| With the advent of the era of big data,a large number of data are facing high storage costs,storage difficultly,and other problems.Volume holographic storage technology develops two-dimensional surface storage into multi-dimensional high-density storage,which can realize the large data storage capacity,fast transmission speed,and parallel reading and writing.As the medium of volume holographic storage,the materials should meet the requirements of high sensitivity,fast response,high resolution,high signal-to-noise ratio,high diffraction efficiency,large dynamic range,small shrinkage,simple preparation and low cost.Phenanthraquinone doped polymethyl methacrylate(PQ/PMMA)photopolymer has low cost,accurate and controllable thickness and model,polarization response ability,and long-term preservation.These commercial values make it possible to become the main force of optical information storage materials in the future.Based on the new polarization holography theory described by tensor,the holographic properties of PQ/PMMA materials are studied under large interference angle.In order to solve the problems of instability,low polarization diffraction efficiency and sensitivity caused by low solubility of photosensitizer,and low refractive index,the following improvement schemes are proposed:(1)Firstly,the influence of different concentration of initiator on the holographic properties of the material is analyzed by experiments,and the best doping ratio(MMA:PQ:AIBN=100:1:1)is determined for the analysis and comparison of subsequent experiments.Because PQ/PMMA materials need two-step polymerization to achieve grating recording,a single thermal initiator can not control the monomer polymerization rate.The self-acceleration phenomenon in the process of thermal polymerization leads to the uneven distribution of polymer molecular weight,which directly affects the stability of holographic properties of photopolymerization.In order to solve this problem,a new type of composite initiator is introduced to control the thermal polymerization process of materials.Two kinds of thermal initiators with different activities can eliminate the self-acceleration phenomenon in the polymerization process,so the polymerization rate can tend to be uniform.Through the test and analysis of the comprehensive properties of materials,it is found that the materials with composite initiators can effectively control the thermal polymerization process.The diffractive efficiency deviation is reduced from5%to less than 2.5%at different recording positions of the material,which makes the material close to the practical specifications.(2)In the recording process of polarization holography,the polarization information of light is mainly controlled by the polarization sensitive rod-shaped phenanthraquinone(PQ)molecule.Because the soluble PQ concentration of the polymer system is only1wt%at 60?,the polarization sensitivity and polarization diffraction efficiency of the material are greatly limited.The second monomer,methyl pyrrolidone(NMP),is introduced to methyl methacrylate(MMA)to form co-monomer.Because the solubility of NMP for PQ is 10 times higher than that of MMA at the same temperature.The solubility of PQ can be effectively increased to 1.3wt%,the polarization diffraction efficiency can be enhanced to 4%,and the sensitivity can be improved to 0.4cm/J.(3)A novel doping method is proposed to improve the polarization holographic properties of PQ/PMMA materials.By introducing graphene oxide(GO)nanosheets,it can initiate MMA polymerization during thermal polymerization.And PMMA is grafted on GO surface,which can improve the conversion of MMA and the molecular weight of PMMA.PQ/PMMA photopolymer grafted on GO surface(GO-g-PQ/PMMA)is successfully prepared.The mechanism of GO doping is analyzed by UV-vis spectrophotometer,Fourier transform infrared spectroscopy,transmission electron microscope,atomic force microscope,and Gaussian calculation.At the same time,the holographic properties of Go-g-PQ/PMMA material are tested and analyzed.The results show that the polarization diffraction efficiency of the material can be effectively improved to 7%,the sensitivity can be enhanced to 0.5cm/J,and the photoinduced birefringence can reach 7×10-5.Through the above research,the problem of material stability is greatly improved.The introduction of comonomer verified that improving the solubility of PQ is an effective way to enhance the holographic properties of materials.The emergence of GO-graft-PQ/PMMA composites materials here is expected to fulfil the requirement of high-performance polarization-sensitive materials in the field of polarization holographic data storage,and provide a facile but effect nanocomposites doping strategy to modulate the holographic performance of photopolymers from micro-and mesoscopic level. |
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