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Exploration of the Fast Response Photorefractive Effect in Liquid Crystall Polymer Composites Based on the Photoconductive Material

Author: LianChao
Tutor: ZhaoHua
School: Harbin Institute of Technology
Course: Optics
Keywords: photorefractive liquid crystal polymer photoconductive materials response time
CLC: O753.2
Type: Master's thesis
Year: 2012
Downloads: 21
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Abstract


Liquid crystalline and polymeric photorefractive materials and associatedresearch activities advanced greatly in the past20years. However, due to the poorperformance in these materials, such as slow response rate, real-time3Dholographic display applications were restricted. Therefore, we carried out thesystematic studies in this dissertation, aiming at solving these problems. Thedissertation is divided into two parts, respectively, for the liquid crystalline andliquid crystal polymeric composites.We conceived and fabricated a new type of liquid crystal cell sandwiched withbare ZnSe substrates. Surprisingly, self-diffraction could be obtained without anexternal electric field, implying that these ZnSe substrates played an important rolein the modulation of the surface charge. When an electric field was applied, a verystrong fanning effect was observed. Even though the depletion of the pumpingenergy depletion, the two-beam gain coefficient reached300cm-1. Encouragingly,the response time of the first-order diffraction was only20ms. Furthermore, toverify the key role played by ZnSe material, ZnSe films were deposited directly ontoITO glass plates with electron beam evaporation. After making liquid crystal cellswith the substrates, as short as tens ms response time were achieved when theoperation voltage decreased an order of magnitude, from their original minutesresponse. Theoretical analysis showed that this effect was due to the chargeconcentration near the surface because of the dc field. The photoconductive ZnSecould enhance the modulation of the surface charge greatly and increase theresponse rate with the lower dc filed. This low-voltage and high-speed liquid crystaldevices were very promising in video rate3D holographic display and many otherapplications.To address the problems encountered in studying liquid crystal polymercomposites PVK:5CB: C60, namely, the performance was lower than the theoreticalvalue, we proposed to introduce PVK: TNF photoconductive films in between theITO and the polymer films. It was found that this modification could improve thediffraction efficiency and the gain coefficient greatly when the external voltage wasonly5KV, from their original ten thousand volts. The double exposureinterferometry experiments showed that the film has good image quality. We alsoperformed the time-of-flight method experiments for the PVK: TNF film whichshowed that the films had the rapid produce and transport ability of the carries.Theoretical analysis revealed that these ability could increase the response rate ofthe liquid crystal polymer and form the strong modulation of the grating. So it could increase the diffraction efficiency and the gain coefficient with the lower applied dcfiled. In order to further explore the reason why PVK: CB: C60polymer performancewas lower than the theoretical value, the ITO was directly sputtered on the PVK:5CB: C60polymer with magnetron sputtering method. It was founded that theresponse time was reduced over two orders of magnitude. Theoretical analysisrevealed that the ion injected from the ITO layer into the PR film restricted theperformance. So the method of sputtering the ITO could effectively reduce theimpact of this process and improve the response rate. This was of great significancein optimization of liquid crystal photorefractive properties.

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CLC: > Mathematical sciences and chemical > Crystallography > Amorphous and class crystalline > Class crystalline > Liquid crystal
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