. Finally, the viscous solusolu-tion was poured into a glass cell and baked at 408C for 3 days. A yellowish bulk photopolymer having a thickness of 2 mm and good optical quality with negligible photochemi-cal shrinkage was obtained [13].
Measurements
To identify the mechanism of action and the recording characteristics of the various codoped PMMA/PQ photo-polymer samples, all measurements were performed indi-vidually both before and after light exposure. UV–Vis spectra of the photopolymer were recorded using a Shi-madzu UV-1800 spectrometer. The transverse electric (TE) and transverse magnetic (TM) refractive indices of the prepared photopolymers were measured using a prism coupler (Metricon Model 2010) at a wavelength of 632.8 nm. The birefringence (Dn), defined as the difference between the values of nTE and nTM, was also calculated. Mass spectra were recorded using a MICROMASS TRIO-2000 mass spectrometer. X-ray photoelectron spectra of the nitrogen and oxygen elements in the exposed photo-polymers were recorded using a Microlab 350 instrument, operated in the constant analyzer energy mode with a pass energy of 40 eV and Mg Ka (1253.6 eV) radiation as the excitation source. X-ray photoelectron spectroscopic anal-ysis was performed at room temperature and under vac-uum at pressures of less than 10–8 torr.
For measurements of holographic characteristic, a col-limated light beam from an Ar laser (wavelength: 514 nm) was divided into two s-polarization beams (intensity of each beam: 5 mW/cm2; ratio: 1:1; beam diameter: 0.7 cm). The intensity of the diffraction beam was detected when the shutter had blocked one of the beams. The two incident beams entered the samples symmetrically with an intersection angle, outside of the sample, of 308. The dif-fraction efficiency is defined as the ratio of the intensity of the diffracted beam to that of the summation of the dif-fracted beam and the transmitted beam. For measurements of the dynamic range, the photopolymers were placed on a rotational stage and multiple holograms were recorded using the perstrophic multiplexing technique. The exposed energy of each hologram was 0.52 J/cm2.
RESULTS AND DISCUSSION
Optical Properties of PMMA/PQ/Nitroaniline Photopolymers
Figure 2a displays typical optical absorption spectra of PMMA/PQ/nitroaniline photopolymers in their unexposed and exposed states (unexposed samples were analyzed im-mediately after fabrication; exposed samples had been illuminated at 514 nm for a suitable length of time). The signals of all of the photopolymers (PMMA/PQ, PMMA/ PQ/pNA, PMMA/PQ/MNA, PMMA/PQ/DMNA) under-went a blue shift to shorter wavelength after exposure. The strong blue shifts imply that the photoproducts were less conjugated than those of the unexposed photopoly-mers, consistent with our previous studies [8, 23]. To observe the effects of doping nitroaniline compounds into PMMA/PQ, we tested the sensitivity and dynamic range of holographic recordings performed at 514 nm.
Diffraction Efficiencies of PMMA/PQ/Nitroaniline Photopolymers
Figure 3 displays the holographic diffraction efficien-cies of the PMMA/PQ/nitroaniline photopolymers. The
samples of PMMA/PQ codoped with DMNA, pNA, and MNA exhibited maximum diffraction efficiencies of 60.3, 50.2, and 44.3%, respectively, significant improvements over that of the PQ-only doped photopolymer (36.1%). We suspect that doping with the nitroaniline compounds increased the birefringence of the photopolymer, such that its diffraction efficiency increased [9, 23]. Our previous study [23] has demonstrated that in the PMMA/PQ/nitro-aniline systems, DMNA increased the birefringence of the photopolymer to a greater extent that did either pNA or MNA. Through this demonstration of the holographic data storage characteristics, we confirm that the properties of the PMMA/PQ photopolymer were further enhanced in the presence of DMNA [23]. On the basis of this result, and taking advantage of previous findings [16–23], we selected DMNA for codoping with the organometallic compound Zn(MA)2into the PMMA/PQ photopolymer.
Optical Properties of Codoped Zn(MA)2Photopolymers Figure 2b presents typical optical absorption spectra of photopolymers codoped with Zn(MA)2. After light expo-sure, the signals of all of the exposed photopolymers
underwent blue shifts to shorter wavelengths, again implying that the photoproducts were less conjugated than the species present in the unexposed photopolymers [8, 9, 13, 20]. The absorption bands of the exposed codoped DMNA samples were close that of the DMNA-only doped photopolymer (Fig. 2c). In their unexposed states, all of the photopolymers in Fig. 2b provided similar absorption at 514 nm, suggesting that PQ served as the only major photosensitizer in the various codoped photopolymers.
The photopolymers doped with Zn(MA)2also exhibited small absorbances from 600 to 750 nm; Fig. 2d displays the corresponding expanded regions of the spectra. The absorption intensity in this range increased after light ex-posure, implying that Zn(MA)2 reacted with PQ to form other photoproducts [17, 20]. Comparing the absorption intensities of the unexposed PMMA/PQ/DMNA/Zn(MA)2 photopolymer and the unexposed PMMA/PQ/Zn(MA)2 photopolymer, we find that the sample codoped with DMNA provided a stronger absorption, suggesting that some of the Zn(MA)2 species may have interacted with DMNA molecules to form other compounds prior to light exposure [28]. Subsequently, after light exposure, the in-tensity of each curve increased, presumably because the remaining Zn(MA)2 species reacted with PQ to form other photoproducts [17, 20]. A variation in molecular structure is the main effect leading top a refractive index
or birefringence change for grating recording in photopol-ymers. To observe the effect of Zn(MA)2 and DMNA doping in the PMMA/PQ, in this study we characterized the sensitivity and dynamic range using 514 nm as the re-cording wavelength.
Table 1 lists the values of the birefringence (Dn ¼ nTE – nTM) of the photopolymers codoped with Zn(MA)2. The
FIG. 2. UV–Vis absorption spectra of the (a) unexposed and exposed PMMA/PQ/nitroaniline photopoly-mers; (b) unexposed and exposed photopolymers and (c) DMNA-only doped photopolymers (without PQ); (d) expanded region of a portion of the long wavelength range.
FIG. 3. Holographic diffraction efficiencies of the PMMA/PQ/nitroani-line photopolymers.
photopolymers doped with Zn(MA)2 all exhibited greater birefringence than did the other photopolymers. Among them, the PMMA/PQ/DMNA/Zn(MA)2photopolymer pro-vided the greatest improvement in birefringence, presum-ably because the presence of Zn(MA)2 react with the PQ and DMNA molecules to form new compounds [25–30], thereby increasing the dipole moments of the Zn(MA)2 -doped photopolymers. With a higher dipole moment led to higher birefringence by the exposed samples and increased the holographic recording characteristics.
Holographic Recording of Codoped Zn(MA)2 Photopolymers
Figure 4 presents plots of the holographic diffraction efficiency as a function of energy. The samples of PMMA/PQ/DMNA/Zn(MA)2, PMMA/PQ/Zn(MA)2, and PMMA/PQ/DMNA exhibited high diffraction efficiencies, reaching maximum values of 86.2, 75.5, and 60.3%, respectively. Relative to that (36.1%) of the photopolymer doped with PQ only, we conclude that the diffraction effi-ciency improved significantly after codoping DMNA and Zn(MA)2 into PMMA/PQ. For the PMMA/DMNA/ Zn(MA)2photopolymer prepared without PQ, the diffrac-tion efficiency was very low (ca. 1.2%). Though, Zn(MA)2 react with DMNA to form the compounds can increased birefringence of sample [25–27]. But the Zn(MA)2molecule is not a photosensitive dye, that
holo-graphic gratings cannot be recorded in PMMA/DMNA/ Zn(MA)2 sample. This result show the holographic char-acteristics was mainly by the photoinduced attachment of PQ molecule to MMA and Zn(MA)2molecules. In which, in PMMA/PQ/DMNA/Zn(MA)2 photopolymer the Zn(MA)2 not only react with PQ, but also react with DMNA form other compounds, the birefringence of pho-topolymer was increased and improved the holographic recording characteristics, as demonstrate in Table 1.
We used perstrophic multiplexing to record 200 holo-grams in one location of the photopolymer. Summation of the square roots of the diffraction efficiency provided a running curve of the cumulative grating strength (Fig. 4). To obtain the holographic parameters, we performed curve fitting with the function
CðEÞ ¼ Csat½1 expðE=EtÞ (1)
where E is total exposed energy; Csat (M#) is the satura-tion value of the curve, giving the dynamic range of the material; andEtgives the exposed energy constant of the material [8, 31]. The material sensitivity (S) is defined as an increment in cumulative grating strength with respect to the amount of exposed energy [8, 9]. When the sample is unexposed, then
S¼ dC=dEjE!0 (2)
According to the running curve function, the sensitivity can be expressed as
S¼ Csat=Et¼ M#=Et (3)
The values of M# and S characterize the ability of the material to be used for volume data storage [8, 9, 31].
Figure 5 displays the running curves for multiple-holo-gram recording in the photopolymers. After curve-fittings
TABLE 1. Birefringence of the exposed photopolymers.
Sample nTE (632.8 nm) nTM (632.8 nm) Dn (nTE–nTM) PMMA/PQ 1.4835 1.4844 20.0009 PMMA/PQ/DMNA 1.4890 1.4915 20.0025 PMMA/PQ/Zn(MA)2 1.4789 1.4824 20.0035 PMMA/PQ/DMNA/Zn(MA)2 1.4842 1.4903 20.0061
FIG. 4. Holographic diffraction efficiencies of the photopolymers (a) PMMA/PQ/DMNA/Zn(MA)2, (b) PMMA/PQ/Zn(MA)2, (c) PMMA/PQ/
DMNA, (d) PMMA/PQ, and (e) PMMA/DMNA/Zn(MA)2.
FIG. 5. Running curves for multiple-hologram recording in PMMA/PQ photopolymers doped with Zn(MA)2and DMNA.
to Fig. 5, we obtained the values of Csat (M#) andEs for the various codoped photopolymers; Table 2 lists the cal-culated values of M# and S for each sample. The photo-polymer doped with both DMNA and Zn(MA)2 provided the highest values of M# (10.68) and S (1.409 cm2/J) among all of the samples, with greater than threefold improvements relative to those of the un-doped PMMA/ PQ sample (2.88 and 0.432 cm2/J, respectively). These results are consistent with the nitroaniline modifying the holographic recording characteristics by increasing the birefringence of the photopolymer, as demonstrated in Table 1.
Chemical Characterization of Zn(MA)2Codoped Photopolymers
During holographic recording, PQ reaches its excited state through electron transfer and then reacts with the MMA monomer to form the photoproduct in the region
exposed to light [8, 9, 32]. The sensitivity of the holo-gram recording process can be improved through a modi-fied recording mechanism after the codoping of Zn(MA)2 into PMMA/PQ photopolymers [17, 20]. On the other hand, in this study we found that introducing DMNA into the PMMA/PQ photopolymer has the advantages of improving the recording sensitivity and dynamic range. Thus, we chemically analyzed the Zn(MA)2 and DMNA codoped photopolymers to clarify the chemical character-istics and the mechanism of their operation. Figure 6 presents the mass spectra of each exposed sample; they provide information regarding the molecular weights of the pyrolyzed fragments of the photopolymers, allowing us to identify the possible photoproducts and deduce a plausible recording mechanism. The molecular weights of PQ, DMNA, Zn(MA)2and MMA are 208, 166, 235, and 100 g/mol, respectively. Figure 6a displays the mass spec-trum of the pyrolyzed fragments of the PMMA/PQ photo-polymer. The signal at m/z 308 corresponds to a photo-product formed from one MMA unit and one PQ unit [8]. Figure 6b, the mass spectrum of PMMA/PQ/DMNA, also features a peak at this molecular weight, presumably rep-resenting the same compound as that in Fig. 6a. In addi-tion, the characteristic peak of DMNA is evident at m/z 165, suggesting that DMNA does not react with either MMA or PQ to form new photoproducts in the PMMA/ PQ/DMNA system after exposure to light. This observa-tion matches the previous study [23, 25, 32, 33] was indi-cated that PQ molecules induce a variation in polarizabil-ity during the photoreaction process. In the PMMA/PQ/ DMNA system, it is likely that the coexisting DMNA
TABLE 2. Dynamic range (M#), exposed energy constant (Es), and material sensitivity (S) for multiplexed volume holographic recording in four photopolymers. Sample M# Es (J/cm2) S (cm2/J)a PMMA/PQ 2.88 6.67 0.432 PMMA/PQ/DMNA 7.19 9.09 0.791 PMMA/PQ/Zn(MA)2 8.21 7.69 1.068 PMMA/PQ/DMNA/Zn(MA)2 10.68 7.58 1.409 a S¼ M#/Es.
FIG. 6. Mass spectra of the light-exposed photopolymers (a) PMMA/PQ, (b) PMMA/DMNA, (c) PMMA/ PQ/Zn(MA)2, and (d) PMMA/PQ/DMNA/Zn(MA)2.
molecules affected the polarizability of PQ during the recording process [23], such that the photoproducts were formed with a more-planar molecular orientation to improve the holographic data storage characteristics. Figure 6c presents the mass spectrum of the PMMA/PQ/ Zn(MA)2 photopolymer; again, the peak at m/z 308 is present, in addition to other peaks in the range m/z 398– 402, which presumably arose from the fragmentation of Zn(MA)2 and further reactions with PQ. The decomposi-tion of Zn(MA)2could involve the loss of one MA mole-cule, with which a PQ molecule might react to form a new photoproduct. The mass spectrum of the PMMA/PQ/ DMNA/Zn(MA)2photopolymer (Fig. 6d) reveals the exis-tence of signals at m/z 295–296 and 308, representing the product formed from one MMA unit and one PQ unit [8]. In which, the peaks at m/z 406–430 was likely the result in Fig. 6c. We attribute the peaks at m/z 357–361 to a molecule comprising one ZnMA unit and one PQ unit.
The peaks in the molecular weight range m/z 267–286; however, might be due to the fragmentation of one ZnMA molecule attached to a NO2 functional group [25–27], which presumably detached from DMNA [25–26, 34]. It means that ZnMA interaction with DMNA. The com-pounds had higher dipole moments and let to higher birefringence by the photopolymers and increased the holographic recording characteristics, it was demonstrated in Fig. 5.
To identify the recording mechanism, we used X-ray pho-toelectron spectroscopy (XPS) to measure the Zn (2p3/2) energy levels as a means of characterizing the chemical compositions of the zinc elements in the photopolymers. Figure 7 displays the Zn (2p3/2) XPS spectra of the PMMA/PQ/Zn(MA)2 and PMMA/PQ/DMNA/Zn(MA)2 photopolymers, with Gaussian deconvolution of the over-lapping peaks. In Fig. 7a, the standard Zn(MA)2 XPS spectrum of Zn (2p3/2), the signal of the free Zn (2p3/2) is
FIG. 7. Zn (2p3/2) core level XPS spectra of (a) standard Zn(MA)2, (b) unexposed PMMA/PQ/Zn(MA)2, (c)
exposed PMMA/PQ/Zn(MA)2, (d) unexposed PMMA/PQ/DMNA/Zn(MA)2, and (e) exposed PMMA/PQ/
located 1021.7 eV. The XPS spectrum of the unexposed PMMA/PQ/Zn(MA)2 photopolymer (Fig. 7b) reveals a peak A (Zn at state A) at 1021.7 eV, which we attribute to free Zn(MA)2, and a peak B (Zn at state B) at 1022.8 eV, which we attribute to the photoproduct formed from Zn(MA)2 interacting with PQ. The XPS signals of the exposed PMMA/PQ/Zn(MA)2 photopolymer (Fig. 7c) were similar to those in Fig. 7b, but the area ratio (B/A) of the two peaks was different. Prior to exposure, the B/A peak area ratio was 33.3/66.7, suggesting that one-third of the Zn species had already interacted with PQ units. After exposure, the peak area ratio was 66.4/33.6, meaning that two-thirds of the Zn species had interacted with PQ moi-eties. Figure 7d displays the Zn (2p3/2) XPS spectrum of the unexposed PMMA/PQ/DMNA/Zn(MA)2 photopoly-mer; we ascribe the peaks at 1021.7 (peak A), 1022.8 (peak B), and 1024.3 (peak C) eV to free Zn(MA)2(state A), Zn(MA)2 having interacted with PQ (state B), and Zn(MA)2having interacted with DMNA (state C), respec-tively. The peak locations in the XPS spectrum of the exposed PMMA/PQ/DMNA/Zn(MA)2 photopolymer (Fig. 7e) were similar to those in Fig. 7d. The results can be further identified through calculating the area of peak. For all of the light-exposed samples, the area of peak A decreased while that of peak B increased significantly, presumably because of the formation of a photoproduct from Zn(MA)2 and PQ after exposure to light. The area of peak C increased slightly, revealing that Zn(MA)2 interacted primarily with PQ and to a much lesser extent with DMNA, in agreement with the conclusions drawn from the mass spectra. This result show the main effect of the holographic recording characteristics was the PQ react with MMA and Zn(MA)2molecules, as demonstrate in Fig. 4. Table 3 lists the peak A ratios (the area of peak A of the unexposed photopolymer divided by that of the exposed sample) of all of the samples. Through the varia-tions of the peak A ratios, we can determine the degrees of reaction of the Zn(MA)2species. The peak A ratios of
the PMMA/PQ/Zn(MA)2 and PMMA/PQ/DMNA/ Zn(MA)2 photopolymers were 1.985 and 3.180, respec-tively; the higher value of the latter implies that more Zn(MA)2molecules reacted with PQ and DMNA to form new compounds during light exposure. In which, the PMMA/PQ/DMNA/Zn(MA)2 photopolymer had greatest birefringence than other photopolymers. At the same time, the PMMA/PQ/DMNA/Zn(MA)2 photopolymer also exhibited superior holographic recording characteristics, as revealed in Fig. 5. Hence, the performance of the dif-fraction efficiency, M#, and the material sensitivity were correlated to the degrees of photoreaction in the samples. Together, our results indicate that the performance of the PMMA/PQ photopolymer can be enhanced by the pres-ence of Zn(MA)2and DMNA in the system.
CONCLUSIONS
The holographic data storage characteristics of PMMA/ PQ photopolymers are significantly enhanced after intro-ducing Zn(MA)2 and DMNA. The results of single-holo-gram recording revealed that the PMMA/PQ/DMNA/ Zn(MA)2 photopolymer provided a diffraction efficiency higher than those of the other tested photopolymers. Fur-thermore, in multiple-hologram recording tests, the PMMA/PQ/DMNA/Zn(MA)2 photopolymer displayed the highest value of M# and sensitivity. The values of gmax and M# improved by 2.39 and 3.69 times, respectively, when Zn(MA)2 and DMNA were codoped into the photo-polymer. Using MS and XPS to investigate the effects of Zn(MA)2 and DMNA on the holographic storage charac-teristics, we found that some of the PQ and DMNA mole-cules bound to Zn(MA)2 in the PMMA/PQ/DMNA/ Zn(MA)2 photopolymer. Our results indicate that the PMMA/PQ photopolymer can display superior holo-graphic recording characteristics after codoping with orga-nometallic and nitroaniline compounds [namely Zn(MA)2 and DMNA, respectively] to further promote the PQ pho-toreaction and birefringence.
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