Increasing photosensitizer concentration has been considered as an effective approach to improve the performance of holographic material. In this paper, we report on new method for increasing the saturated dissolvability of photosensitizer PQ within polymeric media by introducing copolymerization monomer into the PQ/PMMA. The photosensitizer concentration of PQ was increased from 0.7wt% to 1.3wt%, compared with the typical PQ/PMMA sample. Besides, we investigated performance of polarization holographic recordings in typical PQ/PMMA and copolymerization monomer-containing PQ/PMMA with the orthogonally polarized signal and reference waves. And the doping of THFMA component resulted in a significant improvement of diffraction intensity and photosensitivity. In addition, high-quality holographic image reconstruction was realized in our home-made material.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Photopolymers are the key materials for holographic applications like data storage and holographic optical elements due to high optical sensitivity, low fabrication cost, practically unlimited dimension and unnecessary post processing .
In order to improve optical performance, many researchers focused on developing innovative composition by doping additional functional components into recording photopolymer materials , such as nanoparticles , organic-inorganic hybrid , lanthanoid organometallic compounds  and organic metallic component  et al. However, for some applications, for example data storage, these methods were restricted by their limited thickness, high shrinkage during holographic exposure, relatively high cost and complex fabrication .
Among these photopolymers, phenanthrenequinone (PQ) doped poly-(methyl methacrylate) (PMMA) photopolymer has attracted much attention owing to its neglectable shrinkage, high thickness, low cost [8,9], and meanwhile it possesses polarization sensitivity.
However, because of the low solubility (0.7wt%) of the PQ in the MMA solution, the concentration of photosensitizer is limited. Thus, comparing with other materials, the diffraction efficiency and polarization sensitivity of PQ/PMMA are unsatisfactory. To solve these problems, we need to enhance the combination probability of PQ radicals with monomer molecules, which can be achieved by increasing the number of PQ molecules.
H. Liu et al.  reported a method to increase the concentration of PQ by raising the prepolymerization reaction temperature. The results showed that, approximately 1.0wt% PQ were solved into the MMA solution at 60 °C reaction temperature. Mahilny et al.  demonstrated a preparation process where they cast the liquid solution on a substrate and dried to a solid state. With their process, the concentration of PQ can be raised up to 4 mol. % in the photosensitive layer, but the application of this method was limited by its relatively lower thickness (50~180 µm).
In previous work, we have introduced co-monomer BzMA to improve the performance of holographic recording material PQ/PMMA already . However, it is still undefined what kind of the regularity the co-monomer should possess. In this paper, we concentrate on the discussion of photoreaction mechanism and conclude the rules for choosing co-monomer with certain structure. According to the theory that similarities can be solvable easily with each other, we suspect that the dopant structure contains aromatic group that may have high solubility for PQ molecules. Additionally, the unsaturated carbon-carbon double bond in the dopant can make it more efficiency to introduce the dopant into the PQ/PMMA photopolymer system. In accordance with these requirements, we have found the following dopants in Table. 1.
We prepared new materials by introducing above dopants into PQ/PMMA material. Among these new materials, two kinds of dopants (St and PEA) have not succeeded since the viscosity of these two dopants is too much higher than that of MMA solution, resulting in a large number of bubbles in material. However, PQ doped poly (MMA-co-THFMA) photopolymer was successfully prepared by the free radical copolymerization. THFMA is a very interesting monomer as its polymer has a wide range of applicability in the development of biomaterials . By introducing THFMA, the concentration of PQ was improved from 0.7wt% to 1.3wt% in comparing with PQ/PMMA. As this improvement, the material obtained higher diffraction intensity and photosensitivity than typical PQ/PMMA system.
2. Material preparation
All original materials were purchased from Shanghai Macklin Biochemical Company and used as received. In this paper, THFMA and MMA were selected as the co-monomers, 2,2-azobisisobutyronitrile (AIBN) was employed as the thermo-initiator and PQ as the photosensitizer. Figure 1 shows their chemical structures.
In our fabricating process, the monomers MMA and THFMA were mixed firstly with a weight ratio of 7:3, since the PQ have a better solubility in THFMA than MMA monomer. We introduce THFMA monomer into our system for increasing the dissolved concentration of the PQ molecule. The PQ and AIBN (0.8wt%) powders were dissolved in monomers solvent, and doping concentration of PQ in the monomer mixture was determined by measuring their saturation concentration (the concentration of PQ can reach 1.3wt%). The resulting solution was stirred using a magneto stirrer in a glass bottle at 65°C for an appropriate period of time until it became homogeneously viscoid. Such syrup was poured into a glass mold with a 1.5 mm thickness spacer. Then the mold was then baked at 45°C for 10 days to solidify the mixture. This procedure ensures that most of the monomers were polymerized and yellow bulk samples with good optical quality. This material was hereafter named as PQ/P(T3M7) photopolymer.
In this paper, we also prepared PQ/PMMA(PQ:0.7wt%) and PQ/P(T3M7)(PQ:0.7wt%) photopolymers for comparison. Furthermore, we also compared it with the previous system of PQ/P(B3M7) photopolymer .
3. Photochemical reaction
Before the illumination, the PQ molecule presents coplanar structure due to the high conjugate structure . When exposed with linearly polarized wave, the PQ molecules will be oriented dependent on the polarization state after it was exposed by linearly polarized wave. For example, in Fig. 2, it is shown that PQ molecules that parallel to the polarization state of the light have higher opportunity to react with monomer [15,16].
Under the exposure, photons excite 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 monomer molecule to form the photoproduct [17,18]. The polarization anisotropy of photoproduct is better than that of PQ molecules, which only process little polarization anisotropy. Thus, the photoreaction would cause the polarization performance. The more PQ molecules we introduced into the material, the more PQ molecules will be excited by the light to participate into the photoreaction. As above, the PQ molecule play an important role in the PQ-containing polymeric polarization holographic recording material.
4. Results and discussions
4.1 FT-IR spectra measurements
Infrared spectroscopy analysis was performed to gain information about the photochemical reaction of the sample, so we compared the IR spectrum of the reactants (PQ and monomers) before and after (bottom in Fig. 3) photo-irradiation. Infrared spectra of the PQ, MMA, THFMA and exposed PQ/MMA and PQ/P(T3M7) samples were carried out with a Thermo-IS5 Fourier transform infrared spectrophotometer in Fig. 3. Before measurement, the exposed PQ/MMA and PQ/P(T3M7) solutions were dipped on KBr plates. The plate samples were baked at 50 °C for 3 hours such that the unreacted monomer molecules were removed and the photoproducts were left on the KBr plates. This method is the same as Hsiao's method .
A broad absorption between 2800~3300 cm−1, and an absorption band at ~2360 cm−1 are seen on Fig. 3, which are ascribed to the C-H stretching (CH3 and CH2 stretching) and CO2 absorption peak, respectively. The absorption between ~1670 cm−1 corresponds to the C = O of PQ. The strong absorption peak at ~1745, 1725 cm−1 are due to the carbonyl ester group of MMA and THFMA units. The bands at ~1245 and 1078 cm−1 are ascribed to the C-O-C groups of the photoproducts of group on the PQ molecule reacting with the vinyl group on the monomer molecule. Compared with the PQ/MMA (exposed) sample in Fig. 3(a), absorption peaks of the PQ/T3M7 (exposed) sample in Fig. 3(b) also have the same new peaks at wave numbers ~1631, 1245 and 1078 cm−1, which implies that photoproduct was produced by the light exposure . These phenomena show that the THFMA did not affect the photochemical reaction of PQ and monomer molecules.
4.2 UV–Vis spectra measurements
Optical absorption was measured by using U-1901 Double-beam UV-Vis Spectrophotometer. The plot of absorbance versus wavelength is observed in Fig. 4. Its absorption was strong for λ<475 nm and close to zero for λ>600 nm.
We can see from Fig. 4 that the absorption peak appears red shifted with the content of PQ component increasing. It proved that we have succeeded in our strategy to increase PQ's concentration and strengthened optical absorption. The absorbance of PQ/P(T3M7) has very little change with wavelength when the content of PQ is the same as PQ/PMMA. It indicates that the THFMA component has no effect on light absorption. In our experiments, we chose 532 nm as the recording wavelength. The absorbance of PQ/PMMA(PQ:0.7wt%), PQ/P(T3M7)(PQ:0.7wt%) and PQ/P(T3M7)(PQ:1.3wt%) at λ = 532 nm before exposure were α = 0.061, 0.069 and 0.135 cm−1, respectively.
4.3 Photoinduced birefringence
The photoinduced birefringence was investigated with a continuous HeNe laser (λ = 632.8 nm, P = 2.6 mW/cm2) as the probe light, and a diode pumped solid state (DPSS) Nd:YAG laser (λ = 532 nm, P = 52 mW/cm2) as the pump light. The experimental setup and corresponding calculation formula of photoinduced birefringence were shown in the reference .
Figure 5 shows the experimental results of photoinduced birefringence. We can observe that photoinduced birefringence of PQ/P(T3M7)(PQ:1.3wt%) rises much more quickly than that of PQ/P(T3M7)(PQ:0.7wt%). Additionally, higher PQ content in the sample leads to the larger birefringence value. For PQ/P(T3M7) and PQ/PMMA samples with the same concentration of PQ (PQ:0.7wt%), the curve of the photoinduced birefringence was not greatly changed.
4.4 Orthogonal linearly polarized holography
Polarization holography is a promising technology with its unique ability of recording amplify, phase and polarization meanwhile. In contrast to the traditional holography in which information is recorded by the modulation of refractive index, polarization hologram is recorded in polarization-sensitive materials by the modulation of photoinduced anisotropy such as birefringence, optical dichroism and optical activity.
Figure 6 shows the schematic of polarization holography which employs two orthogonal polarization waves for recording information. Compare to traditional hologram where an intensity inference pattern is existed, polarization hologram shows periodically polarizaiton states modulation with a constant intensity distribution in resultant area. And the detail of periodic polarization state modulation of orthogonal linear waves are listed in Table. 2. As the recording material is polarization-sensitive, the hologram is carried out by photoinduced birefringence gratings. And the birefringence modulation comes from the selective photoreaction of reorientation molecules and structure rearrangement by the photochemical reaction. In readout stage as shown in Fig. 6, the polarization hologram is reconstructed by reference wave satisfied Bragg condition similarly to intensity holography.
The material sensitivity S was defined as :
The experimental setup for measurement of orthogonal linearly polarized holography is shown in Fig. 7. Wavelength of the laser used for recording and reconstructing was 532 nm. The green laser beam was split into horizontally polarization (p-pol.) as reference wave and vertically polarization (s-pol.) as signal waves by polarization beam splitter (PBS) with the same intensity of P = 52 mW/cm2. They were incident into the sample symmetrically with a crossing angle of 30°. The diameter of light spot on sample was 6mm.
In the recording process, we opened shutter 1 and shutter 2. Shutter 3 was kept close. The recording waves interfered for 4 seconds with the crossing angle of 30°. In reconstruction process, we closed the shutter 2 and opened the shutter 3. We used the original reference wave to read recorded hologram for 0.4 seconds.
Figure 8(a) shows the typical dependence of the diffraction intensity versus exposure time. The diffraction intensity of each sample was elevated after an increase of exposure time. When the exposure time reached a sufficient value, the diffraction intensity of each sample was comparatively stable.
It can be seen that diffraction intensity of PQ/P(T3M7) increased with increasing concentration of PQ. The data indicated that, the diffraction intensity of PQ/P(T3M7) can be increased by double when the concentration of PQ was increased from 0.7wt% to optimum 1.3wt%. As shown in Fig. 8(a), under the same concentration of PQ was 0.7wt%, the diffraction intensity of PQ/P(T3M7) was slightly increased than the typical sample of PQ/PMMA.
Incorporation of dopant THFMA into the system will lead to the formation of highly concentrated PQ photopolymer material and hence higher diffraction intensity could be maintained.
We also compared the polarization properties of the two series of PQ/P(T3M7) and PQ/P(B3M7) in Fig. 8(b) and Fig. 9. We calculated S of our samples during the 10min from the beginning of recording. The experimental results reveal that photopolymer PQ/P(T3M7) has higher diffraction intensity and higher sensitivity. It can be seen that diffraction intensity and S increase with rising concentration of PQ. Thereby, by this part, it also reveal that diffraction intensity and S is related to concentration of PQ. By increasing PQ’s concentration, more PQ molecules are excited by the light to participate into the polymerization.
5. Application experiments
We investigated the real image reconstruction characteristics of the PQ/P(T3M7) photopolymers. In the experiment, the collimated laser with 532nm wavelength was split into signal and reference wave by PBS. The signal wave is p polarized and the reference wave is s polarized. The crossing angle is exactly 90 degrees. The material we used is PQ/P(T3M7), which is a yellow cube with a size of 10 mm × 10 mm × 15 mm as the recording media. The optical setup was shown in Fig. 10.
Firstly, we received the original transmitted image (Fig. 11(a)) by CMOS. It is the image that the original image (on SLM) went through the material directly without holographic recording and reconstruction. In the holographic recording process, signal wave carried the original image through SLM. Then we opened shutter 1 so that reference wave can interfere with signal wave in our material. In the reconstruction process, we closed shutter 2 and opened shutter 1 to read recorded hologram, we got reconstructed image as shown in Fig. 11(b). The reconstructed image has a very good contrast and retains high fidelity. It is good enough for the application of volume polarization holographic storage.
In this work, we proposed an effective method for enhancing performance of polarization holographic material. By this method, we successfully prepared a new photopolymer based on modified PQ/PMMA, and it showed higher diffraction intensity and higher sensitivity by introducing dopant THFMA. The THFMA is a copolymerization monomer, which can increase the doping weight ratio of PQ. By this method, the combination probability of PQ radicals with monomer molecules was increased. In addition, orthogonal linear polarization recording and high quality reconstruction image were realized in PQ/P(T3M7). The rules we proposed for choosing proper co-monomer are key point to further improve holographic material.
National Natural Science Foundation of China (NSFC) (61205053, 61475019);China Postdoctoral Science Foundation Grant (Grant No.: 2017M620635).
The authors want to thank the International Graduate Exchange Program of Beijing Institute of Technology for support body of the paper.
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