Synthesis and Holographic Recording Properties of the Photorefractive Polymers Exhibiting Photoconductivity and Optical Nonlinearity
|Keywords:||影像儲存;光折變;光導電性;非線性光學;高分子;Holographic Recording;Photorefractive;Photoconductivity;Optical Nonlinearity;Polymers|
|Abstract:||本論文中合成三種類型之光折變高分子材料。第一種材料是將光導電分子混入非線性光學特性之高分子中而製成，透過在極化時之即時(In Situ)量測可了解非線性分子在電場作用下的定向(orientation)機制與熱穩定性。陷阱之位能及對應於不同能量之陷阱密度分布對光折變高分子之特性有決定性之影響, 本研究率先以熱刺激去極化電流(thermally stimulated current TSC) 探討此兩種與有關陷阱之特性，並探討混入之成份及主鏈結構對陷阱之位能及陷阱密度分布之影響。因為光折變高分子的組成成份包括非線性光學分子，電荷生成物種（CGM），電荷傳輸物種（CTM）, 三者之吸光範圍彼此不同, 所以本研究除了改變組成成份之分子結構外, 亦嘗試使用不同波長之雷射作為讀寫光源, 也發現其對光折變高分子的特性有很大的影響。
第二及第三種材料均屬於將非線性光學分子與光導電物種以化學鍵結結合之全官能性光折變高分子材料（Fully Functionalized Materials）, 如此可以解決第一種材料中混入之小分子結晶析出之問題。第二種材料將非線性光學分子與光導電物種透過共單體(comonomer) 結合在一起, 探討光折變特性之最適化及將其用於影像儲存之應用, 嘗試藉由改變分子結構，加入Sensitizer 以加速響應速度，探討影像重覆讀寫之可行性。藉加入交聯劑使原為線性之分子鏈變為網狀結構以增加已定向分子在內部電場消失後再紊亂化(randomization)之阻力, 延長儲存壽命, 探討（交聯劑/高分子）比例及交聯劑鏈長之影響。以角度多工方式使我們可以經由旋轉光折變高分子材料或改變兩道寫入光之夾角而將多張影像儲存於同一點, 增加儲存密度，並評估儲存影像之解析度。
第二種材料有成份將彼此稀釋之問題, 亦即提高某一成份之濃度將導致其他成份之稀釋。但各成份都必須盡量維持高濃度才不致於為提高某種特性而顧此失彼, 採用同時具有光導電性及非線性光學特性之雙功能性發色團(dual functional chromophore)可解決此一問題, 本研究中第三種材料即採用雙功能性發色團, 並引入五元異原子雜環(five-membered heterocyclic)結構及延長共軛長度(conjugation length)以提高二階非線性光學特性, 並觀察其對光折變特性與全像儲存(hologram storage)效果之影響。最近發展之Hyper-Raileigh scattering (HRS)被用以取代傳統之電場誘發二次非線性訊號(EFISH)來量測□ 值, 並評估此材料在影像儲存，清除，多次讀寫，儲存影像解析度方面之表現。|
Three types of PR polymers are prepared in this thesis. The first type polymers are prepared by doping the nonlinear optical host polymer with charge transport agents. The in-situ optical nonlinearity during poling and thermal stability is discussed. Then, trap characteristics of polymeric PR materials are studied for the first time using a thermally stimulated current (TSC) spectrometer. Both the trap depth and the trap density distribution in energy are derived from the TSC spectra. The effect of each component and the main chain structure of the PR polymer system on the trap behaviors are investigated. Since the nonlinear optical polymer and a charge generation material (CGM) exhibit different visible light absorption spectrum, we find that the wavelength of the writing and reading beams also shows a tremendous effect on the photorefractivity. It reveals that choosing appropriate writing and reading laser beams can speed up the response time of PR polymeric materials. To prevent crystallization, the second and the third types of polymers prepared in this thesis are fully functionalized PR polymers where both the charge transporting moieties and NLO moieties are chemically bonded on the main chain. We try to extend the storage lifetime, speed up the response time and increase the storage density of the medium in this chapter. TNF is added into the film as the sensitizer. The images are stored, erased, and overwritten in the PR polymers to investigated the erasure and update capability of the film. The influences of relative component concentrations, the applied electric field, and sensitizers on the PR response time and the diffraction efficiency are also studied. Using the angle multiplexing technique, many pages of data can be stored on the same spot of the medium either by changing the angle between the writing beams or by rotating the medium to different angles for each recording. Thus, specific pages can be accessed by selecting the corresponding angle of the reading beam. By adjusting the polymer/curing agent ratio and choosing a curing agent with proper chain length, a crosslink polymeric network with appropriate cross-linked density is synthesized to slow down the randomization of the oriented chromophores. To achieve large diffraction efficiency and fast response, the third type of PR polymer is prepared from one single active component (dual functional chromophore) to elevate the concentration of each functionality as high as possible. The relationships between the nonlinear optical and the holographic properties are studied. The diffraction efficiencies of the PR polymers are elevated in two ways. First, dual functional chromophores are used to maximize the concentration of each functionality. Second, the second-order optical susceptibility is promoted by extending the conjugation length with an additional electron donating group and an additional electron withdrawing group and/or introducing the five-membered heterocyclic ring onto the chromophore molecules. The influence of the electron acceptor of the chromophores on the PR characteristics is investigated. The resolution of the stored images in the PR polymers and their update capability for data storage are also evaluated.
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