藍相液晶為旋性液晶當中，存在於膽固醇相與均向相之間的中間相。因為藍相液晶本身自聚集的三維光子週期結構，且單位晶格大小約幾百奈米範圍，所以具有可見光波段布拉格反射的現象。此外，它的高度對稱的結構也使它具備光學同向性質。 藍相液晶是由向列型液晶分子所構成的週期結構，所以局部來說，藍相液晶仍然具有異向的物理性質。如此的特性使得我們容易藉由外加場方式控制藍相液晶。因此本文章主要探討藍相液晶在電場、光場、以及溫度下的效應。
首先我們研究負型藍相液晶的雙穩態效應和晶格面指向轉換機制。我們所使用的藍相材料具有16度的寬溫度範圍，並且藉由Kossel 圖判斷出此藍相材料BPI的(110)，(112)，(200)三個晶格面。實驗中發現，反射紅色的(110)和反射藍色 (200)晶格面為穩定的晶格面而且可以施加特別的脈衝電壓達到在 (110)和 (200)晶格面之間的互相切換。另外，我們也探討藍相藉由電場而引致的平面態和電流體動力效應。除了電場下的行為以外，我們發現(200)晶格面在溫度變化下會導致它的晶格形變而改變反射波長，因此可達到調控波長的效果，其調控的範圍從455nm 到545 nm，而且調控的行為是可回復的。
其次我們利用光場調控三維光子晶體的光子能隙。將偶氮染料摻雜於藍相液晶中，藉由偶氮染料的光同素異構化特性控制藍相液晶的光子能隙。本實驗使用M12C以及4MAB兩種不同性質的偶氮染料。M12C的作用主要是改變藍相液晶的晶格參數，然而4MAB主要是要改變藍相液晶的相轉變溫度。在M12C摻雜的藍相液晶樣品當中，當照射473奈米的雷射光時，偶氮染料的光同素異構化反應會使藍相液晶的單位晶格產生形變，使得藍相的光子能隙改變。而照射532奈米的光時，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。以上的晶格變化的行為皆經過kossel繞射圖形而得到驗證。最後我們利用光子能隙調控的行為示範光寫入的藍相反射式顯示器。另一方面，在4MAB摻雜的藍相液晶樣品中，偶氮染料的光同素異構化反應破壞藍相液晶的晶格結構而降低藍相液晶的相轉換溫度。我們進一步觀察藍相液晶在不同紫外光照射時間的相序列變化。實驗結果顯示，藉由控制UV的照射時間以及樣品的溫度，可以輕易切換不同的相狀態，也就是說可以切換這些相對應的光子能隙行為。舉例來說，當樣品在藍相或同均相之間切換時，對應的光子能隙可以被開或關。而當樣品在藍相或膽固醇相之間切換時，對應的光子能隙從三維特性轉化成一維的特性。
最後我們研究藍相液晶和膽固醇液晶相轉化的熱遲滯效應。實驗所使用的旋性液晶具有溫度6度的熱遲滯。因此利用溫度控制的過程，旋性液晶的藍相和膽固醇相可以互相切換，並且可以在室溫下穩定存在。此外，我們藉由不同的表面配向處理示範兩種不同的雙穩態的情況。在具有水平配向的樣品下，膽固醇相與藍相皆具有布拉格反射的行為，膽固醇相的平面結構會反射波長為480-510奈米的光，而具有一致的晶格排列的藍相則會反射630奈米的光。另外在無任何表面處理的樣品中，膽固醇相的焦錐態結構會散射光，而藍相因為它的晶格排列沒有一致性，因此呈現高度的透明情況。

Blue phases have been known to exist in chiral liquid crystals between the cholesteric and isotropic phases. A blue phase as 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 optically isotropy owning to its highly symmetric molecular structure. Locally, blue phases still exhibit local anisotropic physical properties because of anisotropic structure of the nematic liquid crystal molecules, which make it possible to be easily controlled by an external field. This dissertation studies the effects in blue phases under various external fields, including electrical field, optical field, and temperature.
Firstly, we investigated the bistable effect under the influence of an electric field and transition mechanism between various lattice orientations in the negative liquid crystal blue phase. The blue phase exists over a wide temperature range ~16oC, and three lattices (110), (112) and (200) of BPI are confirmed with Kossel diagrams. The red platelet (110) lattice and blue platelet (200) lattice can be stabilized and switched to each other by particular pulse voltages. We also studied the behavior that an electric field induced planar state and electro-hydrodynamatic effect in the blue phase. Additionally, the reflected color of the (200) lattice can be adjusted from 455nm to 545 nm by temperature induced lattice distortions and provided with reversibility.
Secondly, we presented an optically switchable band gap of a 3D photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. Two kinds of azobenzene, M12C and 4MAB, were utilized to switch photonic band gap of blue phases and to change the phase transition temperature of blue phase, respectively. For M12C- doped liquid crystal blue phase, the trans-cis photoisomerization of M12C induced by irradiation using 473nm light caused the deformation of the cubic unit cell of the blue phase and a shift in the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with 532nm light. The crystalline structure was verified using a Kossel diffraction diagram. Moreover, we also demonstrated an optically addressable blue phase display, based on Bragg reflection from the photonic band gap. For 4MAB- doped liquid crystal blue phase, the trans-cis photoisomerization of 4MAB destabilizes cubic unit cell of the blue phase and reduces the phase transition temperature. We observed the phase sequences of the 4MAB-doped blue phase as a function of the time of UV irradiation. Various distinct phases can be switched to another specific phase by controlling irradiated time and temperature of the sample. Therefore, the corresponding bandgap can be switched on and off between blue phase and isotropic phase, or varied from 3D to 1D between blue phase and cholesteric phase.
Finally, we investigated the thermal hysteresis in the phase transition between the cholesteric liquid crystal and the blue phase of liquid crystal. The thermal hysteresis of such a chiral doped nematic liquid crystal occurs over 6oC. Both the CLC phase and the blue phase can stably exist at room temperature and be switched to each other using temperature-controlled processes. Further, we demonstrated two sets of bistable conditions using various surface treatments. In a homogeneous aligned sample, two stable states, CLC with a planar alignment and blue phase with a uniform lattice distribution, reflect light of wavelengths 480-510nm and 630nm, respectively, as determined by the corresponding Bragg’s reflection conditions. In the untreated sample, the CLC phase with a focal conic texture can scatter light and the blue phase with a non-uniform lattice distribution provides high isotropic optical transparency.

Contents
摘要……………………………………………………………….……... IV
Abstract…………………………………………………………….……..VI
Contents……………………..……………………………………….........IX
List of figures……………………………………………………….……XII

Chapter 1 Introduction of Liquid Crystals…………….………………...…1
1.1 Brief History …………………………..……………….………………1
1.2 Liquid crystal phases…………………………………………………...2
1.2.1 Classification of liquid crystals……………...………………….2
1.3 Physical properties of nematic liquid crystal ………………..………...8
1.3.1 Birefrienge…………………………………...……………….....8
1.3.2 Dielectric anisotropy…………………………………………..11
1.3.3 Freedericks Transition………………………….……………...13
1.3.4 Continuum theory……………………………………………...14
Chapter 2 Basic related theories…………………………...……………..16
2.1 Cholesteric liquid crystals………………..…………………………..16
2.1.1 Optical properties of cholesteric liquid crystals………..……...18
2.2 Blue phase liquid crystals…………………………………………......21
2.2.1 Brief history……………………………………………………21
2.2.2 What are Blue Phases?………………………………………...22
2.2.3 Optical properties of blue phase liquid crystals…………..…...27
2.2.4 Methods to analyze blue phase liquid crystals……..………….28
2.2.5 Effect of an electric field to blue phase liquid crystals………..35
2.3 Azobenzene-doped liquid crystals…………………..………………..45
Chapter 3 Sample preparations…………………….……………………..51
3.1 Materials……………………………………...……………...………..51
3.1.1 Nematic liquid crystals (Th-Nge-1, K15, and E48) …………..51
3.1.2 Chiral agent (S811 and R811)……………………..……...…...52
3.1.3 Azobenzene (M12C and 4MAB)……………………………...53
3.2 Fabrication of samples………………………………………………..54
3.2.1 Negative Blue phase sample (used in Chapter four)……...…..54
3.2.2 Azo dye-doped Blue phase sample (used in Chapter five)……55
3.2.3 Positive Blue phase sample (used in Chapter six)……..………56
Chapter 4 Bistable effect in the blue phase liquid crystal………………...57
4.1 Introduction……………………………………………………...57
4.2 Experiment……………………………………….……………...58
4.3 Results and discussion…………………………………………..59

Chapter 5 Optically tuneable blue phase photonic band gaps…………….69
5.1 Introduction……………………………………………………...69
5.2 Experiment………………………………………………….…...72
5.3 Results and discussion…………………………………….……..72
5.3.1 M12C-doped blue phase LC sample……………….……..72
5.3.2 4MAB-doped blue phase LC sample……………….……..78
Chapter 6 Bistable cholesteric-blue phase liquid crystal using thermal hysteresis……………………………………………..………...82
6.1 Introduction……………………………………………………...82
6.2 Experiment………………………………………….…………...83
6.3 Results and discussion…………………………………………..84
Chapter 7 Conclusions and future works…………….…………………...92
7.1 Conclusions……………………..…………………………….....92
7.2 Future work…………………………...………...…………..…...94
Ruplications…..……………………………………………………...…....96
References…………………………………………………………...…....98

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