Elsevier

Optics Communications

Volume 281, Issue 4, 15 February 2008, Pages 559-566
Optics Communications

Fabrication and characterization of poly(methyl methacrylate) photopolymer doped with 9,10-phenanthrenequinone (PQ) based derivatives for volume holographic data storage

https://doi.org/10.1016/j.optcom.2007.10.039Get rights and content

Abstract

We present our studies on the photopolymer of poly(methyl methacrylate) (PMMA) doped with 9,10-phenanthrenequinone based derivatives for volume holographic storage. By introducing different functional groups on the side-chain of 9,10-phenanthrenequinone molecule, the holographic characteristics of the material can be modified. The photoreaction involved with the holographic recording in the samples was investigated by measuring UV–Vis absorption spectrum and mass spectrum. The experimental results show that the similar behaviors were exhibited in these photopolymers. It is found that phase hologram recording in our PQ derivatives doped PMMA photopolymer involves a structure change of the quinone based molecule, which induces a strong change of the refractive index. Experimental characterizations on holographic data storage, including material sensitivity, dynamic range (M#) and bit-error-rate have been performed. We found that, by selecting appropriate functional groups, an improvement in sensitivity and M# for holographic data storage can be achieved.

Introduction

Recently, many experimental works have demonstrated that polymer-based recording materials are very attractive for the holographic recording applications [1], [2], [3], [4], [5], [6]. Photopolymer materials can provide large modulation in refractive index, and they are easy to fabricate because dedicated crystal growth facilities are not required. In contrast, photopolymer materials also possess some disadvantages. Among them, the problem of material shrinkage induced by light exposure is the most serious. This effect induces dimensional distortions on the recorded gratings such that the Bragg conditions for these volume holograms are missed and the recorded information cannot be retrieved completely [7], [8]. Shrinkage effect is to some extent in proportional to the material thickness. Therefore, conventional photopolymerizable material is limited to hundreds μm.

In our group, we have adapted the doped system to produce photopolymer and developed a technique for synthesizing the 9,10-phenanthrenequinone-doped poly(methyl methacrylate) (PQ/PMMA) photopolymer with a few mm thick. The experimental results demonstrated that PQ/PMMA photopolymers possess not only high optical quality but also negligible shrinkage effect under light exposure, thus they are suitable for the volume holographic recording [9], [10], [11]. Our strategy to alleviate the shrinkage problem is to separate the photochemical reaction during holographic recording from polymerization of host monomer molecules during material preparation, since most of shrinkage of material occurs at polymerization of recording elements. This can be achieved if most of the MMA molecules are polymerized to form a host polymer matrix PMMA during material preparation in a way that only a few percentages of un-reacted MMA monomer molecules are left and dispersed in the polymer matrix. The monomer molecules together with the doped photosensitive element, PQ molecules, are responsible for holographic recording. Our previous investigations have demonstrated the successful fabrication of this doped polymer system. However, the recording sensitivity and the dynamic range (M#) are still needed to be improved.

In order to improve the material, chemical analyses have been performed to investigate the physical mechanism of holographic recording on PQ/PMMA photopolymer. It was found that the photochemical process in our PQ/PMMA sample is a one-to-one photoreaction [12]. That is, the o-quinone double bond on the carbonyl functional group of the PQ molecule reacts with the carbon double on vinyl group of the residual MMA monomer. The photo-produced compound is less conjugated than the original molecular structure of PQ, such that the refractive index of the sample has been changed. As the results, a difference in the refractive index between bright and dark areas, following to that of the interference pattern, has formed and which is responsible for phase grating recording. Since such photoreaction does not involve with the backbone of the polymer matrix, hence the holographic recording induces very little influence on the matrix structure of PMMA chains. Thus, volume shrinkage due to photo-polymerization, which always happens in conventional photopolymer materials, can be minimized.

Above information about the process of photoreaction provide some hints for improving the PMMA photopolymer. Since the photoreaction between PQ and MMA molecules play an important role of holographic recording, hence modification of PQ molecules may be able to enhance the optical sensitivity and dynamic range for recording. In this paper, we replace the PQ molecules by doping different 9,10-phenanthrenequinone based derivatives, all with o-quinone double bonds. We hoped that, by doping photosensitive molecules with different side link functional groups, including electron-donor, electron-withdrawing, and aromatic structure, around the o-quinone bond of the molecule, it is possible to modify photo-induced properties of photosensitive elements and then modify the holographic characteristics of materials. Four different 9,10-phenanthrenequinone (PQ) based derivatives are used to fabricate new volume holographic photopolymers. The holographic recording characteristics of the different samples are studied and discussed. The results show that doping photosensitive molecule with electron-donor type can improve not only dynamic range but also sensitivity of the photopolymer material.

Section snippets

Material components and sample fabrication

The monomer, methyl methacrylate (MMA) was distilled under low pressure to remove inhibitor after purchasing from Sigma Aldrich [13]. In addition to 9,10-phenanthrenequinone (PQ), three kinds of PQ-based derivatives are chosen as photosensitive molecules: 1-isopropyl-7methyl-9,10-phenanthrenequinone (named as PQ1), 2-nitro phenanthrenequinone (named as PQ2) and 11,12-dihydrochrysene-11,12-dione (named as PQ3). The azoisobutyronitrile (AIBN), thermo-initiator, was purified through

UV–Vis spectra measurements

Typical optical absorption spectra of our samples are shown in Fig. 2. It can be seen that in Fig. 2a, the unexposed samples possess strong absorption below the blue wavelength (<450 nm), and they are transparent for wavelengths longer than 540 nm. At 514 nm wavelength, the absorption coefficients are 1.94 cm−1, 2.42 cm−1, 0.48 cm−1 and 0.88 cm−1 for PQ, PQ1, PQ2 and PQ3 doped PMMA samples, respectively. The results show that 514 nm wavelength is suitable for holographic recording in these samples.

For

Characterize PQ1/PMMA sample for holographic data storage

Since PQ1/PMMA photopolymer is the best one among the four samples, we performed further experiments on this sample for holographic data storage application. First, we recorded a plane-wave hologram in the sample with different thickness, and then measured the Bragg selectivity of the recorded grating. Fig. 5 shows the experimental results, in which the diffracted power of the grating is plotted as a function of the angular deviation for 1.2 mm and 2.4 mm thick samples, respectively. It can be

Conclusions

We have fabricated four kinds of doped PMMA photopolymers by using different PQ-based derivative molecules. The optical exposure and holographic characteristics of the samples have been measured and compared. Experimental results show that by selecting suitable functional group on photosensitive molecules with suitable doping concentrations, holographic recording properties of photopolymers can be improved. The best one among the four samples is PQ1/PMMA photopolymer, which possesses the

Acknowledgements

The financial supports from the Ministry of Education, Taiwan under Research Excellence Project PhaseII (Contract Number: NSC-95-2752-E-009-007-PAE) and the National Science Council, Taiwan (Contract Number: NSC-95-2112-M-009-007) are gratefully acknowledged.

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