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研究生： 林蕙琪
Lin, Hui-Chi
論文名稱： 利用雙光子Z-掃描技術研究偶氮染料摻雜液晶薄膜之非線性光學性質
Studies of Nonlinear Optical Properties of Azo-Dye-Doped Liquid Crystal Films Using Biphotonic Z-scan Technique
指導教授： 傅永貴
Fuh, Ying-Guey Andy
學位類別： 博士
Doctor
系所名稱： 理學院 - 物理學系
Department of Physics
論文出版年： 2008
畢業學年度： 96
語文別： 英文
論文頁數： 89
中文關鍵詞： 液晶 、偶氮染料 、Z-掃描
外文關鍵詞： liquid crystals, azo-dye, Z-scan
相關次數： 點閱：94  下載：7 
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    • 本論文利用雙光子Z-scan技術探討偶氮染料摻雜液晶薄膜之非線性光學特性。其目的是探究偶氮染料摻雜液晶薄膜中所發生的有趣效應，如雙光子效應、熱效應及偶氮染料引致液晶分子轉動效應等，這些效應都會對Z-scan技術所量測出的光學克爾常數有所影響。Z-scan技術量測光學克爾常數的原理是基於樣本的非線性折射率會產生自聚焦或自發散的現象，利用其自聚焦或自發散的結果推論出其相位及折射率的變化。
    本論文包含著兩個研究主題，第一個研究主題是使用雙光子Z-scan技術探討偶氮染料(DR1)摻雜液晶薄膜的非線性光學克爾效應。我們利用一道線性偏振的紅光聚焦在z方向，使樣本沿著z方向在聚焦後的紅光焦點附近移動。在樣本移動的時候，利用一道線性偏振的綠光同時照射在紅光照射樣本的區域。實驗結果證明，因為光激發偶氮染料同素異構化反應及熱效應的作用，利用紅光聚焦的Z-scan技術所量測之非線性效應可以被一道同時照射在樣本上的綠光所調制或改變。並且基於動態的量測的結果，我們可以知道，在綠光剛開始照射在樣本的時候，是光激發偶氮染料同素異構化反應主導光學克爾常數的變化，而樣本經過綠光照射一段時間之後，則是熱效應主導光學克爾常數的變化。
    另一個研究主題則是利用連續Z-scan技術探討偶氮染料(D2)摻雜液晶薄膜的雙光子效應。在這個研究主題中，紅光與綠光的角色與其在第一個研究主題是相反的。我們是利用一道線性偏振的綠光聚焦在z方向，使樣本沿著z方向在聚焦後的綠光焦點附近移動。樣本是先單獨照射綠光6秒，接著再同時照射綠光與一個均勻的線性偏振紅光6秒。以這樣的照射方式，連續照射在不同z位置的樣本，即所謂的連續Z-scan技術。實驗成果顯示，雙光子效應會對樣本的光學克爾常數有重大影響。在紅光強度較低時，光激發偶氮染料同素異構化反應所引致的液晶分子轉動會主導樣本的光學克爾常數。而在紅光強度較高時，熱效應則會補償液晶分子轉動所引起的光學克爾常數。

    This thesis explores the nonlinear optical property of azo-dye doped nematic liquid crystal films using the biphtonic Z-scan technique, which is a simple but powerful technique to measure the optical Kerr constant of materials. It investigates the effects, such as the biphotonic, the thermal and the liquid crystal reorientation effects induced by the doped azo dye that contribute to the optical Kerr constant. The measurement of the optical Kerr constant using the Z-scan is based on the principle of spatial beam distortion due to the self-focusing or self-defocusing that are associated with the intensity-dependent refractive index of the material.
    This work comprises two main experimental parts. In the first part, the thermal effect of azo-dye (DR1)-doped nematic liquid crystal films is studied using the biphotonic Z-scan technique. A linearly polarized red light is focused in the z direction onto the sample, which is scanned near the beam waist of the red laser. A linearly polarized homogeneous green light simultaneously illuminates the same spot of the sample. The experimental results indicate that the nonlinear effect measured using the Z-scan technique with a red light can be modulated or switched by the simultaneous application of a green light, because of photoisomerization and thermal effects, as determined by dynamic measurements. The former dominates in the early stage when the green light is applied, while the latter dominates in the latter stage.
    The second part is the investigation of the biphotonic effect of azo-dye (D2)-doped liquid crystals using the sequential Z-scan technique. In this part, the roles of the green and red lights are reversed from those in the first part; i.e. a linearly polarized green light is focused in the z direction onto the sample of azo-dye (D2)-doped liquid crystal, which is scanned near the beam waist of the green laser. Sequentially, the sample is illuminated by a focused green laser only, and then simultaneously by a linearly polarized homogeneous red light, in the “sequential Z-scan technique”. The results show that the biphotonic effect significantly affects the nonlinear coefficient of the sample. The measurements demonstrate that the molecular reorientation of the liquid crystals induced by the photoisomerization of the azo dyes dominates at low red-light intensity, but the thermal effect compensates for the molecular reorientational nonlinearity of the sample at high red-light intensity.

    摘要………………………………………………………………………I Abstract………………………………………………………………III Contents…………………………………………………………………V List of Figures……………………………………………………VIII List of Tables………………………………………………………XII Chapter 1 Introduction 1.1 Preface…………………………………………1 1.2 Liquid Crystals………………………………3 1.2.1 Category of liquid crystals………4 1.2.2 Phase of rod-like thermotropic liquid crystals ………………………6 1.2.3 Physical properties of liquid crystals ………………………………11 Chapter 2 Theory 2.1 Molecular Re-orientation Effect in Dye-Doped Liquid Crystals…………………19 2.1.1 Positive torque effect - Jánossy model………………………20 2.1.2 Negative torque effect –Gibbons model………………………23 2.1.3 Photoisomerization effect…………24 2.1.4 Biphotonic effect……………………26 2.1.5 Light-induced thermal effect ……28 2.1.6 Photorefractive effect ……………28 2.2 Z-Scan Technique ……………………………31 2.2.1 Introduction to third-order nonlinear optical process…………31 2.2.2 Nonlinear refractive index in Z-scan …………………………………35 2.2.3 Nonlinear absorption in Z-scan …………………………………………39 2.2.4 Z-scan theory…………………………41 Chapter 3 Experimental Preparations 3.1 Materials Employed in Experiments………46 3.1.1 Nematic liquid crystal – E7 ……46 3.1.2 Azo dye…………………………………48 3.2 Fabrication of DDLC Samples ……………………51 3.3 Inspections of DDLC Alignment - Conoscopic Technology.…………………………53 Chapter 4 Nonlinear Optical Properties of Azo-Dye (DR1)-Doped Liquid Crystals Determined by Biphotonic Z-scan Technique 4.1 Experimental Method…………………………55 4.2 Experimental Results and Discussions 4.2.1 Z-scan measurement …………………57 4.2.2 Temperature measurement……………62 4.2.3 Dynamic Z-scan measurement ………63 Chapter 5 Biphotonic Effect of Azo-Dye (D2)-Doped Liquid Crystals Using the Sequential Z-scan Technique 5.1 Experimental Method ………………………67 5.2 Experimental Results and Discussions 5.2.1 Biphotonic effect …………………71 5.2.2 Thermal effect………………………78 5.2.3 The amplification factor…………………………………79 Chapter 6 Conclusions and Future Studies ……………………81 References……………………………………………………………84 List of Publications…………………………………………………………88

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