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Effect of lanthanoid organometallic compounds on holographic storage characteristics of
doped PQ/poly(hydroxyethyl methacrylate-co-methyl methacrylate) hybrids
View the table of contents for this issue, or go to the journal homepage for more 2009 J. Opt. A: Pure Appl. Opt. 11 125409
(http://iopscience.iop.org/1464-4258/11/12/125409)
J. Opt. A: Pure Appl. Opt. 11 (2009) 125409 (6pp) doi:10.1088/1464-4258/11/12/125409
Effect of lanthanoid organometallic
compounds on holographic storage
characteristics of doped
PQ/poly(hydroxyethyl
methacrylate-co-methyl methacrylate)
hybrids
Yu-Fang Chen
1, Yi-Nan Hsiao
2,4, Shiuan Huei Lin
3, Ken Y Hsu
2,
Wei-Sheng Cheng
1and Wha-Tzong Whang
1,41Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan 300, Republic of China
2Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung
University, Hsinchu, Taiwan 300, Republic of China
3Department of Electro-physics, National Chiao Tung University, Hsinchu, Taiwan 300,
Republic of China
E-mail:[email protected]@mail.nctu.edu.tw Received 30 April 2009, accepted for publication 7 August 2009 Published 21 September 2009
Online atstacks.iop.org/JOptA/11/125409
Abstract
In this research, we fabricate and characterize lanthanide organometallic compounds and 9,10-phenanthrenequinone (PQ) co-doped poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) photopolymers for holographic recording. Five different lanthanoid (Ce3+, Nd3+,
Er3+, Yb3+and Lu3+) organometallic compounds, one at a time, is co-doped with PQ in a
poly(HEMA-co-MMA) matrix, respectively. Holographic experiments demonstrate that lutetium organometallic compounds and a PQ co-doped poly(HEMA-co-MMA) photopolymer can greatly enhance the volume holographic characteristics. The degree of improvement in hologram diffraction efficiency and the recording dynamic range for these co-doped photopolymers follows the order: Lu3+> Yb3+> Er3+ > Nd3+> Ce3+.
Keywords:organometallic compounds, photopolymer, diffraction efficiency, exposure energy, volume holographic data storage, phenanthrenequinone
1. Introduction
Recording media have been a key issue in realizing holographic data storage. Among different materials [1–5], the photopolymer has been considered as a promising medium due to several advantages, such as it being easy to modify the compositions, cheap, simple fabrication and large refractive index change. Several techniques have been
4 Authors to whom any correspondence should be addressed.
developed for fabricating high quality photopolymers for holographic recording. Aromatic monomers that contain heavy atoms [6, 7], or mixing monomers [8, 9] with different functionalities, have been used to enhance the refractive index change. Photosensitizers with different kinds of additives, N -methyldiethanolamine [10, 11] and diphenyliodonium salt [12–14], have been used to improve the holographic recording sensitivity. Different matrices such as epoxy [15–18] and sol–gel [19–21], or photopolymer components mixed with a cross-linking monomer [8,14], have
J. Opt. A: Pure Appl. Opt. 11 (2009) 125409 Y-F Chen et al
been utilized to reduce the shrinkage effect in the recording process. In addition, to improve the sensitivity, radical and cationic photopolymerization with different additives and photosensitizers has been applied to speed up the photoinitiation process, such as sol–gel [19, 20] and two-step thermo-polymerization [22–25]. Neutral components such as bromonaphthalene [3], liquid crystals [4, 5, 26, 27] or nanoparticles [28–31] are introduced into the photopolymer to enhance the diffraction efficiency. This research is aimed at developing a material with large refractive index change, high sensitivity and good dimensional stability.
Recently, doping organometallic compounds [23,32,33] has been an attractive method for improving the holographic recording characteristics of photopolymers. Through the pho-tochemical reaction between metal ions and ligands [34,35], organometallic compounds have been applied widely for chemical reaction catalysts [36, 37], nanoparticle synthe-ses [38–40] and organic syntheses [41–44]. In our previ-ous research, the holographic recording characteristics of the PQ/PMMA photopolymer have been enhanced by co-doping organometallic compounds, zinc methacrylate [23], through the reaction between the metal ion and carbonyl functional groups in PQ molecules [25].
In the PQ/PMMA photopolymer, the PQ molecules react with the residual MMA monomers to form a one-to-one oligomer in the light illuminated region. As a result, the refractive index in the bright region is different from that of the dark region, so the phase grating can be formed in the material. The recording mechanism is that, under light exposure, the PQ molecules absorb photon energy to form a photo-excited triplet state. The excited PQ molecules further form radicals by receiving a hydrogen atom from the residual MMA monomers to form the one-to-one photoproduct of the phase grating. This process is achieved through the electron and the following proton transfers.
Research on the photoreaction between PQ and metal ions indicate that the metal ions, acting as Lewis acids in co-doped organometallics, can be used to accelerate the electron transfer reaction processes [25]. Organometallic compounds with high Lewis acidities can easily react with imine or carbonyl functionalities in organic compounds. In other words, co-doping organometallic components into quinone-based dye-doped photopolymers will provide a means to enhance the photoreaction rate upon light exposure.
In this research, we improve the holographic character-istics of the PQ/P(HEMA-co-MMA) photopolymer through co-doping of the lanthanoid compounds. Five different lan-thanoid (Ce3+, Nd3+, Er3+, Yb3+ and Lu3+)
organometal-lic compounds have been co-doped with PQ with five differ-ent photopolymers. Holographic characteristics, including the recording sensitivity and dynamic range, are experimentally characterized.
2. Experiments
2.1. Material preparationHEMA and MMA, used as the monomers, were purchased from the Showa Chemical Company and the Acros Company,
Figure 1. Chemical structures of compounds used.
respectively. The monomers were purified through distillation under low pressure and then stored in a refrigerator before use. 2,2-azo-bis-isobutyrolnitrile(AIBN), used as the thermo-initiator and purchased from the Tokyo Chemical Industry(TCI), was purified by recrystallization. The photosensitizer, PQ, was purchased from TCI and used as received. The lanthanoid organometallic compounds, lutetium (III) acetate hydrate (99.9%)[Lu(ac)3], ytterbium (III) acetate
tetrahydrate (99%)[Yb(ac)3], erbium (III) acetate tetrahydrate
(99%) [Er(ac)3], neodymium (III) acetate hydrate (99.9%)
[Nd(ac)3] and cerium (III) acetate hydrate (99.9%) [Ce(ac)3]
were purchased from the Sigma-Aldrich Company. All the lanthanoid organometallic compounds were used as received. Figure1shows the chemical structures of all compounds.
2.2. Fabrication of photopolymer samples
The monomers MMA and HEMA were first mixed with a weight ratio of 9:1. which is because the lanthanoid organometallic compounds have better solubility in HEMA than MMA monomers. We introduce HEMA monomers into our system to increase the dissolved concentration of the lanthanoid organometallic compounds. In order to study the effect of doping lanthanoid organometallic compounds, molar concentrations of the solutes were kept identical when making the samples. The lanthanoid organometallic compound of 3.6×10−3mol% was added to the mixed co-monomer solvent. 2
Figure 2. Optical set-ups for measuring volume holographic data recording characteristics.
(This figure is in colour only in the electronic version)
The PQ with 0.36 mol% was dissolved in the solution. Finally, the AIBN with 0.65 mol% was added and stirred under room temperature until the solute dispersed and mixed uniformly. A filter with a hole size of 0.2 μm was used to further filter out impurities from the solution. The modified two-stage thermo-polymerization was then used to fabricate the photopolymer. In the first stage, the mixed solution was stirred at 35◦C for 12 h until it became homogeneously viscous. Then, it was poured into a glass cell with the dimensions 10 cm× 10 cm× 2 mm. In the second stage, the whole set was baked at 45◦C for 3 days until most of the monomer MMA and HEMA were polymerized. All samples were prepared in a dark environment. The 2 mm photopolymer bulk with good optical properties can be prepared by this approach.
2.3. Set-up for holographic recording measurements
An optical set-up for typical non-degenerate four-wave mixing, as shown in figure 2, has been constructed to measure the holographic characteristics of the samples. A diode laser-pumped solid-state laser model Verdi-10 purchased from Coherent Company is used as the light source. The collimated light at 532 nm was spilt into two beams with an intensity ratio 1:1. They were incident on the sample symmetrically with an intersection angle of 30◦outside the sample. By using a shutter to block one of the writing beams at constant time intervals, the intensity of the diffracted light can be measured. The diffraction efficiency is defined as the ratio of the intensity of the diffracted beam to that of the sum of the diffracted and transmitted beams. We used a peristrophic multiplexing technique to measure the thickness of the photopolymer samples was 2 mm. The intensity of each laser beam used to record a single hologram was 7 mW cm−2and the dynamic range was 40 mW cm−2.
Figure 3. UV–vis absorption spectra of lanthanoid organometallic
compounds and PQ co-doped poly(HEMA-co-MMA) photopolymer with different lanthanide elements under different exposure states.
3. Characterization of photopolymer
3.1. UV–vis spectroscopy measurementWe measured the optical absorption spectrum of the lanthanoid organometallic and PQ co-doped photopolymer. The UV– vis–NIR spectrophotometer UV3600 from Shimadzu Co. was used for this purpose. Typical optical absorption spectra of our samples before and after optical exposure under green light (532 nm) are shown in figure 3. It can be seen that, before exposure, all samples with co-doped lanthanoid organometallic compounds demonstrated identical absorption to that of the PQ singly doped sample. The samples
J. Opt. A: Pure Appl. Opt. 11 (2009) 125409 Y-F Chen et al
Figure 4. Temporal response of diffraction efficiency of the recorded
hologram in lanthanoid organometallic compounds and PQ co-doped poly(HEMA-co-MMA) photopolymer with different lanthanide elements.
are totally transparent for wavelengths longer than 540 nm. Adding lanthanoid organometallic compounds does not vary the absorption behaviour in this regime. The results imply that PQ acts as a single photosensitizer in the lanthanoid organometallic co-doped photopolymer and the PQ singly doped photopolymer.
On the other hand, the absorption spectra of the after-exposure samples show much less absorption from 390 to 540 nm. These samples are totally transparent for wavelengths longer than 500 nm. Most parts of the absorption curves are overlapped, except that there is a slight difference from 400 to 475 nm. Comparing with that of a PQ singly doped photopolymer, the absorption band shows a blueshift for the lanthanoid organometallic co-doped photopolymer samples. This implies that the structures of the co-doped photoproducts are less conjugated than that of the PQ singly doped ones. This may result in a larger refractive index change induced by optical exposure. Thus, the lanthanoid organometallic co-doped photopolymers show high potentiality for recording stronger phase gratings.
3.2. Single-hologram recording characteristics
We recorded a single hologram in the photopolymer. Then, the diffraction efficiency was measured. The results are shown in figure 4. It is seen that the diffraction efficiency of all the samples first reaches a maximum, and then it drops with further exposure. It can be seen that the maximal diffraction efficiency of the lanthanoid organometallic compound co-doped photopolymer follows the order as Lu(ac)3 > Yb(ac)3 > Er(ac)3 > Nd(ac)3 > Ce(ac)3. This trend corresponds to the decrease of the atomic number and the increase of the ionic radius according to the periodic table. Among them, it is seen that the maximal diffraction efficiencies for the co-doped samples of Lu(ac)3, Yb(ac)3and Er(ac)3are 98.30%, 74.76% and 61.31%, respectively. Comparing this to 28.81% for the PQ singly doped sample, the diffraction
Table 1. Characteristics of multiplexed volume holographic
recording in lanthanoid organometallic compounds and PQ co-doped poly(HEMA-co-MMA) photopolymer: M# (dynamic range) and S (sensitivity).
Doped component M# Sensitivity (cm2J−1)
PQ 1.87 0.0191 PQ+ Lu(ac)3 3.86 0.0273 PQ+ Yb(ac)3 2.81 0.0296 PQ+ Er(ac)3 2.63 0.0175 PQ+ Nd(ac)3 1.70 0.0021 PQ+ Ce(ac)3 1.56 0.0025
efficiency of co-doped Lu(ac)3 has been improved by more than three times. However, the maximal diffraction efficiencies of Nd(ac)3 and Ce(ac)3 co-doped samples are 23.12% and 22.70%, respectively, which are smaller than that of the PQ singly doped photopolymer.
Figure4can be used to estimate the exposure energy that is required to reach 10% diffraction efficiency. Less exposure energy means the sample is more sensitive in holographic recording. We find that the samples with co-doped Lu(ac)3, Yb(ac)3 and Er(ac)3 are more sensitive than the PQ singly doped photopolymer, but the situation is reversed in the cases with co-doped Nd(ac)3and Ce(ac)3. The best result is obtained for the Lu(ac)3co-doped photopolymer, for which the required energy is 26.7 J cm−2, which is about less than half that of the PQ singly doped photopolymer.
3.3. Multiple-hologram recording characteristics
The characteristics of multiple-hologram recording, including dynamic range and sensitivity, can be measured by the use of the peristrophic multiplexing technique. Dynamic range, or M# [45–47], can be obtained from the saturation value of the cumulative grating strength, C(E) = ni=1√ηi, where ηi is the diffraction efficiency of each hologram and n is the total number of holograms that have been recorded for a total exposure energy of E J cm−2. Through the curve fitting,
C(E) = Csat[1 − exp(−E/Eτ)], the saturation value Csat
and Eτ, the exposure energy constant, respectively, can be calculated. Then, the material sensitivity S, which represents an increment in cumulative grating strength with respective to the amount of exposure energy, can be approximated by
S = Csat/Eτ= M#/Eτ.
In the experiment, 190 plane-wave holograms were recorded at a single location and each hologram was recorded with an exposure energy of 5.66 J cm−2. The running curves for the cumulative grating strength of the samples are shown in figure5. From figure5, M# and sensitivity are calculated, as listed in table1. We see that the M# of samples with co-doped Lu(ac)3, Yb(ac)3and Er(ac)3are higher than that of the PQ singly doped photopolymer. The best result was obtained by co-doping Lu(ac)3, which has been improved by around two times, from 1.87 to 3.86. Again, the magnitude of M# follows the order Lu(ac)3 > Yb(ac)3> Er(ac)3> Nd(ac)3 > Ce(ac)3.
Table 1 also shows the sensitivity of the samples. It is seen that the sensitivity of the co-doped Lu(ac)3 and Yb(ac)3 4
Figure 5. The running curves of the multiple-hologram recording in
lanthanoid organometallic compounds and PQ co-doped poly(HEMA-co-MMA) photopolymer with different lanthanide elements.
photopolymers are 0.0273 cm2J−1 and 0.0296 cm2J−1,
respectively. Comparing to that of the PQ singly doped photopolymer, the sensitivity has been improved by 1.5 times. However, the sensitivity is much smaller for the co-doped Nd(ac)3 and Ce(ac)3 photopolymers, which are 0.0021 cm2J−1and 0.0025 cm2J−1, respectively. The results
show that Lu(ac)3provides the best improvement in sensitivity among the five lanthanide organometallic compound co-doped photopolymers.
The above experimental results indicate that the per-formance for holographic data recording follows the order as: Lu(ac)3 > Yb(ac)3 > Er(ac)3 > Nd(ac)3 > Ce(ac)3. This order corresponds to that of their atomic num-ber (Lu> Yb > Er > Nd > Ce), but is opposite to their ionic radius (Lu3+ < Yb3+ < Er3+ < Nd3+ < Ce3+). It
im-plies that the degree of improvement will be increased if we se-lected a smaller ionic radius of the lanthanide in the lanthanoid organometallic compounds for doping.
From lanthanoid chemistry, we know that the unfilled inner 4f electrons of the lanthanide are shielded by the outer filled 5s and 5p orbitals, which penetrate the subshell of the 4f electrons. The M3+ ions no longer have the outer filled electrons of 5s and 5p orbitals. Hence, the ionic radius of the lanthanide is reduced as the atomic number is increased. This is called the lanthanide contraction, because the effective nuclear charge is increased. Research on the binding mode and catalytic mechanism between PQ and metal ions [48] indicates that the carbonyl groups on the PQ molecule react with metal ions upon light exposure. The photo-induced electron transfer reactive strength will depend on either the Lewis acidity or ionic radius of the metal ions. Our results also show that, when co-doped with lanthanides of a smaller ionic radius, in our case Lu3+, the photoreaction of PQ to produce a
photo-induced phase grating become faster and stronger.
Hence, organometallic compounds and a PQ co-doped photopolymer provide an improvement in the holographic data
recording characteristics. The photoreaction is related to the ionic radius of the metal ions of lanthanoid organometallic compounds. The details of the recording mechanism and their photochemical reaction with PQ molecules will be further investigated by chemical analyses.
4. Conclusion
We have fabricated lanthanoid organometallic compounds and a PQ co-doped photopolymer. The holographic data recording characteristics of these materials have been experimentally characterized. We have performed both single-hologram recording and multiple-hologram recording. Experimental results show that the performance of the holographic recording characteristics follow the order: Lu3+ > Yb3+ > Er3+ >
Nd3+ > Ce3+. The best result is obtained from Lu(ac)3
co-doped molecules. Comparing that with the PQ singly co-doped P(HEMA-co-MMA) photopolymer samples, the maximal diffraction efficiency has been improved by 3.4 times to 98.3%, M/# has been improved by 2 times to 3.86 and the sensitivity was improved by 1.5 times to 0.0296 cm2J−1. These results suggest that adding lanthanoid organometallic molecules with large atomic number of the lanthanoid into a PQ doped P(HEMA-co-MMA) photopolymer system could be an effective way to improve the holographic recording material for volume data storage.
Acknowledgment
The financial support from the National Science Council, Republic of China under NSC 96-2221-E009-015 is gratefully acknowledged.
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