Photopolymers are interesting materials to obtain high-quality performance for the volume holographic data storage with a low noise and high diffraction efficiency. In this paper, the recording of holographic diffraction gratings with a spatial frequency of approximately 1940lines/mm in photopolymerizable epoxy resin materials is experimentally demonstrated. Diffraction efficiency near 92% and an energetic sensitivity of 11.7×10-3cm2/J are achieved by designing the proper structure of matrix and also optimizing photopolymer compositions. The effect of photopolymer compositions on the fundamental optical properties is also discussed.
©2007 Optical Society of America
In recent decades a great deal of research about photopolymers has been carried out in the field of holographic data storage owing to their great advantages: a relatively large refractive index modulation (Δn ∼10-2), high energetic sensitivity, low cost, and no need of post chemical treatment [1–3]. In spite of these merits, photopolymers have still a fundamental problem that the original image is distorted by volume shrinkage that occurs during photopolymerization of the monomer. This volume shrinkage usually takes place in the direction perpendicular to the film surface  and limits the use of photopolymer as a medium of holographic data storage system. To overcome this problem, intensive research has been done including sol-gel system [5–8], cationic ring opening polymerization system [9–10], organic-inorganic hybrid system [11–15], and crosslinked matrix system [16–18]. Among those systems crosslinked matrix system based on epoxy resin would be beneficial to control the thickness of polymer film because monomers of epoxy resin exist as a liquid state, so that it does not need any solvent for preparing photopolymer films. In addition, the photopolymer based on epoxy resin system keeps a good dimensional stability during photopolymerization. In spite of these, a high chain crosslinking density of epoxy resin system still causes a limitation in holographic performance as reported by Timothy et al. They insisted that the high crosslinking density dictates the matrix rigidity and hinders the diffusion of monomers for photopolymerization, resulting in an insufficient energetic sensitivity [3, 16]. In connection with these, in this work, we introduce different epoxy monomer and amine hardener with longer chain length from the one previously used, which is expected to reduce the crosslinking density of matrix and thereby enhance energetic sensitivity. We report the photopolymer system of enhanced energetic sensitivity without sacrificing both diffraction efficiency and dimensional stability on volume shrinkage.
2. Epoxy-resin photopolymer
Polypropylenediglycidylether (PPGDGE, Mn=380, n= 1.464) and polyethyleneimine (PEI, Mn ∼10,000 n=1.512) for the synthesis of epoxy resin were purchased from Aldrich and Wako Chemicals, respectively. Acrylamide (AA, n= 1.550) that is a photopolymerizable monomer, triethanolamine (TEA, n= 1.485) of a co-initiator, and yellowish eosin (YE, λmax=524nm) as a sensitizer were all purchased from Aldrich. All reagents were used as received without further purification. Molecular structures of photopolymer components are shown in Fig. 1. Reactants for photopolymerization were prepared by mixing a proper amount of the constituents. The PPGDGE and PEI are mixed with 1:1 molar ratio of amine to epoxide groups. All the reactants were prepared under red light and stored in the dark to prevent polymerization of acrylamide during the epoxy curing at room temperature. The photopolymer films were prepared by placing a few drops of reactant solutions on the glass plates between stainless steel spacers. The PPGDGE/PEI are cured at room temperature for 12 h and completion of the reaction was confirmed by identifying the disappearance of the characteristic peak centered at 890cm-1 corresponding to epoxide ring vibration with FT-IR (Bruker Tensor27). The cast film thickness was 200∼240μm measured on micrometer and thermal analysis was conducted with a DSC (TA Instruments DSC2010).
3. Holographic recording
A diode-pumped solid-state (DPSS) microchip laser with a wavelength of 532nm was used to record diffraction gratings by means of continuous laser exposure. Output from Melles Griot laser (model Green laser 532) was spatially filtered and collimated to provide the holographic exposures. The laser beam was split into two secondary beams with an intensity ratio of 1:1. The working intensity of each beam was 1.48mW/cm2, and both beams were recombined on the sample at an angle of 20° to the normal which results in the spatial frequency of approximately 1940lines/mm. The diffracted and transmitted intensity was monitored with an Ophir optical meter (model PD300-SH) through shutter blocking setup, which is described in our previous work . In order to obtain diffraction efficiency as a function of angle for reconstruction, the sample was rotated using a computer-controlled motorized stage from Suruga Seiki (model D250) with resolution of 0.01°.
3.1 Optimization of the YE concentration
One of critical factors determining the diffraction efficiency and energetic sensitivity is the relative amount of sensitizer in the reactants. Generally, energetic sensitivity can be improved with increasing the amount of sensitizer that induces photoinitiation, however, it may be deteriorated when the amount exceeds a certain critical value because additional energy is needed to photobleach the sensitizer in excess. Figure 2(a) shows the temporal traces of diffraction efficiency for epoxy resin-based photopolymers with various amount of sensitizer. The detailed compositions are summarized in Table 1.
The diffraction efficiency is defined as Id/Ii, where Id and Ii are intensities of a diffracted beam and an incident beam, respectively. Maximum diffraction efficiency of the samples decreased with increase of the amount of the sensitizer. This is attributed to the lower molecular weight of grating polymer that results from fast termination rate due to the presence of a larger amount of sensitizer, which in turn reduces the modulation of refractive index [20–21].
Table 2 lists energetic sensitivity of the photopolymers with various amount of sensitizer. The energetic sensitivity is defined as a modulation of refractive index divided by a total energy to modulate that refractive index , which is given by following Eq. (1).
where η is the diffraction efficiency, d is the film thickness, λ is the wavelength of incident beam and θ is the angle of reconstruction in the recording medium. The energetic sensitivity reaches maximum for YE2 and then decreases with further addition of the sensitizer. The energetic sensitivity of YE1 is lower than that of YE2 because of low polymerization rate of acrylamide due to the less amount of sensitizer. The reduction in the sensitivity of YE3 and YE4 compared to YE2 is attributed to light absorption of the sensitizer which results in a photobleaching. The energetic sensitivity usually depends on the amount of sensitizer, which influences both photo-polymerizaion rate and degree of photobleaching for the given radiation period. The polymerization rate of monomer will be enhanced as the amount of sensitizer increases, while the time required to completely bleach the sensitizer will be also increased. As mentioned above, for YE3, YE4, the energetic sensitivity is decreased, even with more amount of the sensitizer than YE2. This means that the required time to bleach the sensitizer is increased for YE3 and YE4. Figure 2(b) shows transmission efficiency behavior of the photopolymers, which reveals that the remarkable difference in the photobleached degree at the given period is originated from the sensitizer in excess.
3.2 Optimization of the AA/TEA concentration
For the photopolymers having different relative ratio in the amount of AA and TEA, holographic recording was performed. The compositions of the different photopolymers are summarized in Table 3. Figure 3 shows that diffraction efficiencies of the samples are significantly dependent upon the relative amount of TEA. Among the four different photopolymers, the diffraction efficiency is highest for the photopolymer based on TEA3 and lowest for the one based on TEA1. Usually TEA acts as a co-initiator as well as plasticizer in the photopolymer . In this study, the effect of TEA as a co-initiator is not so significant because the amount of sensitizer, which absorbs radiation energy and also activates the coinitiator, TEA, is so small that its effect on the change of diffraction efficiency can be ignored compared to the contribution from TEA. Meanwhile the effect of TEA as a plasticizer was investigated by thermal analysis using DSC, of which results are summarized in Table 4. The glass transition temperature of photopolymer shifts to the lower temperature due to the increase of free volume in photopolymers as the plasticizer is added. This implies that the monomer is able to diffuse and be photopolymerized more readily with addition of the plasticizer. As a result, volumetric gratings are formed more rapidly accompanied by a higher modulation of refractive index. Accordingly, the diffraction efficiency and energetic sensitivity could be enhanced with addition of the plasticizer for the TEA1, TEA2 and TEA3.
One thing particularly to note is that the diffraction efficiency and energetic sensitivity of TEA4 are lower than those of TEA3 even if the content of plasticizer in TEA4 is larger than that in TEA3. We conjecture that it could be due to either a decrease in the relative content of photopolymerizable acrylamide in AA/TEA or the specific interaction between AA and TEA for TEA4 . As for the interaction, the hydroxyl group of TEA is able to be hydrogen bonded to the amide group of acrylamide. This may restrict acrylamide to freely move and result in a decrease of both diffraction efficiency and energetic sensitivity. It was shown from the results of FT-IR measurements in ATR-mode that the characteristic peak intensity of acrylamide (1670cm-1 C=O stretch, 1605cm-1 NH2 deformation in amide) does not change in all formulations of TEA as shown in Fig. 5, indicating that the deterioration in the optical properties for TEA4 compared to TEA3 is mainly affected by the relative content of AA.
We listed some comparative values of optical properties of photopolymer films reported by other groups in Table 5. Our result of energetic sensitivity seems to be higher than the others’ results except for the one from the inorganic-organic hybrid system . The result of the inorganic-organic hybrid system is especially for the thinner film and so it cannot be properly compared with other results. Furthermore, it was reported for the above hybrid system that it exhibits rather high scattering.
4. Angular response of epoxy-resin photopolymer
Figure 5(b) illustrates the angular response of epoxy-resin photopolymer of which thickness is 81μm. The angular response represents the refractive index normalized by the index modulation at the surface, as a function of the rotation angle. The recording was performed asymmetrically at the angle of 0, 7.5 and 15 degree perpendicular to the photopolymer film surface. The volume shrinkage phenomenon, which distorts the recorded original data, is more severely affected when the stress is exerted to the normal direction of film surface during photopolymerization at an asymmetric angle of recording . This can be problematic for angular multiplexing technique where the asymmetric recording is essential. In Fig. 5, the deviated angle degree from Bragg’s angle was 0.01, 0.05, 0.15 degree at 0, 7.5, 15 degree of asymmetric angle, respectively. In present study, the volume shrinkage coefficient is calculated from Eq. (3) .
where σ, ø, Δø are the volume shrinkage coefficient, asymmetric angle perpendicular to film surface and deviation from the reconstruction angle, respectively, which are schematically described in Fig. 5(a). Table 6 shows that volume shrinkage of the photopolymer is sufficiently low for practical consideration.
We report the epoxy-resin based crosslink photopolymer with low crosslinking density, which could significantly enhance the energetic sensitivity without sacrificing both the diffraction efficiency and dimensional stability. In this work, for the new photopolymer, we could achieve a high diffraction efficiency of 92%, low volume shrinkage of 0.67% at the asymmetric angle of 7.5°, and energetic sensitivity of 11.72×10-3cm2/J by means of optimizing the concentrations of sensitizer and plasticizer, which seems to be very promising for practical application.
This research was supported by a grant (code#: 06K1501-02700) from ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Programs’ of the Ministry of Science and Technology, Korea.
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