Preparation and Characterization of Volume Holographic Data Storage by using the Doped PMMA Photopolymers

• Main Authors: Chen, Po-Lin, 陳柏霖
• Other Authors: Hsu, Ken-Yuh
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/80856872307220638972

博士 === 國立交通大學 === 光電工程所 === 100 === In this dissertation, we focus on the holographic characterization of the doped PMMA photopolymer for volume holographic data storage. First of all, we elaborate the design strategy and preparation technique to fabricate a photopolymer with low photochemical shrinkage, called phenanthrenequinone doped poly(methylmethacrylate) (PQ/PMMA). The high-priority material requirements for holographic data storage, including optical characteristics, dynamic range (M#), sensitivity and photo-induced dimensional stability, are investigated on it. An experimental demonstration of multiple storage of digital data pages with non-distorted readout is presented. Based on the diffusion model of holographic recording in PQ/PMMA, the recoded volume holograms are predicted to be self-developed in dark and fixed by optical exposure. These results indicate that the PQ/PMMA can be considered as a promisingmaterial for volume holographic data storage application. However, the related characterization of PQ/PMMA such as dynamic range and sensitivity still need to be improved. From our previous research on physical mechanism of holographic recording in PQ/PMMA, the refractive index modulation between the bright and dark zones of interference patterns, is attributed to the adduct of one PQ molecule with one MMA molecule in the bright zone. Therefore, we proposed two methods to improve the holographic characteristics of the material： (1) By introducing different functional groups on the side-chain of PQ molecule, the holographic characteristics of the material can be modified. We found that, by selecting appropriate functional groups, an improvement in sensitivity and M# for holographic data storage can be achieved. (2) By co-doping different acrylate-based monomer in the PMMA matrix, holographic characteristics of the material can also be modified. We anticipated that the co-doped multi-acrylate /highly reactive monomers can enhance the combination ability of monomer molecules with PQ radicals, as well as the sensitivity and M#. Further, we found that the planar structure of PQ molecules dispersed in amorphous PMMA polymer possessed the photo induced anisotropy with linear polarized light. The resulting photo-induced birefringence in a 2-mm thick sample is measured by a phase-modulated ellipsometry, which can reaches 1.2 x10-5 by linearly polarized beam at 514 nm. We have also performed volume polarization holographic recording with two different polarization configurations. The experimental results show that with circular polarization configuration, the maximal diffraction efficiency of hologram can reach ~40%. A clear sinc-squared Bragg selectivity curve has been obtained so that the shrinkage is not a suspect in our PQ/PMMA photopolymer. The capability of multiplexing polarization holograms in our PQ/PMMA samples is also tested. These results suggest our PQ/PMMA can be attractive candidate for permanent volume polarization holographic recording. Due to the negligible shrinkage effect and good long-term stability of PQ/PMMA photopolymer, it is suitable to record the volume holographic hologram as narrow-bandwidth filter for optical communication and bio-sensing application. Unfortunately, the material can be used at only a limited spectral range shorter than 550 nm. In terms of chemical formula, the PQ is a-diketone based derivatives so that using the same idea proposed by Brauchle et al., it is possible to perform two-wavelength recording. Then, we investigated the two-wavelength holographic recording in 2-mm thick PQ/PMMA. The hologram was recorded at 647 nm in the sample with gating by an additional laser at 325 nm, significant 104-fold increase in material sensitivity and diffraction efficiency is achieved. The maximal diffraction efficiency can reach ~4%. In addition, Bragg selectivity curve and ability for image reconstruction is demonstrated in a 2-mm thick sample supporting further applications as recording media for volume holographic device with extended spectral response and selective recording property.