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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1497–1504
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Holographic diffraction gratings with enhanced sensitivity based on epoxy-resin photopolymers

Yong-Cheol Jeong, Seungwoo Lee, and Jung-Ki Park  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1497-1504 (2007)
http://dx.doi.org/10.1364/OE.15.001497


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Abstract

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

1. Introduction

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

1. J. R. Lawrence, F. T. O’Neill, and J. T. Sheridan, “Photopolymer holographic recording material,” Optik 112,449–463 (2001). [CrossRef]

3

3. L. Dhar, “High-performance polymer recording materials for holographic data storage,” MRS Bull. 31,324–328 (2006). [CrossRef]

]. 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 [4

4. R. M. Shelby, D. A. Waldman, and R. T. Ingwall, “Distortions in pixel-matched holographic data storage due to lateral dimensional change of photopolymer storage media,” Opt. Lett. 25,713–715 (2000). [CrossRef]

] 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

5. P. Cheben and M. L. Calvo, “A photopolymerizable glass with diffraction efficiency near 100% for holographic storage,” Appl. Phys. Lett. 78,1490–1492 (2001). [CrossRef]

8

8. G. Ramos, A. A. Herrero, T. Belenguer, F. del Monte, and D. Levy, “Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording,” Appl. Opt. 43,4018–4024 (2004). [CrossRef] [PubMed]

], cationic ring opening polymerization system [9

9. D. A. Waldman, H. -Y. S. Li, and M. G. Horner, “Volume shrinkage in slant fringe gratings of a cationic ring-opening holographic recording material,” J. Imaging Sci. Technol. 41,497–514 (1997).

10

10. D. A. Waldman, C. I. Butler, and D. H. Raguin, “CROP holographic storage media for optical data storage at greater than 100bits/sq. micron,” Proc. SPIE 5216,10–25 (2003). [CrossRef]

], organic-inorganic hybrid system [11

11. N. Suzuki, Y. Tomita, and T. Kojima, “Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films,” Appl. Phys. Lett. 81,4121–4123 (2002). [CrossRef]

15

15. F. del Monte, O. Martinez, J. A. Rodrigo, M. L. Calvo, and P. Cheben, “A volume holographic sol-gel material with large enhancement of dynamic range by incorporation of high refractive index species,” Adv. Mater. 18,2014–2017 (2006). [CrossRef]

], and crosslinked matrix system [16

16. T. J. Trentler, J. E. Boyd, and V. L. Colvin, “Epoxy resin-photopolymer composites for volume holography,” Chem. Mater. 12,1431–1438 (2000). [CrossRef]

18

18. B. P. Iguanero, A. O. Perez, and I. F. Tapia, “Holographic material film composed by Norland Noa 65 adhesive,” Opt. Mater. 29,225–232 (2002). [CrossRef]

]. 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

3. L. Dhar, “High-performance polymer recording materials for holographic data storage,” MRS Bull. 31,324–328 (2006). [CrossRef]

, 16

16. T. J. Trentler, J. E. Boyd, and V. L. Colvin, “Epoxy resin-photopolymer composites for volume holography,” Chem. Mater. 12,1431–1438 (2000). [CrossRef]

]. 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).

Fig. 1. Molecular structures of component materials for photopolymer (a) PPGDGE (b) PEI (c) AA (d) TEA (e) YE

3. Holographic recording

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

20. M. Ortuno, S. Gallego, C. Garcia, C. Neipp, A. Belendez, and I. Pascual, “Optimization of a 1mm thick PVA/acrylamide recording material to obtain holographic memories: method of preparation and holographic properties,” Appl. Phys. B 76,851–857 (2003). [CrossRef]

21

21. H. Yao, M. Huang, Z. Chen, L. Hou, and F. Gan, “Optimization of two-monomer-based photopolymer used for holographic recording,” Mater. Lett. 56,3–8 (2002). [CrossRef]

].

Table 1. Compositions of photopolymers with various amounts of YE

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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 [12

12. Y. Tomita and H. Nishibiraki, “Improvement of holographic recording sensitivities in the green in SiO2 nanoparticle-dispersed methacrylate photopolymers doped with pyrromethene dyes,” Appl. Phys. Lett. 83,410–412 (2003). [CrossRef]

], which is given by following Eq. (1).

S(cm2/J)=ΔnE
(1)

where E is the total exposure energy and Δn is a value of refractive index modulation obtained by Eq. (2). Δn can be derived from Kogelnik’s coupled wave theory [22

22. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Sys. Technol. J. 48,2909–2947 (1969).

].

η=sin2Δndλcosθ
(2)

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.

Fig. 2. The optical properties of photopolymers with different content of sensitizer (a) Diffraction efficiency behavior of photopolymers with various amount of YE (b) Transmission efficiency behavior of photopolymers with various amount of YE

Table 2. Diffraction efficiency, film thickness of photopolymer, and refractive index modulation, energetic sensitivity calculated from Eq. (1) and Eq. (2)

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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 [21

21. H. Yao, M. Huang, Z. Chen, L. Hou, and F. Gan, “Optimization of two-monomer-based photopolymer used for holographic recording,” Mater. Lett. 56,3–8 (2002). [CrossRef]

]. 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.

Table 3. Compositions of photopolymers with various TEA contents

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Fig. 3. The diffraction efficiency behavior of photopolymers with various TEA contents

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 [23

23. W. S. Kim, Y. -C. Jeong, and J. -K. Park, “Diffraction efficiency behavior of photopolymer based on P(MMA-co-MAA) copolymer matrix,” Opt. Mater. accepted (2006).

]. 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.

Table 4. Diffraction efficiency, film thickness of photopolymer, glass transition temperature, and refractive index modulation, energetic sensitivity that are calculated from Eq. (1) and Eq. (2)

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Fig. 4. FT-IR analysis in ATR mode of photopolymer with variation of TEA, which shows the interaction between AA and TEA

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 [14

14. C. Sanchez, M. J. Escuti, C. V. Heesch, C. W. M. Bastiaansen, D. J. Broer, J. Loos, and R. Nussbaumer, “TiO2 nanoparticle-photopolymer composites for volume holographic recording,” Adv. Funct. Mater. 15,1623–1629 (2005). [CrossRef]

]. 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.

Table 5. Diffraction efficiency and energetic sensitivity with important parameters, spatial frequency and film thickness of recent works by others reported in the literature

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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 [4

4. R. M. Shelby, D. A. Waldman, and R. T. Ingwall, “Distortions in pixel-matched holographic data storage due to lateral dimensional change of photopolymer storage media,” Opt. Lett. 25,713–715 (2000). [CrossRef]

]. 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) [24

24. L. Dhar, M. G. Schnoes, T. L. Wysocki, H. Bair, M. Schilling, and C. Boyd, “Temperature-induced changes in photopolymer volume holograms,” Appl. Phys. Lett. 73,1337–1339 (1998). [CrossRef]

].

σ=1tanøtan(ø+Δø)
(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.

Fig. 5. (a). Schematic figure of asymmetric recording, where the long dash line is a K-vector of incident beams and short dash line is perpendicular to the film surface. (b). Angular response of photopolymer with film thickness, 81μm The asymmetric angle is represented as x in the ‘Asym-x’.

Table 6. Results of angular response at asymmetric geometry and calculated volume shrinkage factor.

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5. Conclusions

Acknowledgments

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.

References and links

1.

J. R. Lawrence, F. T. O’Neill, and J. T. Sheridan, “Photopolymer holographic recording material,” Optik 112,449–463 (2001). [CrossRef]

2.

L. Hesselink, S. S. Orlov, and M. C. Bashaw, “Holographic data storage systems,” Proc. IEEE 92,1231–1280 (2004). [CrossRef]

3.

L. Dhar, “High-performance polymer recording materials for holographic data storage,” MRS Bull. 31,324–328 (2006). [CrossRef]

4.

R. M. Shelby, D. A. Waldman, and R. T. Ingwall, “Distortions in pixel-matched holographic data storage due to lateral dimensional change of photopolymer storage media,” Opt. Lett. 25,713–715 (2000). [CrossRef]

5.

P. Cheben and M. L. Calvo, “A photopolymerizable glass with diffraction efficiency near 100% for holographic storage,” Appl. Phys. Lett. 78,1490–1492 (2001). [CrossRef]

6.

M. G. Schnoes, L. Dhar, M. L. Schilling, S. S. Patel, and P. Wiltzius, “Photopolymer-filled nanoporous glass as a dimensionally stable holographic recording medium,” Opt. Lett. 24,658–660 (1999). [CrossRef]

7.

L. Carretero, A. Murciano, S. Blaya, M. Ulibarrena, and A. Fimia, “Acrylamide-N,N’-methylenebisacrylamide silica glass holographic recording material,” Opt. Express 12,1780–1787 (2004). [CrossRef] [PubMed]

8.

G. Ramos, A. A. Herrero, T. Belenguer, F. del Monte, and D. Levy, “Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording,” Appl. Opt. 43,4018–4024 (2004). [CrossRef] [PubMed]

9.

D. A. Waldman, H. -Y. S. Li, and M. G. Horner, “Volume shrinkage in slant fringe gratings of a cationic ring-opening holographic recording material,” J. Imaging Sci. Technol. 41,497–514 (1997).

10.

D. A. Waldman, C. I. Butler, and D. H. Raguin, “CROP holographic storage media for optical data storage at greater than 100bits/sq. micron,” Proc. SPIE 5216,10–25 (2003). [CrossRef]

11.

N. Suzuki, Y. Tomita, and T. Kojima, “Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films,” Appl. Phys. Lett. 81,4121–4123 (2002). [CrossRef]

12.

Y. Tomita and H. Nishibiraki, “Improvement of holographic recording sensitivities in the green in SiO2 nanoparticle-dispersed methacrylate photopolymers doped with pyrromethene dyes,” Appl. Phys. Lett. 83,410–412 (2003). [CrossRef]

13.

W. S. Kim, Y. -C. Jeong, and J. -K. Park, “Organic-inorganic hybrid photopolymer with reduced volume shrinkage,” Appl. Phys. Lett. 87, 012106 (2005).

14.

C. Sanchez, M. J. Escuti, C. V. Heesch, C. W. M. Bastiaansen, D. J. Broer, J. Loos, and R. Nussbaumer, “TiO2 nanoparticle-photopolymer composites for volume holographic recording,” Adv. Funct. Mater. 15,1623–1629 (2005). [CrossRef]

15.

F. del Monte, O. Martinez, J. A. Rodrigo, M. L. Calvo, and P. Cheben, “A volume holographic sol-gel material with large enhancement of dynamic range by incorporation of high refractive index species,” Adv. Mater. 18,2014–2017 (2006). [CrossRef]

16.

T. J. Trentler, J. E. Boyd, and V. L. Colvin, “Epoxy resin-photopolymer composites for volume holography,” Chem. Mater. 12,1431–1438 (2000). [CrossRef]

17.

T. J. Trentler, J. E. Boyd, and V. L. Colvin, “Epoxy-photopolymer composites: thick recording media for holographic data storage,” Proc. SPIE 4296,259–266 (2001). [CrossRef]

18.

B. P. Iguanero, A. O. Perez, and I. F. Tapia, “Holographic material film composed by Norland Noa 65 adhesive,” Opt. Mater. 29,225–232 (2002). [CrossRef]

19.

W. S. Kim, Y. -C. Jeong, and J. -K. Park, “Nanoparticle-induced refractive index modulation of organic-inorganic hybrid photopolymer,” Opt. Express 14,8967–8973 (2006). [CrossRef] [PubMed]

20.

M. Ortuno, S. Gallego, C. Garcia, C. Neipp, A. Belendez, and I. Pascual, “Optimization of a 1mm thick PVA/acrylamide recording material to obtain holographic memories: method of preparation and holographic properties,” Appl. Phys. B 76,851–857 (2003). [CrossRef]

21.

H. Yao, M. Huang, Z. Chen, L. Hou, and F. Gan, “Optimization of two-monomer-based photopolymer used for holographic recording,” Mater. Lett. 56,3–8 (2002). [CrossRef]

22.

H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Sys. Technol. J. 48,2909–2947 (1969).

23.

W. S. Kim, Y. -C. Jeong, and J. -K. Park, “Diffraction efficiency behavior of photopolymer based on P(MMA-co-MAA) copolymer matrix,” Opt. Mater. accepted (2006).

24.

L. Dhar, M. G. Schnoes, T. L. Wysocki, H. Bair, M. Schilling, and C. Boyd, “Temperature-induced changes in photopolymer volume holograms,” Appl. Phys. Lett. 73,1337–1339 (1998). [CrossRef]

OCIS Codes
(090.0090) Holography : Holography
(090.2900) Holography : Optical storage materials
(090.7330) Holography : Volume gratings

ToC Category:
Holography

History
Original Manuscript: December 21, 2006
Revised Manuscript: January 26, 2007
Manuscript Accepted: January 29, 2007
Published: February 19, 2007

Citation
Yong-Cheol Jeong, Seungwoo Lee, and Jung-Ki Park, "Holographic diffraction gratings with enhanced sensitivity based on epoxy-resin photopolymers," Opt. Express 15, 1497-1504 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1497


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References

  1. J. R. Lawrence, F. T. O`Neill and J. T. Sheridan, "Photopolymer holographic recording material," Optik 112, 449-463 (2001). [CrossRef]
  2. L. Hesselink, S. S. Orlov and M. C. Bashaw, "Holographic data storage systems," Proc. IEEE 92, 1231-1280 (2004). [CrossRef]
  3. L. Dhar, "High-performance polymer recording materials for holographic data storage," MRS Bull. 31, 324-328 (2006). [CrossRef]
  4. R. M. Shelby, D. A. Waldman and R. T. Ingwall, "Distortions in pixel-matched holographic data storage due to lateral dimensional change of photopolymer storage media," Opt. Lett. 25, 713-715 (2000). [CrossRef]
  5. P. Cheben and M. L. Calvo, "A photopolymerizable glass with diffraction efficiency near 100% for holographic storage," Appl. Phys. Lett. 78, 1490-1492 (2001). [CrossRef]
  6. M. G. Schnoes, L. Dhar, M. L. Schilling, S. S. Patel and P. Wiltzius, "Photopolymer-filled nanoporous glass as a dimensionally stable holographic recording medium," Opt. Lett. 24, 658-660 (1999). [CrossRef]
  7. L. Carretero, A. Murciano, S. Blaya, M. Ulibarrena and A. Fimia, "Acrylamide-N,N’-methylenebisacrylamide silica glass holographic recording material," Opt. Express 12, 1780-1787 (2004). [CrossRef] [PubMed]
  8. G. Ramos, A. A. Herrero, T. Belenguer, F. del Monte, and D. Levy, "Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording," Appl. Opt. 43, 4018-4024 (2004). [CrossRef] [PubMed]
  9. D. A. Waldman, H. -Y. S. Li and M. G. Horner, "Volume shrinkage in slant fringe gratings of a cationic ring-opening holographic recording material," J. Imaging Sci. Technol. 41, 497-514 (1997).
  10. D. A. Waldman, C. I. Butler and D. H. Raguin, "CROP holographic storage media for optical data storage at greater than 100bits/sq. micron," Proc. SPIE 5216, 10-25 (2003). [CrossRef]
  11. N. Suzuki, Y. Tomita and T. Kojima, "Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films," Appl. Phys. Lett. 81, 4121-4123 (2002). [CrossRef]
  12. Y. Tomita and H. Nishibiraki, "Improvement of holographic recording sensitivities in the green in SiO2 nanoparticle-dispersed methacrylate photopolymers doped with pyrromethene dyes," Appl. Phys. Lett. 83, 410-412 (2003). [CrossRef]
  13. W. S. Kim, Y. -C. Jeong and J. -K. Park, "Organic-inorganic hybrid photopolymer with reduced volume shrinkage," Appl. Phys. Lett. 87, 012106 (2005).
  14. C. Sanchez, M. J. Escuti, C. V. Heesch, C. W. M. Bastiaansen, D. J. Broer, J. Loos and R. Nussbaumer, "TiO2 nanoparticle-photopolymer composites for volume holographic recording," Adv. Funct. Mater. 15, 1623-1629 (2005). [CrossRef]
  15. F. del Monte, O. Martinez, J. A. Rodrigo, M. L. Calvo, and P. Cheben, "A volume holographic sol-gel material with large enhancement of dynamic range by incorporation of high refractive index species," Adv. Mater. 18, 2014-2017 (2006). [CrossRef]
  16. T. J. Trentler, J. E. Boyd and V. L. Colvin, "Epoxy resin-photopolymer composites for volume holography," Chem. Mater. 12, 1431-1438 (2000). [CrossRef]
  17. T. J. Trentler, J. E. Boyd and V. L. Colvin, "Epoxy-photopolymer composites: thick recording media for holographic data storage," Proc. SPIE 4296, 259-266 (2001). [CrossRef]
  18. B. P. Iguanero, A. O. Perez and I. F. Tapia, "Holographic material film composed by Norland Noa 65 adhesive," Opt. Mater. 29, 225-232 (2002). [CrossRef]
  19. W. S. Kim, Y. -C. Jeong and J. -K. Park, "Nanoparticle-induced refractive index modulation of organic-inorganic hybrid photopolymer," Opt. Express 14, 8967-8973 (2006). [CrossRef] [PubMed]
  20. M. Ortuno, S. Gallego, C. Garcia, C. Neipp, A. Belendez and I. Pascual, "Optimization of a 1mm thick PVA/acrylamide recording material to obtain holographic memories: method of preparation and holographic properties," Appl. Phys. B 76, 851-857 (2003). [CrossRef]
  21. H. Yao, M. Huang, Z. Chen, L. Hou and F. Gan, "Optimization of two-monomer-based photopolymer used for holographic recording," Mater. Lett. 56, 3-8 (2002). [CrossRef]
  22. H. Kogelnik, "Coupled wave theory for thick hologram gratings," Bell Sys. Technol. J. 48, 2909-2947 (1969).
  23. W. S. Kim, Y. -C. Jeong and J. -K. Park, "Diffraction efficiency behavior of photopolymer based on P(MMA-co-MAA) copolymer matrix," Opt. Mater. accepted (2006).
  24. L. Dhar, M. G. Schnoes, T. L. Wysocki, H. Bair, M. Schilling and C. Boyd, "Temperature-induced changes in photopolymer volume holograms," Appl. Phys. Lett. 73, 1337-1339 (1998). [CrossRef]

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