Photoinitiator plays a crucial role on photopolymer. Unlike photoinitiator phenanthrenequinone (PQ), the solubility of Irgacure 784 dissolved in MMA is very high. In this paper, we use Irgacure 784 as photoinitiator doped in poly(methyl methacrylate)(PMMA)to make a bulk photopolymer with high photoinitiator concentration for holographic data storage. The effect of concentration of photoinitiator and record intensity are experimentally investigated. The results reveal the material has an optimum condition in holographic recording. A comparison between our material and the material of PQ doped PMMA is carried out by experiment. It is found that our material has better performance on diffraction efficiency, refractive index modulation and recording sensitivity. Besides, this material also has polarization sensitivity, which can be applied to polarization holography. With the capacity of recording polarization holography and conventional intensity holography simultaneously, the Irgacure 784 doped PMMA material is expected to be applied in holographic data storage.
© 2017 Optical Society of America
Volume holography optical storage known as high-density three-dimensional structure optical storage with large storage capacity and high transfer rate is considered as a potential choice for next generation storage technologies [1–4]. The characteristic of light is not limited to its intensity, and polarization information is also an important characteristic that can record in polarization-sensitive materials as the information code to increase the recording capacity of the medium [5–9]. Because of various restrictions, polarization information recording is always ignored in volume holography storage. Recently, polarization holography theory based on tensor theory which can analyze recording and reconstruction of polarized light with large cross-angle is extensive proposed [10–15]. It is urgent to find a suitable volume holographic material with excellent optical properties used for both traditional holography and polarization holography.
Photopolymers are one kind of holographic recording media for high-density optical data storage system due to their high diffraction efficiency, low cost, high sensitivity, high refractive index modulation, and simple process . Photopolymers with excellent holographic characteristic have been extensively proposed [17–20]. Polymethylmethacrylate (PMMA) is a famous polymer matrix used to record three-dimensional phase holograms. Phenanthrenequinone(PQ) doped PMMA is the subject of a large number of investigations [21–23]. By separating the thermo-polymerization reaction and photo-polymerization reaction, the volume shrinkage of the sample is greatly inhibited. The thickness of the sample can be in the order of millimeters. Although solubility of PQ doped in liquid MMA monomer is relatively few, using PMMA as polymer matrix is a good choice.
Photosensitizer Irgacure 784 is an excellent photoinitiator used in a variety of holography material system [24–26].Unlike photoinitiatorPQ, Irgacure 784 has high solubility doped in liquid MMA monomer. In this paper, we investigated the holography characteristic of Irgacure 784-doped PMMA photopolymer (Irgacure 784/PMMA) and compared with PQ/PMMA. We found that Irgacure 784/PMMA had eminent optical holographic characteristic and was sensitive to polarization. For purpose of choosing appropriate material using for traditional and polarization holography, the concentration of photo initiator and the effects of different light intensity used in the process of recording were studied in this paper. We used the material in both traditional and polarization system, holographic record and reconstruction process of the image are commendably achieved, respectively.
2. Material Preparation
The photopolymer material consisted of methyl methacrylate (MMA) as photopolymerizable monomer, Irgacure 784 as photoinitiator, and 2,2-azo-bis-isobutyronitrile (AIBN) as thermal initiator. The chemical structures of all compounds were shown in Figs. 1(a)-1(c). The material contained 0.8wt% thermal initiator (on the basis of the weight of total monomers). After weighing each composition, they were mixed in a sample bottle. Then we put the bottle into ultrasonic cleaner for about 1h, making mixture mixed thoroughly to form a uniform solution. Subsequently, the bottle was placed in a thermostat water bath with magnetic stirring of 333K for about 1.5h until the solution turned viscous. Viscous solution was poured into a container composed of silicon and glass. The thickness of silicon was the determinant of our material’s thickness. Then the container with viscous solution was put in drying oven of 333K for about 20h to complete the polymerization process. Finally, the container removed from the material. The samples with clear optical transmission are pale orange.
3. Material characteristic
The UV/V is absorption spectra of the samples with different concentration ratios of Irgacure 784 in the PMMA and different thickness of material were shown in Figs. 2(a) and 2(b), respectively. The absorption of the samples compared with PQ/PMMA (the concentration of PQ was 1wt% dissolved in MMA monomer)in Fig. 2(a). As shown in Fig. 2(a), the thickness of material was 1.5mm, as concentration of photosensitizer further increased, the absorption of the material was increased correspondingly. The absorption of Irgacure 784/PMMA was higher than PQ/PMMA with same concentration of photoinitiator doped in a wide range of spectrum. Figure 2(b) showed the effect of material thickness on the absorption spectra with 5wt% concentration of the photosensitizer. Increasing the material thickness from 0.5mm to 2mm resulted a large increase in absorption level in range of 500-550nm. The absorption of the samples had a great relationship with the concentration of material photo initiator and material thickness. It is hard for light pass through the material with high absorption. Furthermore, absorption of material could affect the response rate of the material. The material absorption would restrict the option of the concentration of material photoinitiator and material thickness. It could conclude that laser of 532 nm was very suitable as the recording light when the weight ratio of the photo sensitizer was 5wt% with the material thickness was 1.5mm.
To study the holographic characteristics of the material, we designed an experimental setup, as shown in Fig. 3.This experimental setup could measure the diffraction efficiency as a function of recording time t. The green laser (532nm) beam was split into horizontally polarized (p-pol.) and vertically polarized (s-pol.) waves by polarization beam splitter. PBS with the same intensity of 20mW, and they were incident into the sample symmetrically with an include angle of 60°. The spot size illuminated was 6mm. The polarization state of signal beam was s-pol. In order to explore situation of both traditional holography and polarization holography, polarization state of reference beam was controlled by half-wave plate2 (HWP2). The intensities of transmitted and diffracted beams were measured by power-meter. We stopped exposure and measured after every different exposure time to control recording and reconstruction process. In the recording process, we opened shutters 1 and 2. Shutter 3 was kept close. The recording waves interfered for 4s with the cross-angle of 60°. In reconstruction process, we closed the shutter 2 and opened the shutter 3. We used the original reference wave to retrieve the grating for 0.4s. These two processes were repeated until the reconstructed power saturation.
The diffraction efficiencydefined as I+1/(I0 + I+1), where I0 and I+1are the intensities of the transmitted and the 1st-order diffracted beam, respectively. In order to explore the influence of the content of Irgacure 784 dissolved in MMA solvent, diffraction efficiency temporal evolution of photo sensitizer concentration of 1, 3, 5 and 7wt% with material thickness of 1.5mm were measured. As was depicted in Fig. 4, diffraction efficiency was not always augmented following the increase of photo sensitizer concentration of the material. After the weight ratio of photo initiator dissolved in monomer was higher than 5wt%, diffraction efficiency approach saturation point. The diffraction efficiency was not rose as expected. It was useless to aggrandize the photo initiator. The material thickness of 1.5mm, concentration photo sensitizer of 5wt% was the relatively appropriate state, its corresponding rate and diffraction efficiency were more suitable.
In order to make a comparison of holographic characteristics between the two kinds of photopolymers (PQ/PMMA and Irgacure 784/PMMA), concentration of the Irgacure 784 added into liquid MMA monomer were 1wt% and 5wt%. PQ with 1wt% was added into the liquid MMA monomer which was the maximum saturated solubility of PQ dissolved in MMA solution at 333K. The fabrication process of material PQ/PMMA was the same as material Irgacure 784/PMMA. The running curve of diffraction efficiency was shown in Fig. 5. When the concentration of Irgacure 784 was the same as that of the PQ in MMA monomer, it could be seen that the Irgacure 784/PMMA material possessed a higher diffraction efficiency and faster response which means the time to achieve equivalent diffraction efficiency value is shorter. On account of the restriction of the quantity of PQ dissolved in MMA monomer, the diffraction efficiency was greatly restricted. Along with the increase of the concentration of Irgacure 784 dissolved in MMA monomer, the diffraction efficiency could reach to more than 50% approximately. Saturated refractive index modulation Δnsat and a series of other characteristics were computed in Table 1. We defined recording sensitivity (S) as the slope of square root of the diffraction efficiency at a certain time divided by the product of the writing beam intensity and material thickness. It could be calculated by the Eq. (1):Table 1.Response is the time which diffraction efficiency of material reach to the 3.3% (the saturated diffraction efficiency of the material PQ/PMMA). The defect of material Irgacure 784/PMMA was that the time of diffraction efficiency attaining to the peak value was somewhat long. It is necessary to further enhance sensitivity in future study.
For choosing befitting recorded exposure intensity in volume holography storage system, equal intensities of 10, 20, 30, 40mW were set for the two recording beams, respectively. Material thickness was 1.5mm. As shown in Fig. 6, exposure intensity had little impact on the saturated diffraction efficiency of the material, the diffraction efficiency could reach to higher than 50% for all the material.Whereasas the recording intensity further increased, the response rate of the material has a certain improvement. So the material used in the storage is more suitable for highlight recording. Considering the energy savings, for the material performance measurement, we choose the equal intensities of 20mW for the two recording beams, respectively.
The collinear holography system  in our laboratory was used to test the image recording capability of the material. As the system was a reflective system, we attached a mirror to the material. Original reflected image was recorded in our material and transformed to digital signal by a picture receiver (CMOS), finally send to computer. The original reflected image, reconstructed image of experimental results of image reconstruction, reconstructed image of translation 5multiplexing once and fifth time were shown in Figs. 7(a)-7(d), respectively. The images were the same 630 × 630 pixels. High fidelity reconstructed image could be acquired. This supports possibilities for further applications in optical information. But after multiple reused, the quality of reconstructed image had a decline in certain degree.
As it reported that Irgacure 784/PMMA photopolymer fabricated by two-step thermo-polymerization technique followed a similar recording mechanism to that of the PQ/PMMA photopolymer [22,26,27]. While photopolymer was under the illumination of the laser of 532nm, the Irgacure 784 molecules decomposed to multiple intermediate radicals which had benzene ring plane structure. They were activated and attached with PMMA polymer matrix or MMA monomers to become the photoproduct. The difference of refractive index between the photoproduct and the polymer matrix is the main reason for hologram formation in Irgacure 784/PMMA photopolymer. The structural rearrangements induced by the photochemical reaction of Irgacure 784 molecules were mainly attributed to the photoinduced birefringence in the Irgacure 784 doped PMMA. Induced anisotropy material was sensitive to the polarization.
Polarization holography could record polarization information in material with two different polarizations for recording information. In order to measure the polarization characteristic of the material, the representative orthogonal polarization waves were used to record and reconstruct. S-polarized wave was used for the signal wave, p-polarized wave was used for the reference wave to record, and same p-polarized wave as reconstruct wave, schematic diagram of polarization holography was shown in Fig. 8(a). The thickness of material was 1.5mm. The intensity of each record wave was 20mW. The weight ratio of Irgacure 784 dissolved in MMA was 5wt%. It was shown in Fig. 8(b) that the material was sensitive to polarization and could achieve orthogonal reconstruction with large angle recording. Compared with the material PQ/PMMA (the weight ratio of PQ dissolved in MMA was 1wt%), diffraction efficiency and response rate had markedly improved. It is worth mentioned that the time to achieve maximum diffraction efficiency was reduced by about 10 times, greatly enhance the recording speed. Therefore, the material could be used to investigate polarization holography.
In order to better illustrate the material could use in polarization holographic, we recorded and reconstructed image in a polarization holography system .This polarization holography system demonstrated the feasibility of polarization multiplexed holograms. Original transmitted image was recorded in our material and transformed to digital signal by a picture receiver (CMOS). The original transmitted image and reconstructed image of experimental results of image reconstruction were shown in Figs. 9(a) and 9(b), respectively. The images were the same 300 × 300 pixels. The object image was reconstructed faithfully. These results support possibilities for further application in polarization holography.
We present a holographic photopolymer that the photoinitiatorIrgacure 784 has high dissolvability (far more than the photoinitiator PQ) with MMA monomer. Through a series of experiments, we can conclude that the weight ratio of Irgacure 784 dissolved in MMA monomer, exposure intensity is significant to diffraction efficiency. Our Material has good optical properties, 1.5mm thick, 5 wt% photo sensitizer dissolved in the material is better. The diffraction efficiency is capable of exceeding 50% which is much higher than PQ/PMMA material can achieve. We have demonstrated that permanent volume holographic storage in our material has good quality. Traditional and polarization holographic recording are successfully performed with a 532 nm laser beam in Irgacure 784/PMMA and translation multiplexing in traditional holographic recording are well achieving. We expect that Irgacure 784/PMMA volume holography photopolymer material is useful for many holographic applications.
National Natural Science Foundation of China(NSFC) (61475019,61675020).
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