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Journal of Modern Optics
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Co-doping with polysquaraine enhances the
holographic optical data storage of PMMA/PQ
photopolymers
Cheng-Jung Ko a , Yeh-Lu Chang a , Yi-Nan Hsiao b , Po-Lin Chen b , Shiuan-Huei Lin c , Wha-Tzong Whang a , Ken-Y. Hsu b , Mei-Hui Tsai d & Wen-Yen Tsang d
a
Department of Materials Science and Engineering , National Chiao Tung University , Hsin-Chu 300 , Taiwan , ROC
b
Department of Photonics and Institute of Electro-Optical Engineering , National Chiao Tung University , Hsin-Chu 300 , Taiwan , ROC
c
Department of Electrophysics , National Chiao Tung University , Hsin-Chu 300 , Taiwan , ROC
d
Department of Chemical and Materials Engineering , National Chin-Yi University of Technology , Taichung 411 , Taiwan , ROC
Published online: 04 Aug 2011.
To cite this article: Cheng-Jung Ko , Yeh-Lu Chang , Yi-Nan Hsiao , Po-Lin Chen , Shiuan-Huei Lin , Wha-Tzong Whang , Ken-Y. Hsu , Mei-Hui Tsai & Wen-Yen Tsang (2011) Co-doping with polysquaraine enhances the holographic optical data storage of PMMA/PQ photopolymers, Journal of Modern Optics, 58:14, 1215-1219, DOI: 10.1080/09500340.2011.581765
To link to this article: http://dx.doi.org/10.1080/09500340.2011.581765
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Vol. 58, No. 14, 10 August 2011, 1215–1219
Co-doping with polysquaraine enhances the holographic optical data storage
of PMMA/PQ photopolymers
Cheng-Jung Koa, Yeh-Lu Changa, Yi-Nan Hsiaob, Po-Lin Chenb, Shiuan-Huei Linc, Wha-Tzong Whanga*, Ken-Y. Hsub, Mei-Hui Tsaidand Wen-Yen Tsangd
aDepartment of Materials Science and Engineering, National Chiao Tung University, Hsin-Chu 300, Taiwan, ROC; b
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsin-Chu 300, Taiwan, ROC;cDepartment of Electrophysics, National Chiao Tung University,
Hsin-Chu 300, Taiwan, ROC;dDepartment of Chemical and Materials Engineering, National Chin-Yi University of Technology,
Taichung 411, Taiwan, ROC
(Received 6 January 2011; final version received 12 April 2011)
In this study, we synthesised poly(3-octylpyrrole-co-squaric acid) (PSQ3), a polysquaraine, through the reaction of 3-octylpyrrole and squaric acid, and then co-doped it with phenanthrenequinone (PQ) into poly(methyl methacrylate) (PMMA) to improve the holographic data storage characteristics of the photopolymer. The photopolymers co-doped with relatively small amounts of PSQ3 exhibited greatly improved holographic recording characteristics, including superior diffraction efficiency and dynamic range (M#). Among the samples co-doped with PQ and PSQ3, the maximum diffraction efficiency reached 54.8% (cf. 9.0% for PMMA/PQ) without further downgrade and the value of M# reached 1.05 (cf. 0.46 for PMMA/PQ). Therefore, the holographic data storage characteristics of the photopolymer PMMA/PQ were improved through co-doping with PSQ3.
Keywords: polysquaraine; phenanthrenequinone; poly(methyl methacrylate); holographic recording; optical data storage
1. Introduction
Holographic data storage is one of the most promising next-generation data storage technologies because it combines large storage capacities with fast data access rates. Photopolymer recording materials are particu-larly attractive for holographic recording applications [1–6]; compared with inorganic materials, photopoly-mers can provide larger refractive index modulation, higher sensitivity and better flexibility in terms of composition. In previous studies, we found that 9,10-phenanthrenequinone (PQ)–doped poly(methyl meth-acrylate) (PMMA) serves as a good recording material for holographic data storage [6–9]. During such a holographic recording process, the PQ molecules absorb energy to form an excited state, which under-goes further transfer to a radical state that reacts with residual MMA monomer to form the photoproduct; at the same time, a modulated refractive index change occurs [10]. Faster photoreaction processes and larger modulated refractive index changes of the photopoly-mer are expected to result in superior holographic recording characteristics.
Polysquaraines are -conjugated polymers that possess consecutive donor/acceptor repeating units in the polymer chain [11,12]; they feature considerable charge-transfer interactions, which could be used to affect the photoreaction between PQ and MMA and, thereby, improve the holographic characteristics. In this study, we synthesised poly(3-octylpyrrole-co-squa-ric acid) (PSQ3) and co-doped it with PQ into PMMA as the photopolymer. We used a UV–visible (Vis) spectrometer and an Abbe refractometer to measure the optical absorption and refractive index data, respectively. From a study of the holographic record-ing characteristics of the photopolymer incorporatrecord-ing different doped concentrations of PSQ3, we found that the co-doped polysquaraine improved the dynamic range and sensitivity of the photopolymer.
2. Experimental 2.1. Materials
High-purity 3-octylpyrrole and squaric acid were obtained from Tokyo Chemical Industry and Acros
*Corresponding author. Email: [email protected] ISSN 0950–0340 print/ISSN 1362–3044 online
ß 2011 Taylor & Francis DOI: 10.1080/09500340.2011.581765 http://www.informaworld.com
Organics, respectively. Benzene was acquired from Showa Chemical. Butyl alcohol, ethyl ether and
CH2Cl2 were purchased from Echo Chemical
Company. PMMA powder (average molecular
weight: 120,000) was obtained from Sigma–Aldrich. PQ was acquired from Tokyo Chemical Industry and used as received.
2.2. Poly(3-octylpyrrole-co-squaric acid) (PSQ3) 3-Octylpyrrole (0.5 mmol) and squaric acid (0.5 mmol) were heated under reflux in benzene (10 ml) and butyl alcohol (20 ml) under a N2atmosphere with stirring for
24 h, with azeotropic removal of water in a Dean–Stark apparatus [11,13]. After cooling, the mixture was concentrated under vacuum and the product poured into ethyl ether. The precipitate was collected by filtration, dissolved in CH2Cl2 and re-precipitated
from ethyl ether to provide the final product, which was dried in a vacuum oven at 30C for 24 h prior to use.
2.3. Photopolymer fabrication
PMMA powder (20 wt%) and polysquaraine powder were dissolved in CH2Cl2by stirring at room
temper-ature for 2 h; the dissolved PSQ3 concentrations within the PMMA matrix were ca. 0.05, 0.07, and 0.1 wt%. PQ (5 wt%) was then added into the solution, which was stirred for 2 h [14]. This precursor solution was coated on a dust-free glass plate using a 650 mm wide doctor blade. The gel film was further heated at 35C for 30 min to remove the CH
2Cl2, providing
PMMA/PQ/PSQ3 photopolymer films having an aver-age final thickness of 110 mm.
2.4. Measurements
Fourier transform infrared (FTIR) absorption spectra were recorded between 4000 and 500 cm1 using a PerkinElmer 100 spectrophotometer. To identify the photoreactions occurring in our photopolymer, the samples were measured in unexposed and exposed states. UV–Vis spectra of the photopolymer were recorded using a Shimadzu UV-1800 instrument. Refractive indices were measured using an Abbe refractometer (ATAGO-DR-M2) and a sodium light source (589 nm) at room temperature.
A typical four-wave mixing optical setup was constructed to measure the holographic characteristics (Figure 1). A collimated light beam from an Ar laser (wavelength: 514 nm) was s-polarised and divided into two beams with an intensity ratio of 1:1 (intensity of each beam: 6 mW/cm2; diameter: 0.7 cm). The intensity of the diffraction beam was detected when the shutter
blocked one of the beams. The light beams were aligned incident and symmetrically to the sam-ple with an intersection angle of 30 outside of the
sample. The diffraction efficiency was defined as the ratio of the intensity of the diffracted beam to the sum of the intensities of the diffracted and transmitted beams. To measure the dynamic range, the photopolymer was placed on a rotational stage to allow multiple hologram recordings using a peri-strephic multiplexing technique. The exposure energy of each hologram was 0.312 J/cm2.
3. Results and discussion 3.1. FTIR spectra
We prepared PSQ3 through the reaction of 3-octyl-pyrrole and squaric acid and characterised its chemical structure using FTIR spectroscopy (Figure 2). The signals at 1610 and 1730 cm1represent the C–O and C¼O stretchings of the cyclobutenediylium-1,3-diolate moiety, respectively [11,15,16]. The strong C–H stretching of the C8H17 units appeared as absorption
bands at ca. 2800–3000 cm1[16]. The broad absorp-tion band at 3300 cm1 was produced as a result of intramolecular hydrogen bonding of the PSQ3 struc-tures. These spectral features were consistent with the proposed structure of PSQ3.
3.2. Optical chemical measurements
Figure 3 presents typical optical absorption spectra of
the various photopolymers before and after
exposure. PSQ3 itself exhibited its maximum
Figure 1. Optical setup for measuring holographic data recording characteristics. (The colour version of this figure is included in the online version of the journal.)
1216 C.-J. Ko et al.
absorption at 549 nm. Prior to exposure, the photo-polymers co-doped with PSQ3 all provided similar absorption curves. All of the samples had lower absorbance at 514 nm than they did at 532 nm. Notably, the absorption coefficients at 514 and 549 nm increased upon increasing the doped content of PSQ3. After exposure to green light (514 nm), the entire absorption band of the each photopolymer underwent a blue shift. At the same time, the absorp-tion peak of PSQ3 disappeared, predictably because PSQ3 reacted with PQ and PMMA to form various photoproducts [10,17,18]. This substantial variation in absorbance was clearly different from that of the unmodified PMMA/PQ photopolymer. To monitor the effect of co-doping PSQ3 in the PMMA/PQ photopolymer in terms of the holographic data storage characteristics, we selected 514 nm as the recording wavelength for our subsequent studies.
Table 1 reveals that the refractive indices of the photopolymers increased upon increasing the content of PSQ3. The change in refractive index after exposure of the co-doped PMMA/PQ photopolymer to radia-tion increased when the concentraradia-tion of PSQ3 increased from 0 to 0.1 wt%. For the PQ-only and PSQ3-only doped photopolymers, the refractive index changes were much lower. These results suggested that the holographic data storage characteristics of the PMMA/PQ photopolymer might be enhanced after co-doping with PSQ3.
3.3. Holographic data storage characteristics
Figure 4 presents the diffraction efficiencies of the photopolymers plotted as a function of the exposure energy. The co-doped PMMA/PQ/PSQ3 photopoly-mers all exhibited higher holographic diffraction effi-ciencies than did the PMMA/PQ photopolymer. The maximum diffraction efficiencies at PSQ3 co-doping levels of 0.1, 0.07, 0.05 and 0 wt% reached 54.8, 44.9, 19.9 and 9.0%, respectively. For the PMMA/0.1 wt% PSQ3 photopolymer prepared without PQ, the diffrac-tion efficiency was very low (ca. 0.2%). These varia-tions are consistent with the changes in refractive index.
The two most important parameters for holo-graphic data storage materials are their dynamic range and sensitivity. The dynamic range of a holographic material provides an estimate of the recording capac-ity; the larger the dynamic range of a material, the larger its storage capacity. The sensitivity is a gauge of the recording data rate; because a material with high sensitivity allows faster recording, less energy is required when recording each hologram.
We applied a peristrephic multiplexing technique to record 200 holograms in a single location of each photopolymer. The sum of the square roots of the diffraction efficiency forms a running curve of the cumulative grating strength, expressed as
CðEÞ ¼X n i¼1 ffiffiffiffi i p , ð1Þ
where n is total number of holograms that have been recorded for a total exposure energy density of EJ/cm2. By curve fitting with the function
CðEÞ ¼ Csat½1 expðE=EÞ, ð2Þ
where E is total exposure energy, the saturation value Csat(M#) of the curve gives the dynamic range of the
material, and Egives the exposure energy constant of
the material [7,19]. The material sensitivity (S) is an increment in cumulative grating strength with respect to the amount of exposure energy. When the sample
300 400 500 600 700 800 900 0.0 0.5 1.0 1.5 2.0 2.5 Absorbance Wavelength (nm) unexposed PMMA/PQ/0.1wt% PSQ3 PMMA/PQ/0.07wt% PSQ3 PMMA/PQ/0.05wt% PSQ3 PMMA/PQ exposure PMMA/PQ/0.1wt% PSQ3 PMMA/PQ/0.07wt% PSQ3 PMMA/PQ/0.05wt% PSQ3 PMMA/PQ
Figure 3. UV–Vis absorption spectra of the unexposed and exposure photopolymers. (The colour version of this figure is included in the online version of the journal.)
4000 3500 3000 2500 2000 1500 1000 500
Absorbance (a.u.)
Wavenumber (cm–1)
1610
1730
Figure 2. FTIR spectra of PSQ3.
is unexposed, S ¼ dC=dEjE!0. According to the run-ning curve function, the sensitivity can be written as
S ¼ Csat=E ¼M#=E: ð3Þ
Figure 5 displays the running curves for multiple-hologram recording in PMMA/PQ photopolymers co-doped with PSQ3. Each curve reached saturation when the exposure energy exceeded a certain value. Using the formulae above for curve-fitting, we
obtained the values of M#, E and S listed in
Table 2. All of the photopolymers co-doped with PSQ3 had higher values of M# and S than did the PQ-only doped photopolymer. The PMMA/PQ/0.1 wt% PSQ3 photopolymer exhibited the highest value of M# (1.05) among all of the tested samples (cf. 0.47 for PMMA/PQ). Moreover, the value of S of the PMMA/ PQ/0.1 wt% PSQ3 system was 2.6 times greater than that of the PMMA/PQ photopolymer.
These experimental results reveal that co-doping with PSQ3 enhanced the holographic data storage
characteristics of the PMMA/PQ photopolymer
system. Upon exposure to light, the -conjugated structure of PSQ3 predictably changed or was destroyed; the absorption peaks of PSQ3 disappeared, consistent with it having undergone photochemical reactions with PQ and PMMA, in which the PSQ3 was
reacted with the residule monomer of PMMA [17,18] and the C¼C of PSQ3 was furthermore reacted with the C¼O group of PQ [10]. These new photoproducts would be different from those in the PMMA/PQ sample, providing a larger refractive index change and superior holographic recording performance.
4. Conclusions
Co-doped PMMA/PQ/polysquaraine photopolymers exhibit superior holographic performance relative to
Table 1. Refractive indices of PSQ3/PQ-doped PMMA photopolymers.
Sample Unexposed refractive index Exposed refractive index Refractive index change (Dn) PMMA/PQ 1.4925 1.4901 0.0024 PMMA/PQ/0.05 wt% PSQ3 1.4969 1.4921 0.0048 PMMA/PQ/0.07 wt% PSQ3 1.4983 1.4930 0.0053 PMMA/PQ/0.10 wt% PSQ3 1.5030 1.4962 0.0068 PMMA/0.10 wt% PSQ3 1.4984 1.4978 0.0006 0 5 10 15 20 25 0 20 40 60 80 Diffraction efficiency (%) Exposure energy (J/cm2) PMMA/PQ/0.1wt% PSQ3 PMMA/PQ/0.07wt% PSQ3 PMMA/PQ/0.05wt% PSQ3 PMMA/PQ PMMA/0.1wt% PSQ3
Figure 4. Holographic diffraction efficiencies of the photo-polymers. (The colour version of this figure is included in the online version of the journal.)
0 10 20 30 40 50 60 0.0 0.5 1.0 1.5 Cumulative strength Exposed energy (J/cm2) PMMA/PQ/0.1wt% PSQ3 PMMA/PQ/0.07wt% PSQ3 PMMA/PQ/0.05wt% PSQ3 PMMA/PQ
Figure 5. Running curves for multiple-hologram recording in the photopolymers. (The color version of this figure is included in the online version of the journal.)
Table 2. Characteristics of multiplexed volume holographic recording in PSQ3/PQ-doped PMMA photopolymers.
Sample M# E(J/cm 2 ) Sensitivity (cm2/J)a PMMA/PQ 0.47 8.33 0.056 PMMA/PQ/0.05 wt% PSQ3 0.53 7.69 0.069 PMMA/PQ/0.07 wt% PSQ3 0.62 5.88 0.105 PMMA/PQ/0.10 wt% PSQ3 1.05 7.14 0.147 a Sensitivity (S) ¼ M#/E. 1218 C.-J. Ko et al.
that of the corresponding PMMA/PQ photopolymer. Single-hologram recording revealed that the diffraction efficiency of the PMMA/PQ/0.1 wt% PSQ3 photo-polymer was higher than those of any of the other tested photopolymers. Furthermore, in a multiple-hologram recording scheme, the PMMA/PQ/0.1 wt% PSQ3 photopolymer also exhibited the greatest value of M# and sensitivity. The refractive index change of the PMMA/PQ photopolymer was enhanced after co-doping with the polysquaraine, providing greater holographic recording characteristics.
Acknowledgements
We thank the National Science Council Republic of China, Taiwan, for supporting in this research financially under Grant NSC97-2221-E009-012-MY3.
References
[1] Dhar, L.; Hale, A.; Katz, H.E.; Schilling, M.L.; Schnoes, M.G.; Schilling, F.C. Opt. Lett. 1999, 24, 487–489. [2] Ono, H.; Tamoto, T.; Emoto, A.; Kawatsuki, N. Jpn. J.
Appl. Phys. 2005, 44, 1781–1786.
[3] Mahilny, U.V.; Marmysh, D.N.; Stankevich, A.I.; Tolstik, A.L.; Matusevich, V.; Kowarschik, R. Appl. Phys. B: Lasers Opt. 2006, 82, 299–302.
[4] Xue, S.S.; Manivannan, G.; Lessard, R.A. Thin Solid Films 1994, 253, 228–232.
[5] Pham, V.P.; Manivannan, G.; Lessard, R.A. Thin Solid Films 1995, 270, 295–299.
[6] Lin, S.H.; Chen, P.L.; Hsiao, Y.N.; Whang, W.T. Opt. Commun. 2008, 281, 559–566.
[7] Hsiao, Y.N.; Whang, W.T.; Lin, S.H. Opt. Eng. 2004, 43,1993–2002.
[8] Sukhanov, V.I. J. Opt. Technol. 1994, 61, 49–56. [9] Steckman, G.J.; Solomatine, I.; Zhou, G.; Psaltis, D.
Opt. Lett. 1998, 23, 1310–1312.
[10] Horspool, W.M.; Song, P.S. CRC Handbook of Organic Photochemistry and Photobiology; CRC Press: Boca Raton, FL, 1995.
[11] Chenthamarakshan, C.R.; Eldo, J.; Ajayaghosh, A. Macromolecules 1999, 32, 251–257.
[12] Eldo, J.; Ajayaghosh, A. Chem. Mater. 2002, 14, 410–418. [13] Lu, H.C.; Whang, W.T.; Cheng, B.M. Synth. Met. 2010,
160,1002–1007.
[14] Hsiao, Y.N.; Whang, W.T.; Lin, S.H. Jpn. J. Appl. Phys. 2005, 44, 914–919.
[15] Silva, C.E.; Diniz, R.; Rodrigues, B.L.; de Oliveira, L.F.C. J. Mol. Struct. 2007, 831, 187–194.
[16] Sant’Ana, A.C.; de Siqueira, L.J.A.; Santos, P.S.; Temperini, M.L.A. J. Raman Spectrosc. 2006, 37, 1346–1353.
[17] He, Y.; Zhou, W.; Wu, F.; Li, M.; Wang, E. J. Photochem. Photobiol., A 2004, 162, 463–471. [18] Mohr, G.J. Sens. Actuators, B 2005, 107, 2–13. [19] Pu, A.; Psaltis, D. Appl. Opt. 1996, 35, 2389–2398.
