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Abstract
This work proposed a new kind of photosensitizer doped photopolymer and an optimized fabrication method. Holographic gratings are recorded in a glass-like photopolymer based on methyl methacrylate (MMA) as monomer, 2,2-azo-bisisobutyrolnitrile (AIBN) as thermo-initiator and a cationic initiator with high sensitivity and solubility, titanocene (Irgacure 784, BASF) (TI), as photo-initiator. In our fabrication, an optimized three-step thermal-polymerization method is investigated and depicted. 3 mm thick TI/PMMA photopolymers with different concentrations of AIBN and TI molecules are examined in detail. The photo-physical and photo-chemical processes inside the sample during continuous exposure are analyzed. In our photopolymers, permanent volume holographic gratings with diffraction efficiency approaching 74% and response time close to 20s, which makes this photopolymer available in volume holographic data storage.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Volume holographic permanent memory in bulk photopolymers is a potential storage technology with high density storage, low cost, high thickness and neglectable shrinkage [1–3]. The phenanthrenequinone (PQ) doped poly (methyl methacrylate) (PMMA) photopolymer is considered as a suitable holographic storage photopolymer due to its memory stability and density [3–6]. This kind of material also has a characteristic of dark enhanced diffusion which means it can improve the holographic properties after exposure in the surroundings of darkness [7,8]. The photo-physical and photo-chemical mechanisms of PQ/PMMA polymers have been researched in detail [9–11]. In our previous work, PQ/PMMA photopolymers can record holograms under both continuous exposure [12] and pulsed exposure [13–15]. However, low diffraction efficiency and slow response time are two main disadvantages of PQ/PMMA photopolymers. This is because the PQ molecules have low solubility in MMA monomers (up to 1.0wt% PQ can be dissolved in MMA at 60 °C in our previous work) [16,17]. In order to enhance both grating strength and response time, metal nanoparticles dispersed PMMA caused plasmon-induced holographic absorption system [18–21] and oxide nanoparticles mutual diffusion system [22,23] have been proposed and researched. However, complex fabrication process and holographic scattering caused by nanoparticles are severe flaws. Discovering a new kind photo-initiator to replace PQ molecules becomes an available approach.
Titanocene (Irgacure 784, BASF) (TI) is a potential photo-initiator with high solubility in organic solutions and large absorption coefficient in the range of 400 to 550 nm wavelength [24]. In photo-induced process, PQ molecules absorb photons to transform into a more stable triple state 3PQ. PMMA macromolecules is then initiated by 3PQ and transformed into its corresponding radicals. Meanwhile, the PQ radicals get attached with PMMA radicals to form PQ-PMMA chains [16,17]. Different from PQ molecules, TI molecules absorb photons to create the photo-induced isomers. The isomers are then replaced by the ester carbonyl groups of acrylate, producing free radicals, which initiate polymerization and crosslinking [25]. Some studies have confirmed that TI molecules doped photopolymers exhibit better holographic performance than dyes photo-initiators [24–28]. Therefore, TI molecules are considered as a next generation photo-initiator in PMMA matrix photopolymers.
In this paper, we present an optimized three-step thermal-polymerization method to fabricate TI molecules doped PMMA photopolymers. Firstly, the precise holographic performance influence of different thermo-initiator (AIBN) concentrations is investigated (from 0.5wt% to 3.0wt%). Then, TI molecules doped with different concentrations (from 1.5wt% to 4.5wt%) are also examined. Finally, the Bragg angular selectivity of different AIBN and TI ratios doped PMMA photopolymer is analyzed. All the researches in this paper provide a modified bulk TI/PMMA photopolymer with fast response and high grating strength for volume holographic data storage.
2. Preparations and experiments
In our three-step thermal-polymerization method, there are three main processes, pre-polymerization, high temperature polymerization and low temperature polymerization. In the first stage, the photo-initiator TI molecules (1.5wt%~4.5wt%) and thermo-initiator AIBN (0.5wt%~3.0wt%) are dissolved into MMA monomers. The viscoid liquid mixture is then stirred at a particular temperature for 24h in order to remove the nitrogen produced by thermal decomposition of AIBN. Secondly, the homogeneous solution is filtered and moved to the incubator with higher temperature for the rapid polymerization of MMA monomers. Finally, the viscoid liquid mixture is solidified at 45°C for 48h. 3 mm TI/PMMA photopolymers are obtained after polishing, as shown in Fig. 1(a). In addition, to obtain TI/PMMA photopolymers with good optical quality, different stirring temperatures and incubator temperatures are chosen under different doping concentrations of TI molecules and AIBN. The detailed parameter conditions are shown in Table 1. In this paper, we want to make a detail research on holographic properties of thick titanocene dispersed poly (methylmethacrylate) photopolymer. In the material system, the content of AIBN and TI molecules mainly effect the holographic properties. Therefore, we examined the holographic properties with different TI and AIBN molecules doped PMMA photopolymers. Sample 1-6 were selected to investigate the effect of different AIBN ratios on the holographic characteristics with the same TI ratio (1.5wt%). Meanwhile, sample 4 and 7-12 were selected to examine the influence on different TI ratios on the holographic properties with an optimized AIBN ratio (2.0wt%). In our fabrication process, pre-polymerization and high temperature polymerization are the two main factor to effect the curing process of our samples. AIBN molecules play the role to absorb the heat and form free radicals to promote monomers polymerization. Therefore, changing the stirring and incubator temperature influence the degree of thermal decomposition of AIBN, more heat can lead to bubbles in solidified samples due to the nitrogen produced by thermal decomposition of AIBN, while less heat can make it impossible to solidify the sample. At the same time, the dispersion of TI molecules in the system will hinder the thermal decomposition of AIBN. As a result, different combinations of TI and AIBN need to synthesize samples under different experimental conditions. The parameter of stirring and incubator temperature should match the corresponding content. The temperature parameter in Table. 1 is the optimum temperature determined by a lot of experimental tests. All of our samples were fabricated under standard atmospheric pressure. We know MMA is very volatile, but the volatile content under thermal polymerization is very small (up to 10wt% in our fabrication process), this is acceptable because we need to polish the sample to 3mm or even thinner, the original thickness of the bulk can reach 6mm with our method and dosage.
Fig. 1 (a) 3 mm TI/PMMA photopolymers; (b) Two beams coupling interference system, PBS, polarization beam splitter.
Table 1. Detailed parameters in preparations of TI/PMMA photopolymers.
A two-beam coupling interference system was used to investigate holographic performances, as shown in Fig. 1(b). The holographic recording and reconstruction process can be depicted as follows: Firstly, we open shutter 1 and close shutter 2, the 532 nm green-light CW laser beam is divided into two beam with equal power, the same optical path and polarization state. Unslanted transmission gratings are recorded with the incident angle of 20° into the sample with different TI and AIBN ratios. Secondly, we turn off shutter 1 and open shutter 2 to let 633 nm red-light CW laser beam go through the sample with the corresponding Bragg’s matched angle. The diffraction can be observed through the detector. In the experiment, we examined the diffraction every 5s of green beam interference recording.
3. Results and discussion
3.1 Visible absorption spectra
Figure 2 shows the visible absorption spectra of different concentration ratios of TI molecules and AIBN. The absorption coefficient is not obviously affected by the variation of AIBN content, as shown in Fig. 2(a). As the AIBN ratios were being increased, more PMMA short chains were generated. In the process of photo polymerization, the photosensitive molecules not only polymerized with the MMA monomers, but also with the short-chain PMMAs. It is indicated that higher AIBN concentrations will give rise to more opportunities for the polymerization of photosensitive molecules, which eventually leads to an enhancement of grating strength. Figure 2(b) shows the absorption coefficient influenced by different TI molecule ratios. A significant enhancement of absorption in TI/PMMA photopolymers is generated with the increasing amount of TI molecules, which provides the basis for further improvement of the diffraction efficiency and response time.
3.2 Holographic parameters
An unslanted transmission grating was recorded in TI/PMMA photopolymer with different exposure energy. The diffraction efficiency plays an important role in describing holographic storage performance, which was firstly investigated. In the experiment, the first-order diffracted intensity, Id, the incident intensity, II, and the reflected intensity IR of the probe beam were measured. The diffraction efficiency could be expressed as,
Secondly, we could calculate the refractive index modulation according to Kogelnik theory [29], as shown in Eq. (2). where d is the effective thickness of samples, is the recording wavelength and represents the included angle between the reference beam and the object beam with the perpendicular to the recording media measured inside the photopolymer, which can be calculated with the Snell’s law.
In addition, the temporal variation of while recording could set the trend of the response time, described as
where is the maximum diffraction efficiency, is defined as the response time when the square root of diffraction efficiency increases to 1-1/e of the square root of maximum diffraction efficiency . The response time is obtained by exponential fitting through experimental data. Static and dynamic sensitivities were defined according to the refractive index modulation . expressed as, Here, and are the static and dynamic sensitivities, respectively, and E is the total exposure flux.The response time was calculated by exponential function fitting of diffraction efficiency curves, which could be used to indicate the response characteristic. We measured the static and the dynamic sensitivities to describe the stability state and the growth rate of grating formations, respectively [30]. Holographic performances of samples with different TI molecule and AIBN ratios were examined and calculated according to the above formulas.
3.3 Holographic performances influenced by thermo-initiator (AIBN) ratios
Firstly, we examined influence of sample thickness on holographic properties with sample 1, as shown in Fig. 3. With the increment of thickness, the grating strength has an obvious promotion. Therefore, 3mm thickness is selected to investigate the TI/PMMA photopolymers with different AIBN and TI ratios. Meanwhile, it does not mean the 3mm thick sample is the optimum thickness, we only want to research the holographic properties of TI/PMMA photopolymers with no consideration of the influence of thickness.
From the internal components of TI/PMMA photopolymer we could see, its optical properties were affected by two aspects, TI molecules and AIBN ratios. In this section, TI/PMMA photopolymers with different AIBN ratios were investigated and analyzed. In order to compare holographic performances in different AIBN ratios dispersed TI/PMMA photopolymers, samples were illuminated under the same exposure flux with different laser intensities, as shown in Fig. 4. Different AIBN ratios in samples affected polymerization degree of MMA monomers, which indicated that higher AIBN concentrations led to more short-chain PMMAs formation. In the experiment, TI/PMMA photopolymer with different AIBN concentrations changed from 0.5 to 3.0wt % were examined. As is illustrated, the diffraction efficiency was enhanced with the increasing of AIBN ratios (from 0.5 to 2.0wt %). However, a decline of diffraction efficiency was occurred while doping AIBN more than 2.0wt%. It was mainly because when increasing the content of AIBN, there were more residual AIBN radicals could not be consumed, eventually hindered the diffusion of photosensitive molecules. Another possible process is related to termination reactions. More AIBN radicals could enhance termination reactions hindering the growing polymer chains. The linear absorption and holographic scattering were the main reasons for transmission losses in samples. According to the results, 2.0wt % AIBN dispersed ratio was a better choice in TI/PMMA photopolymers with good holographic performances. Meanwhile, we examined the holographic properties of PQ/PMMA polymers to make a contrast with the TI/PMMA polymers. The TI molecules with 1.5wt% were first added into the MMA matrix in order to make the same molar concentration as PQ molecules. As shown in Fig. 4(d), the grating strength of TI/PMMA polymer is lower than that of PQ/PMMA with the developed AIBN concentration, which means the thermo-initiator is not a key factor in the matrix to avoid the holographic properties. All of the above analysis is only a preliminary explanation, the further explanation needs to measure AIBN radicals and the length of PMMA in our future work.
Fig. 4 Temporal evolution of diffraction efficiency under holographic recording with different AIBN ratios, (a) exposure intensity: 115mW/cm2, (b) exposure intensity: 96mW/cm2, (c) exposure intensity: 64mW/cm2, (d) comparison with traditional PQ/PMMA polymer.
The response time of TI/PMMA photopolymers were also influenced by the AIBN ratios. Although the increase of AIBN ratios made a contribution to the diffraction efficiency, the formation of transmission gratings was hindered due to the excessive AIBN radicals. With the increasing amount of AIBN ratio, the response time was rising, as shown in Fig. 5.
Furthermore, the static and dynamic sensitivities of samples with different AIBN ratios were also examined, as shown in Fig. 6. As previously illustrated, an attenuation of the static and dynamic sensitivities occurred due to the increasing amount of residual AIBN radicals. Therefore, the change of AIBN ratio in photopolymers would affect many holographic characteristics. After a comprehensive evaluation of various indicators, 2.0wt% AIBN was a better doping concentration.
Fig. 6 Sensitivities of TI/PMMA photopolymers with different AIBN ratios, (a) dynamic sensitivity; (b) static sensitivity.
In addition, we examined the refractive index modulation in different AIBN ratios dispersed samples, as shown in Fig. 7. In comparison to the traditional AIBN doping concentration in PQ/PMMA photopolymers (0.5wt%) [12,13], the optimized AIBN concentration (2.0wt%) enhanced the refractive index modulation by 25%. This results indicated that TI/PMMA samples with higher concentration of AIBN (2.0 wt%) possessed an efficient PMMA surrounding matrix for transmission grating recordings.
3.4 Holographic performances influenced by photo-initiator (TI) ratios
The proportion of different components in the photopolymers had a great influence on their holographic performances. In the TI/PMMA composition system, the ratio of AIBN determined the degree of polymerization in the process of thermo-polymerization, which led to more short-chain PMMAs generation, so as to make up for the shortcomings of insufficient monomers in this system. The photo-initiator TI molecules was a key factor affecting the holographic performances in photo-polymerization as compared with the thermo-initiator AIBN. The TI molecules were a kind of cationic photo-initiator. Complex photo-polymerization process was succinctly expressed as [23,24],
where B represented the by-products from the splitting decomposition of [TI]*. There were two approaches to form the transmission grating. One was due to the diffusions of [TI]* and free TI molecules, which had the main contribution to grating formation. This was a photo-physical process. The other one was the photo-chemical process between the TI molecules and the PMMA/MMA monomers. TI molecules absorbed photons and transited into excited state [TI]* at an initiation rate ka. However, the excited [TI]* turned back into TI at a rate of kb. Then, the [TI]* photo-cleaved into unstable diradicals and by-products (stable aryl compounds) with the cracking rate kd. The unstable diradicals interacted with two PMMA or MMA monomers to form stable photo-products at a polymerization rate ke. In addition, there were two main reactions in photo-chemical process, one was the above interactions between unstable diradicals and monomers, and the other was a direct polymerization of [TI]* and monomers. These two photo-chemical reactions eventually led to the formation of photo-products. Finally, both the photo-products and the diffusion of TI molecules maintained the stability of transmission gratings. It is a tentative explanation since we did not measure TI radical concentrations or other intermediates while exposure.This photo-initiator (TI molecules) had a high solubility in MMA solution (~up to 10 wt%). However, increasing the content of photo-initiator did not always improve its holographic performances. In this section, our aim was to find an optimized dopant concentration of TI molecules. Firstly, we examined the temporal evolution of diffraction efficiency of photopolymers with different TI ratios, as shown in Fig. 8. With an increasing amount of TI molecules in the samples, there was a significant improvement on diffraction efficiency, as was previously demonstrated, there were two contributions to the grating formation, diffusions of photo-initiators and creations of photo-products. Therefore, increasing the content of photo-initiator greatly enhanced the diffraction efficiency. In the experiment, the diffraction efficiency was enhanced from 40% to 75.6% by doping additional 3.0 wt% photo-initiator. Also, another approach to improve diffraction efficiency was to increase the exposure power. Higher power excited more TI molecules, which led to an enhancement of grating formation and a reduction of response time.
Fig. 8 Temporal evolution of diffraction efficiency under holographic recording with different TI molecule ratios, (a) exposure intensity: 115mW/cm2, (b) exposure intensity: 96mW/cm2, (c) exposure intensity: 64mW/cm2, (d) comparison with traditional PQ/PMMA polymer.
We obtained the maximum diffraction efficiency 75.5% in samples with 4.5 wt% TI molecules under 115 mW/cm2 energy level. However, the response time had a severe growth. Therefore, the response time with samples of different TI molecule ratios were examined, as shown in Fig. 8. In the experiment, the photopolymer with 4.0 wt% TI molecules got the shortest response time of 20.5 s with the diffraction efficiency of 74%, as shown in Fig. 9. Although the sample with 4.5wt % TI molecules got better diffraction efficiency, its response time was 1.75 times longer than that with 4.0 wt % TI molecules. Meanwhile, the response time was not significantly shortened with the increase of photo-initiator content, which was not as we expected. The main reason was that, different from PQ molecules, the TI molecules generated photo-cleavage phenomena after absorbing photons. Among these products, only the unstable diradicals contributed to the grating formation. The remaining by-products became barriers to the grating formation. In the case of low photo-initiator content (1.5-4.0 wt%) in the sample, the photo-initiator could sufficiently absorb photons, the main reaction was the photo-cleavage and the less hindered by-products. In the case of high photo-initiator content (>4.0 wt%) in the sample, TI molecules could not transit into excited state [TI]* completely, because there were residual photosensitive molecules which were not involved in photochemical reactions. Also, the generated by-products created more obstacles to the grating formation. These two aspects together led to the growth of response time. In our experimental conditions, photopolymers with 4.0 wt% TI molecules kept the balance between diffraction efficiency and response time, was selected as an optimized concentration. Meanwhile, Fig. 8(d) depicts the grating strength difference between TI/PMMA and PQ/PMMA polymers. From the experimental result we can see, the PQ/PMMA polymers get 46% diffraction efficiency with the response time of 43s, while the TI/PMMA polymers get 74% with the response time close to 20s, which indicates that the concentration of photo-initiator in the matrix is a main factor to avoid the holographic performances.
Then, we examined the static and dynamic sensitivities of samples with different TI molecule ratios, as shown in Fig. 10. The sensitivities of TI/PMMA photopolymers mainly described the formation capacity of gratings. On the one hand, the static sensitivity described the required exposure energy when the grating strength reached the maximum value. The static sensitivity directly reflected the ability of material in refractive index modulation. On the other hand, the dynamic sensitivity reflected the growth rate of the grating formation after the material was illuminated, which depicted the velocity of grating formations. Unlike the influence of AIBN ratios in TI/PMMA photopolymers, with the increase of photo-initiator content, both the static and the dynamic sensitivity were enhanced. As observed in Fig. 10, there is a dynamic turn at 4.0wt % TI molecules doped samples, which was previously analyzed. According to different holographic performances in TI/PMMA photopolymers, a better photo-initiator concentration, 4.0 wt%, was selected.
Fig. 10 Sensitivities of TI/PMMA photopolymers with different TI molecule ratios, (a) dynamic sensitivity; (b) static sensitivity.
Figure 11 depicted the change of refractive index modulation with increasing energy flux in different TI molecule ratios dispersed samples. The refractive index modulation 7.61 × 10−5 with the exposure flux of 10J/cm2 was obtained in 4.0 wt% TI molecules dispersed TI/PMMA photopolymers. According to experimental results, TI/PMMA photopolymers with 4.0 wt% TI molecules and 2.0 wt% AIBN exhibited a better holographic performance.
3.5 Bragg angular selectivity of different TI&AIBN doped PMMA photopolymers
In this part, we only want to examine the influence of different AIBN and TI molecules doping concentrations on the Bragg angular selectivity. Also, it is known that the thickness of sample is a significant factor for the Bragg angular selectivity Therefore, in the experiment, we polished all the samples to 3 mm. The influence of sample thickness can be eliminated. To examine the holographic storage capacity, the Bragg angular selectivity of storage medium is a main index. We use the full width at half maxima (FWHM) of grating strength to describe the angular selectivity. The density of storage medium will be higher when the FWHM is smaller.
Figure 12(a) and 12(b) depicts the Bragg angular selectivity curves with different TI&AIBN ratios doped PMMA photopolymers. Samples with different doping photo-initiators and thermo-initiators have different Bragg angular width. However, the relationship between the angular with and the initiator (TI&AIBN) concentration is nonlinear. We use the full width at half maxima of saturation grating strength to describe the nonlinear angle width, as shown in Fig. 12(c) and 12(d). From the experimental result we can see, different doping concentration of AIBN and TI molecules do not show much influence on Bragg angular selectivity with the same thickness of 3 mm. The main reason for the diversity of angular width is because of the slightly deviation in thickness while polishing. The FWHM for our TI/PMMA photopolymers is around 0.0634°~0.0845°, while the PQ/PMMA is around 0.0533°. Though the FWHM for TI/PMMA is a little higher, it meets a basic need for holographic memory, which indicates that this kind of material owns good optical properties and high storage density.
Fig. 12 (a) Bragg angular selectivity scan with different AIBN doping ratios, (b) Bragg angular selectivity scan with different TI doping ratios, (c) nonlinear angle width with different AIBN doping ratios, (d) nonlinear angle width with different TI doping ratios.
4. Summary
In this paper, a three-step thermal-polymerization method was proposed to fabricate TI molecules dispersed PMMA photopolymers. 3 mm thick samples were prepared at different fabrication conditions. The effect of different components in photopolymers on holographic memory performances have been investigated in detail. Contents of AIBN decide the degree of polymerization in the process of thermo-polymerization, which leads to more short-chain PMMAs (available monomers) generation. Contents of TI molecules determine the rate and intensity of photo-polymerization process, which can promote the speed and strength of the transmission grating formation. We give a tentative explanation on the photo-polymerization process of TI/PMMA photopolymers, a detail analysis can be achieved after measuring the radical consumption and the length of PMMA while exposure in a further research. By adjusting their contents in the substrate, sufficient monomers and photo-initiators can be supplied. At the same time the hindrance caused by by-products can be minimized. An optimized concentration of 4.0wt% TI molecules and 2.0 wt% AIBN in TI/PMMA photopolymers was obtained with diffraction efficiency approaching 74% and response time close to 20s. Also, the refractive index modulation reached 7.61 × 10−5 with the exposure flux of 10J/cm2. The angular width can reach 0.0634°, which means it has a good storage capacity. This photopolymer can be seen as an upgraded product of PQ/PMMA photopolymers, which is more competitive in further improvement of holographic data storage.
Funding
National Basic Research Program of China (2013CB328702); National Natural Science Foundation of China (11374074).
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