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Abstract
We reveal that polymerization degree is a significant factor that affects the holographic properties of glass-like photopolymer. By modulating polymerization degree, as well as changing the photosensitizer concentration, we fabricate multilayer PQ/PMMA photopolymer, exhibiting excellent performances in storage fidelity and reproduction resolution. A reflection geometry holographic disk system is established using the multilayer sample, in which a high storage density of 40.5GB/cm3 is realized. The reproduced images exhibit high quality with an average diffraction efficiency of 7.4 × 10−4 and average peak SNR of 27.4. This study provides a paradigm for the fabrication and holographic application of other photopolymers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The development of photopolymer in the past decade has triggered increasing interest in holographic application in data storage and holographic optical element due to its advantages in flexible design, high photosensitivity and low cost [1–3]. Many efforts have been made to improve the optical functional characteristics, including in the composition optimization by doping functional components of nanoparticles and organic-inorganic hybrid [4,5], multi-layer optimization on dye concentrations for each layer [6], and synthesis of various matrix systems, such as polyvinyl alcohol (PVA)-based [7], cyclic allylic sulfide-based [8] and poly - (methyl methacrylate) (PMMA)-based [9] polymers. As for the application on volume holography, bulk material where thickness is much larger than light wavelength provides a huge capacity potential for multiplexing techniques [10], therefore, fabricating thick photopolymer with uniform optical quality, low shrinkage during exposure and high optical sensitivity is a significant consideration for large-capacity and high-density data storage.
Notably, among many photopolymers, glass-like phenanthrenequinone (PQ) doped PMMA photopolymer (PQ/PMMA) [11,12] has rapidly drawn research interest for its unique advantages in negligible shrinkage, controllable thickness up to several millimeters and polarization sensitivity, making it suitable as multidimensional high-density holography storage material. Recent years have witnessed some improvement in holographic properties, such as increasing the net diffraction efficiency and linear photoinduced birefringence by doping silica-nanoparticle [13], gold-nanoparticle [14] and SiO2 nanoparticles [15], improving the polarization-holography performance by doping Tetrahydrofurfuryl methacrylate (THMFA) [16]. In the previous work, our Lab has improved the dynamic range and photosensitivity by increasing the concentration of PQ photosensitizer [17], doping SiO2 nanoparticles [18] and ZnMA organic-inorganic hybrid [19], and analyzed the photochemical reaction process [20].
The unique properties of PQ/PMMA glass-like polymer are attributed to the separation of polymerization and photochemical reactions [17]. Polymerization occurs in the material fabrication process to produce PMMA chains, in which the chain length can be modulated by the fabricating parameters. Photochemical reaction occurs in the holographic recording process, when PQ molecules react with other molecules containing unsaturated bonds, resulting in the refractive index modulation due to the difference of PQ concentration in bright and dark regions. The previous work mainly focused on the photochemical reaction, the influence of the polymerization on holographic properties was seldom considered. In this paper, we experimentally reveal the holographic properties of diffraction efficiency, sensitivity, scattering and volume stability in PQ/PMMA samples with various polymerization degree (PD), and propose a novel method to fabricate optimized multilayer PQ/PMMA sample to improve the holographic properties. In addition, a holographic disk storage system suitable for multilayer photopolymer is designed and established based on peristrophic multiplexing combined with shift multiplexing, and the storage density and the quality of reproduced images are studied.
2. Photopolymer preparation
2.1 PQ/PMMA photopolymer fabrication with various PD
The components for fabricating PQ/PMMA photopolymer are PQ (P106382, Aladdin Industrial Corporation, USA) as sensitizer, 2,2'-Azobis(2-methylpropionitrile) (AIBN) (A104256, Aladdin Industrial Corporation, USA) as thermal initiator, and methyl methacrylate (MMA) (M109626, Aladdin Industrial Corporation, USA) as polymerizable monomer. The chemical structures of the components are shown in Fig. 1.
During material synthesis, MMA monomers are initiated by AIBN into PMMA matrix of polymer, and short-chain iMMA (i > 1) molecules that fail to be fully polymerized [12,20]. The average chain length and distribution of these components, as well as the content of residual MMA monomer can be modulated by the fabricating parameters of AIBN content, initiating temperature and initiating duration time. According to the photochemical reaction model [20], PQ molecules are excited into free radicals under exposure and react with unsaturated bond in residual MMA monomer or iMMA components during holographic recording. Therefore, it is worth investigating that PD may affect the photochemical reaction properties of polymers. Experimentally, 3 pieces of samples at the same thickness of 2mm with various fabrication parameters are synthesized, as shown in Table 1.
Table 1. Samples of PQ/PMMA photopolymers
These samples are doped with 0.7wt% PQ content and synthesized at the same polymerization temperature of 60°C and polymerization duration time of 48 hours. Compared with sample 1, sample 2 increases AIBN content from 0.4 to 0.5 mass percent, and sample 3 increases initiating temperature from 75°C to 85°C, in addition, the initiating duration time is reduced by 2 minutes to avoid explosive polymerization.
2.2 Holographic properties of PQ/PMMA with various PD
The number-average molecular weight $\overline {{M_n}} = \frac{{\mathop \sum \nolimits_i {N_i}{M_i}}}{{\mathop \sum \nolimits_i {N_i}}}$ and weight-average molecular weight $\overline {{M_w}} = \frac{{\mathop \sum \nolimits_i {W_i}{M_i}}}{{\mathop \sum \nolimits_i {W_i}}}$ are used to evaluate the PD of PMMA matrix [21], where ${M_i}$ is molecular weight of iMMA, ${N_i}$ is moles of iMMA molecules, ${W_i}$ is the weight corresponding to ${M_i}$. $D = \frac{{\overline {{M_w}} }}{{\overline {{M_n}} }}$ is defined to describe the uniformity of iMMA molecular chains. It should be emphasized that the closer D is to 1, the more uniform the molecular chains are. $\overline {{M_n}} $ and $\overline {{M_w}} $ of the samples are measured by Agilent 1100 gel permeation chromatograph at the working temperature of 30 °C, using Agilent Plgel MIXED-C as column model, and the solvent is polystyrene mixed with tetrahydrofuran. Table 1 shows that $\overline {{M_n}} $ and $\overline {{M_w}} $ of sample 1 are the highest, and sample 3 are the lowest, moreover, the length of polymer chain in sample 3 is the most uniform. We also derive the normalized outflow curves for the 3 samples [Fig. 2], which reveals that the polymerization of MMA molecules in sample 1 is the most sufficient, and the content of short-chain PMMA component is the least due to the peak of the curve tilts to the left, in contrast, the content of short chain PMMA components in sample 3 is the highest.
In order to clarify the effect of PD on the basic holographic performance of materials, we measure the properties of diffraction efficiency (DE), sensitivity, scattering and volume stability for the 3 samples, respectively. A grating is recorded by two equal intensity 532 nm beams at an included angle of 30 degrees and symmetrical incidence in the 3 samples, respectively. DE is defined as the intensity ratio of diffracted light to incident light when the gratings are illuminated using a 633nm laser at the Bragg matched angle. Figure 3(a), (b) and (c) are DE curves versus recording time at the exposure intensity of 56.6mW/cm2, 113 mW/cm2 and 170 mW/cm2 for the 3 samples. The results indicate that DE increases with exposure intensity and decreases with PD, moreover, the photochemical start-up in sample 1 is the slowest, and that in sample 3 is the fastest. Response time (RT), defined as the time from the beginning of exposure to the (1-1/e) of the maximum DE, is used to evaluate the sensitivity. RTs of the 3 samples are 1626, 1410 and 390 seconds in 56.6mW/cm2, 1604, 1002 and 312 seconds in 113 mW/cm2, 1388, 956 and 254 seconds in 170 mW/cm2, respectively. These results indicate that the sensitivity decreases with the increase of PD under the same exposure energy.
Scattering is a significant factor to reduce the quality of reconstructed image during holographic application. We measure the scattering property by focusing a beam in the material [Fig. 3(d)], and define scattering ration (SR) as the power of scattering ${P_s}(t )$ divided by the sum of transmission and scattering ${P_{s,t}}(t )$,
where ${P_t}(t )$ is the transmission power as a function of time. ${P_{s,t}}(t )$ and ${P_t}(t )$ are measured corresponding to the distance of d = 0 and d = f, as seen in Fig. 3(d), respectively. Figure 3(e) shows the SR versus exposure energy for the 3 samples, which indicates that the scattering increases with the decline of PD under the same exposure energy.
Fig. 3. Holographic performances for PQ/PMMA samples. (a), (b), (c) DE versus recording time in sample 1, 2 and 3, respectively. (d) Setup for measuring scattering property, where Ap is aperture, PM is power meter, f is focal length of the Lens. (e) SR as the function of exposure energy for the 3 samples. (f) The Bragg mismatched angle versus dark reaction time for the 3 samples.
The dark reaction [17], a phenomenon that the grating intensity keeps changing for a long time after exposure, can be used to characterize the volume stability of photopolymer. During diffraction reproduction, the angle of incident beam should be adjusted to meet Bragg diffraction condition. We measure the Bragg mismatched angles as a function of dark reaction time for the 3 samples [Fig. 3(f)], which indicates that the volume stability decreases with the decline of PD.
To sum up, the PD can effectively modulate the holographic properties of PQ/PMMA photopolymer. The increase of PD decreases the DE and sensitivity, on the contrary, leads to the improvement of volume stability and the decrease of scattering, and vice versa. These results can be explained by the photochemical reaction process of the photopolymer, in which PQ molecules are more likely to react with small molecules, i.e. residual MMA monomers and short-chain iMMA molecules, due to the more unsaturated bond, the better mobility and diffusion performance compared with long-chain PMMA molecules. The increase of PD reduces the content of small molecules, resulting in the decrease of photochemical reaction rate and speed, as a result, scattering decreases and the volume stability increases. Considering the trade-off in the properties of DE, sensitivity, scattering and volume stability for holographic storage application, the optimized number-average molecular weight for PMMA matrix should be between $1.0 \times {10^5}$ and $2.0 \times {10^5}$ for PQ/PMMA photopolymer.
2.3 Samples with multilayer optimization
The light intensity declines with the propagation distance in bulk material, resulting in recording a nonuniform grating along the direction of thickness when two beams interfere. As a result, the distortion occurs and the resolution decreases after dark reaction when images are recorded. Here we propose a method to mitigate the distortion by use of multilayer PQ/PMMA photopolymer.
According to Lambert-Beer’s law, the light absorbance ${I_a}(t )$ in bulk material can be described by [19],
where ${I_0}$ is incidence intensity, d is propagation distance, $\varepsilon [{\textrm{PQ}} ](t )$ is molar absorption coefficient, which is a function of PQ sensitizer concentration. By differentiating Eq. (2), the absorbance $\Delta {I_a}(t )$ when light propagates $\Delta d$ is obtained, Based on experimentally measurement $\varepsilon [{\textrm{PQ}} ](0 )= 0.11/mm$ at the PQ concentration of 0.7wt%, we theoretically analyzed the relationship between $\Delta {I_a}/\Delta d$ and the propagation distance. Figure 4(a) shows the result of propagation from 1mm to 3mm at an interval of 0.5mm. In contrast, if the $\varepsilon [{\textrm{PQ}} ]$ linearly increases from 0.11 to 0.15, corresponding to propagation distance, the uniformity of light absorption will be greatly improved [Fig. 4(b)]. According to the photochemical reaction kinetics [20], the improvement of $\varepsilon [{\textrm{PQ}} ]$ requires more chemical reaction of PQ molecules and iMMA molecules, and thus, we correspondingly reduce the PD at the direction of thickness and design an optimized 5-layer configuration, as shown in Fig. 4(c).Fig. 4. Analysis on absorbance and configuration for PQ/PMMA photopolymer. Theoretical analysis on absorbance in homogeneous material (a) and multilayer material (b). Optimized 5-layer configuration (c).
Experimentally, 5 pieces of PQ/PMMA samples with various PQ concentration are prepared, respectively, by changing AIBN content, initiating temperature and initiating duration time. PD measured and PQ concentration are (1.88${\times} {10^5}$, 0.6wt%), (1.74${\times} {10^5}$, 0.7wt%), (1.65${\times} {10^5}$, 0.8wt%), (1.50${\times} {10^5}$, 0.9wt%) and (1.39${\times} {10^5}$, 1.0wt%). Then, the samples are dissolved with tetrahydrofuran to colloidal state and stacked in a mold layer by layer with the same thickness. After each stack, we place the material in a thermostat 10 to 15 minutes at 60 °C to evaporate the solvent and make the top layer in a semi solidified state. Finally, the 5-layer sample is cured in the thermostat at 60 °C for more than 48 hours. This process does not change the molecular weight and distribution of each layer and maintain good optical transparency and physical connection.
3. High-density and high-quality holographic storage
3.1 Setup for high-density holographic storage
A reflection geometry holographic disk storage system was established [Fig. 5]. It can record grey patterns by using peristrophic multiplexing [22] combined with shift multiplexing, and capture retrieved images adaptively from continuously rotating and moving disk radially. A DPL laser with the wavelength of 532nm is used as light source. The laser beam is expanded by a spatial filter (SF) and a lens (L1), then split by a polarized beam splitter (PBS) into object beam and reference beam, whose ratio of intensity can be changed by use of a half wave plate (HP1) mounted behind the light source. The object beam is modulated by a transmission spatial light modulator (SLM, CRL XGA1L12 with 1024 × 768 pixels), then the Fourier spectrum of the SLM pattern appears on the back focal plane of the FL1 lens. On the reference path, HP2 is placed to adjust the polarization direction to realize the best interference with the object beam. The sample is made into disc shape, mounted through a metal rod on a rotation mechanism with resolution of $0.004^\circ $, driven by a SURUGA SEIKI K401-60M rotary stage. The rotation mechanism is mounted on a 1D motorized stage KS101-20HD with the resolution of $0.02\mu m$. Two shutters and a CCD (IPX-1M48-L camera with 1004 × 1004 pixels) are used as on-off switches and output device, respectively. All these electronic devices connect with a computer and controlled by a self-made software. During holographic recording, Shutter1 and Shutter2 switch on and the Fourier spectrums of the SLM patterns are recorded in the sample. During holographic reconstruction, Shutter1 switches off and Shutter2 switches on, then the spectrum is diffracted and transformed by Fourier lens FL2 into the reconstructed image.
Fig. 5. Setup for high-density holographic storage and reconstruction, A is the enlarged red ring area. HP: Half wave plate, SF: Spatial filter, L: lens, PBS: Polarized beam splitter, SLM: Spatial light modulator, FL: Fourier lens.
In order to realize the high-density and high-quality holographic storage, we have made some optimization as follows. First, with the reference beam perpendicular to the sample, the angle between the object beam and the normal of the material should be reduced as much as possible, resulting in a small exposure spot of approximately 0.25mm2 at the angle of 150 degrees between the reference and object beams. Second, a convergent spherical reference beam was produced by using the lens L2 with focal length of 88.9mm, as same as that of FL1, achieved better angle selectivity compared with Gaussian beam, and improved the storage density. Furthermore, ipsilateral out-of-focus geometry, as shown in the enlarged red ring area of Fig. 5, where the two focal points are located outside the sample of approximately 0.5 mm, not only reduced the scattering effect, but also matched the multi-layer material design to obtain high-quality holographic reconstruction.
3.2 Experiments and analysis
We first studied the holography performance of the multi-layer sample. A gratings image [ Fig. 6(a)], which can evaluate the resolution of 80 lines/mm, 100 lines/mm, 120 lines/mm, 140 lines/mm and 160 lines/mm in 4 different directions, was recorded in sample 2 and a 2 mm thick 5-layer sample with each layer of 0.4 mm thickness, respectively, using the reflection geometry holographic disk storage system at the exposure energy of 10mJ. 2 hours later, the images were reproduced, exhibiting a serious distortion in sample 2 and the resolution was less than 80 lines/mm [Fig.6b)], in contrast, the distortion was well suppressed and the resolution reached more than 100 lines/mm in the 5-layer sample [Fig.6c]. The results indicate that the multilayer material with variable PD is conducive to improve the storage fidelity and reproduction resolution, and also achieve a trade-off in sensitivity and volume stability.
Fig. 6. Comparation of original image (a), reproduced image in a 2 mm thick conventional (b) and multilayer (c) PQ/PMMA photopolymer under the dark reaction of 2 hours. Part of image is enlarged in the red circle. Scale bars: 100 $\mathrm{\mu m}$.
Then, we studied the holographic multiplexing performance of the storage system based on a 1.5 mm thick 5-layer PQ/PMMA sample at the same configuration as described in section 2.2 with each layer of 0.3 mm thickness. In order to avoid multiplexing image interference, the selectivity of rotation and radial movement were measured. At the condition that the radial moving direction was at an angle of 45 degrees with the interference plane, we obtained the optimized multiplexing of rotation selectivity 0.052 degrees and radial movement selectivity 0.17 mm around the radius of 11 mm. To demonstrate the holographic storage performance of the system, 18000 binary and grayscale images with resolution of 400 × 300 pixels, which was available for the resolution of material, were recorded in three adjacent tracks. Each track recorded 6000 images with angular spacing of 0.06 degrees and track spacing of 0.17 mm. The recording time was 0.5 second, exposure energy of approximately 2mJ for each image, resulting in the storage density of 40.5GB/cm3. Accordingly, it could be estimated that more than 680GB capacity was realized in a standard 120 mm diameter disc. In practice, the capacity would be further increased by improving the resolution of the input images. After the dark reaction of 2 hours, the images were retrieved and the diffraction efficiency for the retrieved images were measured, as Fig. 7(a), (b) and (c) show. The average diffraction efficiency was 7.4 × 10−4.
Fig. 7. Retrieval from three adjacent tracks in a 5-layer PQ/PMMA sample. (a) (b) (c) corresponding to the diffraction efficiency of the 6000 images in each track. (d) The PSNRs for the images in the 1st track. (e) Some reproduced images from the 3rd track.
To evaluate the holographic reconstruction performance, the peak signal-to-noise ratio (PSNR) [23] was used by comparing the gray values of each pixel of the original image and the retrieved image. It is defined as $\textrm{PSNR} = 10 \cdot \lg \frac{{{{255}^2}}}{{\textrm{MSE}}}$, and $\textrm{MSE} = \frac{1}{{MN}}\mathop \sum \nolimits_{x = 0}^{M - 1} \mathop \sum \nolimits_{y = 0}^{N - 1} {[I({x,y} )- {I_0}({x,y} )]^2}$, where MSE is the mean squared error, ${I_0}({x,y} )$ and $I({x,y} )$ are grey level of the original and retrieved pixel, respectively. Part of the PSNR of 6000 patterns in the 1st track is calculated and the results are shown in Fig. 7(d), from which it could be seen that the PSNR of these patterns are in the range from 25 to 28, and the average PSNR is approximately 27.4. The retrieved images exhibit good quality. Some of them in the 3rd track are shown in Fig. 7(e).
4. Conclusion
We experimentally verify that PD is a significant factor affecting the holographic properties of glass-like photopolymer. As PD increases, the volume stability and anti-scattering properties of the material increase, and the refractive index modulation and sensitivity decrease. In order to achieve high fidelity and high-density volume holographic storage, we fabricated 5-layers PQ/PMMA sample by changing the PD and PQ molecular concentration in each layer, and established a reflection geometry holographic disk storage system by using peristrophic multiplexing combined with shift multiplexing, achieved a storage density of 40.5GB/cm3, estimating more than 680GB data stored in a standard 120mm diameter disc with the thickness of 1.5mm. In the reproduction experiment of 18000 images stored in three adjacent tracks, high-quality image reconstruction with average diffraction efficiency of 7.4 × 10−4 and average peak signal-to-noise ratio of 27.4 was realized. This study provides a paradigm for the manufacture and holographic application of other photopolymers.
Funding
National Natural Science Foundation of China (21973023, 62175052).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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