近年隨著顯示技術領域的發達，以及液晶材料及光學研究進步，廣泛應用在於顯示科技，而不僅在顯示方面，更應用在光電元件。液晶光柵是可以外場來控制液晶行為，來進行分光的元件，具有低成本、低耗能和能做為光開關的優勢。目前液晶光柵的元件由於設計結構的關係，而使得繞射效率與入射光波長與偏振有關。
　　藍相液晶為存在於膽固醇相與均向相之間的中間相。由於藍相的高度對稱的結構，在未加外場時具有光學同向性質以及次毫秒的反應時間的優點。因此本文章利用藍相液晶來製作光柵，並且探討其電光特性。
　　首先我們利用聚合物穩定藍相液晶在不同溫度下聚合所形成的結構不同，所產生光電特性的差異來製作相位光柵。實驗中發現，在相同驅動電壓下，高溫聚合穩定藍相液晶與在低溫聚合穩定藍相液晶所引致的折射率不同，造成相位差而形成光柵，而產生繞射的效果。此外，由於是使用垂直電場來驅動元件，所引致折射率方向與電場方向相同。因此，入射光行進方向與電場方向相同時，此光柵達到純相位調製的功能，不改變入射光的偏振特性。所以它具有與入射光偏振無關的且具有快速時間的優點。
　　其次我們利用偶氮染料摻雜於聚合穩定藍相液晶中光引致相變之機制，來製作光柵。當照射UV的雷射光時，偶氮材料的光同素異構化反應會使聚合物穩定藍相液晶相變，原本藍相液晶相變為光學各向同性特性，未曝光區域則保持原本藍相液晶光學特性來製作光柵。當照射綠色的光時，光引致的光同素異構化反應被引致而使得藍相液晶回到原始狀態。實驗結果顯示，這元件不但保有前一個光柵偏振無關及快速反應時間的特性，而且其繞射效率大幅提升以及驅動電壓也大幅降低。另外，藉由改變照射的的波長可以複寫任意製作所需要的光柵圖形之特性對於元件的應用上具有相當大的價值。

　　The advantages of liquid crystal (LC) diffraction gratings include their low cost and low-power electrical switchability. Liquid crystal is easily controlled by exploiting common electro-optical behaviors. To form a phase grating profile, the LC must have a periodically varying refractive index. Previous works by the authors showed that, because of the intrinsic optical anisotropy of LCs, the optical properties of
these gratings are highly dependent on the polarization state of incident light. Some polarization-independent gratings use conventional orthogonally aligned nematic LC in one cell. However, even though the diffraction intensity is independent of light polarization, the diffraction beam polarization still depends on the angle between the polarization of the incident beam and the LC alignment.
　　Blue phases (BPs) have been in chiral liquid crystals between the cholesteric and isotropic phases The BP of a self-assembled three-dimensional cubic structure with lattice periods of several hundred nanometers exhibits not only selective Bragg reflections of light in the visible wavelength, but also optical isotropy owing to its highly symmetric molecular structure. A BPLC is optically isotropic when no voltage is
applied, and phase-only modulation occurs when an electric field is applied parallel to the direction of propagated light. However, the use of in-plane switching (IPS) electrodes makes the grating polarization-dependent and retains background diffraction from the patterned electrode.
　　This dissertation discusses the control of a polarization-independent and rapidly responding polymer-stabilized blue phase liquid crystal (PSBP) phase grating by exploiting the variation in the voltage-induced birefringence of the LC under a varying phase. The electro-optical properties, including the voltage-dependent diffraction intensity, polarization-independence and response time, are also discussed.
　　Firstly, the hybrid PSBP liquid crystal phase grating was studied. This grating consists of hybrid PSBPs with different Kerr constants that are determined by varied phase separation conditions. When cured at a high temperature, a loose PSBP network is formed and a large Kerr effect is obtained. Accordingly, the electric-field-induced birefringence of PSBPI is lower than that of PSBPII at a given driving voltage. The
non-patterned electrode and optical isotropy of both PSBPI and PSBPII enable elimination of the diffraction effect when the voltage is off. The diffraction intensity increases with the applied voltage up to a maximum at 150V.
　　Secondly, a polarization-independent and rapidly responding dye-doped (DD) PSBP liquid crystal phase grating was realized with two different phase states. The non-patterned electrode and the optical isotropy of the PSBP completely eliminate the diffraction effect when the voltage is off. The diffraction intensity can be increased by applying a uniform electrical field, which induces a phase difference in the DDPSBP. The diffraction efficiency exceeds 6%, which is 7-fold higher than those of most hybrid PSBP gratings and the device also supports optical writing, erasing, and rewriting. Additionally, the driving voltage is much smaller than that of hybrid PSBP grating. The phase grating is completely independent of polarized incident light. Finally, switching response time is in the sub-millisecond range.

摘要……………………………………………………………….……….II
Abstract……………………………………...…………….……….……..III
致謝……………………………………………………………….………VI
Contents……………………..…………………………………….........VII
List of figures……………………………………………………….….. XI
Chapter 1 Liquid Crystals…………….…………......................………...…1
1.1 Introduction of Liquid Crystals………..........................….…………...……1
1.2 Category of liquid crystals…………………………………………......2
1.3 Physical of nematic liquid crystal ………………...................………...8
1.3.1 Birefrienge…………………………….........………….…….....8
1.3.2 Effects of Temperature on Nematics………………........……..12
1.3.3 Fréedericksz Transition…………….....….…..………...……...14
Chapter 2 Basic related theories…………………………...………….…..16
2.1 Cholesteric liquid crystals………………..………………...…..……..16
2.1.1 Optical properties of cholesteric liquid crystals…………..…...17
2.2 Blue phase liquid crystals…………………………………………......20
2.2.1 Brief history……………………………………………………20
2.2.2 What are Blue Phases?………………………………………...21
2.2.3 Blue phase liquid crystals formed by birefringence induced by
electric field……………………………………………………...…...25
2.3 Azobenzene-doped liquid crystals…………………..………….……..28
2.4 Liquid-crystal gratings…………….…………………………….........30
2.4.1 Characteristics of diffraction gratings……….…………...……30
2.4.2 The theory of binary diffraction grating…………...……..…...34
Chapter 3 Sample preparations…………………….………………….…..38
3.1 Materials……………………………………...……………...………..38
3.1.1 Nematic liquid crystals…………………………...………..…..38
3.1.2 Chiral agent ……………….……………………..…….....…...39
3.1.3 Azobenzene …………………..…………………………..…...39
3.2 Fabrication of samples…………………………………………….…..40
3.2.1 Hybrid BP liquid crystal phase grating (used in Chapter
four)……………………………………………………………...…..40
3.2.2 Azo dye-doped polymer-stabilized BP liquid crystal phase
grating (used in Chapter five)……………………….…………….…41
Chapter 4 Hybrid BP liquid crystal phase grating………………………...42
4.1 Introduction……………………………………………………...42
4.2 Experiment…………………………………….…..………..…...44
4.3 Results and discussion…………………………..…………….....45
Chapter 5 Azo dye-doped polymer-stabilized BP liquid crystal
(DDPSBPLC) phase grating………………………………..…….52
5.1 Introduction……………………………………………………...52
5.2 Experiment………………………………………….…….…......54
5.3 Results and discussion…………………………………..…….....56
Chapter 6 Conclusions…………….…………………………………...62
References…………………………………………………………...…....64

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