A new photopolymer for volume holographic recording

Po-Lin Chen
National Chiao Tung University, Taiwan
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Photopolymers have been applied in different applications on holographic recording. The advantages of this material are easy to synthesize, high refractive index change and high sensitivity. For volume holograms recorded with thick materials, good optical quality and dimensional stability of the recording material are the most stringent requirements. Recently, a novel technique for fabricating bulk phenanthrenequinone-doped poly(methyl methacrylate) (PQ:PMMA) photopolymers with thickness of ~cm has been developed in our laboratory1. According to our investigations, the physical mechanism of holographic recording in our PQ:PMMA can be summarized as follows2: in the bright region of an interference pattern, the quinone double bond on the carbonyl functional group of a PQ molecule is excited to react with the carbonic double bond on the vinyl group of a residual MMA molecule in the PMMA matrix to form a new compound. This compound is less conjugated than the original molecular structure, and thus the refractive index of the material is changed locally. In other words, the attachment of a PQ molecule and a MMA molecule plays a key role. Nevertheless, this compound is a relatively small molecule and because of the concentration difference between the two regions in the polymer matrix, it may diffuse from the bright to the dark region. Thus, the modulation of the refractive index degraded gradually as time goes. Further, after recording, the residual PQ molecules that are unexposed in the dark region will react to light illumination during the reconstruction stage. Then, photo-induced attachment of the PQ molecule with the MMA molecule will occur accordingly. As a result, the refractive index modulation between the dark and bright regions of the original interference pattern will be reduced further, and the hologram behaves as unfixed. Therefore, the long-term stability, or, fixing of holograms in PQ-PMMA photopolymers is an important issue for applications. In this paper, we present the principle and experimental results for improving long-term stability of holograms in PQ-PMMA photopolymers. In this method, in addition to MMA, another monomer with multi-acrylate groups is added during the material fabrication stage. These new monomer molecules can be incorporated with the long-chain of PMMA polymer in a way to form co-polymer matrix with a pendent carbonic double bond of the vinyl group. As a result, the photo-induced attachment of PQ molecules can occur not only with the residual monomers but also with the copolymer matrix. Since the copolymer matrix can not diffuse at room temperatures or slightly higher, thus long-term stability of the hologram will be anticipated. In the meantime during the reconstruction stage, the PQ molecules will diffuse from dark to bright region and distribute uniformly in the sample, the photo-induced reaction between this molecule and the residual molecules will occur until all PQ molecules in the material are photo-chemically exhausted so that no more photoreaction occurs. Then, the hologram can remain stable under light illumination, thus it can be fixed. In this study, the trimethylolpropane triacrylate (TMPTA), which has three acrylate groups, has been added to fabricate copolymer samples. Experimental results show that when the ratio of TMPTA to MMA is 2:3, the hologram recorded in co-polymer sample remains stable during the reconstruction. The fabrication and characterization of the new PQ doped poly(methyl methacrylate-co- trimethylolpropane triacrylate) photopolymer will be described in this presentation. 

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