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A new photopolymer for volume holographic recording

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A new photopolymer for volume holographic recording
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Shenzhen, China

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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.