臺灣博碩士論文加值系統

近年來，液晶領域科學家們利用具旋性之液晶高分子製作新穎之膽固醇聚合物模板 (簡稱模板)，此模板可有效改善膽固醇液晶模板之天生光學侷限性，例如:提高反射率超過傳統膽固醇液晶之反射極限50%、在單一樣品中具有多個光子能隙、可彈性置換回填材料改變其光學特性等。
本篇論文乃製作兩個具有螺距梯度且相反旋性之膽固醇液晶模板，組合成一個可空間調控且可同時反射相反旋性圓偏振光之複合式膽固醇聚合物模板，其可空間調控波段範圍涵蓋整個白光區。此研究亦進一步探討此模板反射率受限之可能原因與增進反射率之方法，此方法可藉由將模板樣品升溫高於回灌液晶之澄清點以上，使得此模板之回灌液晶於液晶相時之散射可大大地有效降低，其反射率亦可明顯提高。
最後，本篇論文利用模板製作出擁有高反射率與可寬頻帶空間調控之光子能隙元件，其最高反射率可超過85%，可空間調控光子能隙波段涵蓋400 nm至800 nm，其覆蓋整個全白光範圍。未來，此具有全白光區高反射率之可空間調控光子能隙模板元件不只可應用於可全白光區之空間濾波器亦可發展成可全白光區空間調控、無須反射鏡、可同時產生左右旋圓偏振雷射之低能量閾值雷射器。

The scientists in the field of liquid crystal (LC) exploited chiral LC polymer to fabricate novel cholesteric LC (CLC) polymer template (simply called template) in recent years. The template can effectively overcome the limitation in the optical features of traditional CLCs, such as enhancement of reflectivity over 50%, multiple photonic bandgaps (PBGs), and changeable optical characteristics by flexibly replacing the refillingLC materials, and so on.
This thesis fabricates two gradient-pitched CLC templates with two opposite handednesses, which are then merged as a spatially tunable and hyper-reflective CLC template sample. This sample can simultaneously reflect right- and left-circularly polarized lights and the tunable spectral range includes the entire visible region. In addition, this study investigates the causes to limit the reflectance of the template sample and a method to improve the reflectance. By increasing the temperature of the template sample exceeding the clearing point of the refilling LC, the light scattering significantly decreases and the reflectance effectively increases.
In summary, this study fabricates a merged template sample to develop a wide-band spatially tunable and hyper-reflective PBG device. This device has a maximum reflectance over 85% and a wide-band spatial tunability in PBG between 400 nm and 800 nm which covers the entire visible region. This hyper-reflective PBG template device with a wide-band tunability over entire visible region can not only be employed as a wide-band spatially tunable filter, but also used to develop a low-threshold mirror-less laser with a spatial tunability at entire visible region and simultaneous emission of left- and right-circular polarizations.

Contents
摘要 I
Abstract II
Acknowledgements IV
Contents V
List of Figures VIII
List of Tables XV
Chapter 1 Introduction 1
Chapter2 Introduction to Liquid Crystals 3
2.1 The Discovery of Liquid Crystals 3
2.2 Classification of Liquid Crystals 4
2.2.1 Lyotropic Liquid Crystals 4
2.2.2 Thermotropic Liquid Crystals 4
2.3 The Physical Properties of Liquid Crystals 10
2.3.1 Birefringence and Optical Anisotropy 10
2.3.2 Dielectric Anisotropy 14
2.3.3 Elastic Continuum Theory of Liquid Crystals 15
Chapter 3 Cholesteric liquid crystals and polymer templates 17
3.1 Optical properties of cholesteric liquid crystals 17
3.2 Influences of various factors on the pitch 18
3.2.1 Temperature 19
3.2.2 Magnetic and electric fields 19
3.2.3 Optical field 22
3.3 Liquid crystalline polymer template 22
3.3.1 Addition polymerization 22
3.3.2 Fabrication of LC polymer template 23
3.4 Review of important literatures associated with LC polymer templates 25
3.4.1 Single refilled template reflecting both right- and left-circularly polarized lights 25
3.4.2 Thermally-induced multicolored hyper-reflective cholesteric liquid crystals 26
3.4.3 Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications 28
Chapter 4 Sample Preparation and Experimental Setups 31
4.1 Materials 31
4.2 Sample preparation 35
4.2.1 Cleaning of glass slides 36
4.2.2 Fabrication of empty cells 36
4.2.3 Mixture for fabrication of CLC polymer template 37
4.2.4 Fabrication of NLC-refilling CLC polymer template cells 38
4.3 Experimental setup 43
Chapter 5 Results and Discussion 44
5.1 PBG properties of template samples 44
5.1.1 Reflection spectra of template samples with uniform pitches before and after curing 44
5.1.2 Differences in the reflection spectra of the template samples measured from opposite sides 46
5.2 Spatially tunable and hyper-reflective refilling CLC template samples 49
5.2.1 Gradient-pitched L-CLC template samples 49
5.2.2 Gradient-pitched R-CLC template samples 54
5.2.3 Gradient-pitched composite CLC template samples 58
5.3 Improvement of the reflectance from refilling CLC template samples 61
5.3.1 Scattering from refilling CLC template samples 61
5.3.2 Temperature-dependent reflectivity of the refilling CLC template samples 63
5.3.3 Spatially tunable and hyper-reflective CLC template samples 64
Chapter 6 Conclusion and Future Works 67
6.1 Conclusion 67
6.2 Future works 67
References 69

 
List of Figures
Figure 2.1 Schematic representation of molecular order in the crystalline, mesomorphic and isotropic phase. 3
Figure 2.2 Schematic representation of molecular arrangement for nematic liquid crystals in space. 6
Figure 2.3 Schematic representation of molecular arrangement for cholesteric liquid crystals. The director n rotates gradually along the helical axis(z-axis). 7
Figure 2.4 Structures of semectic liquid crystals in (a) semctic A (b) semctic C, whrer n is the director of liquid crystal. 9
Figure 2.5 Liquid crystalline phase in a temperature scale. TM and TC are the melting and clearing temperatures, respectively. 10
Fig. 2.6 (a) Schema of refractive index ellipsoid with n_x=〖 n〗_y=n_o and n_z=〖 n〗_e for the uniaxial crystal and k is the propagating direction of a plane wave. The refractive indices and allowed polarization of light depend on the direction of k vector and can be obtained by using the schema (b) intersection of ellipsoid and y-z plane. n_e(θ) is effective refractive index when the k has an angle θ related to optic axis. 13
Figure 2.7 Temperature dependence of the refractive indices, n_e and n_o, for nematic liquid crystal. 14
Figure 2.8 Three basic types of deformation of Nematic Liquid Crystal : (a) splay, (b) twist, (c) bend. 16
Figure 3.1 Schematic representation of selective reflection with left-circularly polarized light for a non-polarized incident light through a planar CLC with a left-handedness spiral structure. 17
Figure 3.2 Typical reflection spectrum of a planar cholesteric liquid crystal cell for normal incidence. 18
Figure 3.3 Schematic representation of the CLC texture under the action of external magnetic field with increasing the strength of the magnetic field. Hc is the critical value of the field. 21
Figure 3.4 Three steps of free-radical addition polymerization. 23
Figure 3.5 Schematic diagram for fabricating a typical CLC polymer template. (a) The cell is radiated with UV light to form polymer network. (b) The template is obtained by removing the nonreactive residues. (c) The cell is refilled with a specific NLC. 24
Figure 3.6 Schematic of the method to prepare the PSLC film. (a) The cell containing mixture of L-CLC and monomers (b) The polymer network tempalte with a left-handed helical structure (c) The cell containing the polymer network template were refilled with mixtures with a right-handed helical structure R: reflection; T: transmission. 26
Figure 3.7 (a) Schematic representation of the fabrication process where polymerization from one side of the cell causes growth of a chiral polymer structure from the top side of the cell only. The middle panel indicates that the polymer structure only traverses a fraction of the cell thickness and that when refilled the bottom half of the cell exhibits properties of the filling mixture. (b) The unpolarized transmission spectra of the STPN cell refilled with a left-handed SmA* mixture at 27 °C. (c) The texture of the polymer-rich side of the cell and d) the texture from the back side of the cell. 27
Figure 3.8 Unpolarized transmission spectra showing thermal tuning of a cell composed of a left-handed SmA*-CLC mixture and STPN that was templated in the presence of a right-handed CLC with a reflective notch at 670 nm. 28
Figure 3.9 Formation of the 3D refilling nanostructured polymer template sample. (a) Schematic diagram of the procedure. (b) Transmission polarizing optical microscopy images (scale bar, 100 m) (c) Photographs of cell (scale bar, 5 mm). 30
Figure 3.10 Thermal stability of the templated BP. Polarizing optical microscope images indicate the range of thermal stability (scale bar, 100 m). (a) Images with equal illumination indicate gradual loss of birefringence at low and high temperatures. (b) Intensity-enhanced images clarify that the structure persists over a range of at least 250 °C ( –125 – 125 °C). 30
Figure 4.1 Chemical structure of E7. 32
Figure 4.2 Chemical structure of 5CB. 32
Figure 4.3 Chemical structure of chiral dopant S1011. 33
Figure 4.4 Chemical structure of chiral dopant R1011. 33
Figure 4.5 Chemical structure of chiral dopant S811. 33
Figure 4.6 Chemical structure of chiral dopant R811. 34
Figure 4.7 Chemical structure of RMM257. 34
Figure 4.8 Chemical structure of Irg184. 35
Figure 4.9 Schematic representation for the fabrication process of CLC template sample. 35
Figure 4.10 Schematic presentation of empty cell 1. 37
Figure 4.11 Schematic presentation of empty cell 2. 37
Figure 4.12 Two CLC-monomer mixtures which can reflect light at red and blue regions are injected from the right and left edges of empty cell 2. The two mixtures then diffuse slowly in reversed directions so as to create a cell which has a rainbow-like reflection. 40
Figure 4.13 A large pitch gradient in the sample shown in Fig. 4.12 generates along the diffusion direction of the mixtures. 40
Figure 4.14 Formed gradient-pitched CLC polymer cell is completely cured under UV irradiation and then immersed in acetone for 16–24 hours. 40
Figure 4.15 Left: Schematic L-CLC and R-CLC polymer templates both with a large gradient after washing out the nonreactive residues from the completely cured gradient-pitched L-CLC- and R-CLC-polymer cells, respectively. The two templates are then combined to form a merged template, in which the regions reflecting red or blue light consistently overlapped. Right: A NLC is injected into and refilled in the entire merged template sample to form a merged refilling template sample. 41
Figure 4.16 After washing-out sample with two unsealed gaps. When the substrates of the sample (a) are not and (b) are separated, in which the template adheres partly on each of the two substrates. 42
Figure 4.17 After washing-out the sample with one unsealed and one pre-sealed gaps. When the substrates of the sample (a) are not and (b) are separated, in which the template adheres completely on one of the substrates. 42
Figure 4.18 Experimental setup for measuring the reflection spectra of the sample at every steps of fabrication. 43
Figure 5.1 Reflection spectra of the CLC-monomer (mixture III) sample before and after UV-curing. 46
Figure 5.2 Definition of the front-side and rear-side of the samples after curing or after refilling. 48
Figure 5.3 (a) Reflection spectra of the sample before UV curing (black curve) and after UV curing when the sample is measured from front-side (blue curve) and rear-side (red curve). (b) Reflection spectra measured from front-side (blue curve) and rear-side (red curve) of the sample and corresponding transmission spectrum of the sample (black curve) after washing and refilling. 48
Figure 5.5 (a) Reflection spectra of sample I (long pitch) measured at before curing stage (black curve) and from front-side and rear-side at after curing stage (blue and red curves, respectively). (b) Reflection spectra of sample I measured from front-side and rear-side (blue and red curves, respectively) and corresponding transmission spectrum (black curve) at after-refilling stage. 50
Figure 5.6 (a) Reflection spectra of sample II (short pitch) measured at before-curing stage (black curve) and from front-side and rear-side at after-curing stage (blue and red curves, respectively). (b) Reflection spectra of sample II measured from front-side and rear-side (blue and red curves, respectively) and corresponding transmission spectrum (black curve) at after-refilling stage. 51
Figure 5.7 Formed gradient-pitched L-CLC sample. The cell positions reflecting a rainbow-like color from blue (461 nm) to red (812 nm) in order are labeled as x = 0 mm to x = 10 mm. 52
Figure 5.8 Schematic structure of the spatially tunable CLC template sample which is formed 52
Figure 5.9 PBG spatial distribution of the L-CLC sample measured at positions x = 0 mm to x = 10 mm at before-curing stage. 53
Figure 5.10 PBG spatial distribution of the L-CLC sample measured at positions x = 0 mm to x = 10 mm at after-curing stage. 53
Figure 5.11 PBG spatial distribution of the L-CLC sample measured at positions x = 0 mm to x = 10 mm at after-refilling stage, where c = 433 nm measured at x = 0 mm and c = 761 nm measured at x = 10 mm. 54
Figure 5.12 (a) Reflection spectra of sample III (long pitch) measured at before-curing stage (black curve) and from front-side and rear-side at after-curing stage (blue and red curves, respectively). (b) Reflection spectra of sample III measured from front-side and rear-side (blue and red curves, respectively) and corresponding transmission spectrum (black curve) at after-refilling stage. 55
Figure 5.13 (a) Reflection spectra of sample IV (short pitch) measured at before-curing stage (black curve) and from front-side and rear-side at after-curing stage (blue and red curves, respectively). (b) Reflection spectra of sample IV measured from front-side and rear-side (blue and red curves, respectively) and corresponding transmission spectrum (black curve) at after-refilling stage. 55
Figure 5.14 Formed gradient-pitched R-CLC sample. The cell positions reflecting a rainbow-like color from blue (435 nm) to red (826 nm) in order are labeled as x = 0 mm to x = 11.5 mm. 56
Figure 5.15 PBG spatial distribution of the R-CLC sample measured at positions x = 0 mm to x = 11.5 mm at before-curing stage. 57
Figure 5.16 PBG spatial distribution of the R-CLC sample measured at positions x = 0 mm to x = 11.5 mm at after-curing stage. 57
Figure 5.17 PBG spatial distribution of the L-CLC sample measured at positions x = 0 mm to x = 11.5 mm at after-refilling stage, where c = 437 nm measured at x = 0 mm and c = 758 nm measured at x = 10 mm. 58
Figure 5.18 Summarized spectral range for each PBG and its c measured at various positions from the front-side of the after-refilling L-CLC template sample and from the rear-side of the after-refilling R-CLC template sample. The error bars measured at each position indicate the spectral range of corresponding PBG. 60
Figure 5.19 PBG spatial distribution of the gradient-pitched composite CLC template sample measured at x = 0 – 10 mm. 60
Figure 5.20 Transmission spectra of the gradient- pitched composite CLC template sample measured at x = 0 – 10 mm. 61
Figure 5.23 Images of samples I and II at after-refilling stage. 62
Figure 5.24 Top-viewed SEM images of the CLC templates of (a) sample I, (b) sample II, (c) sample III, and (d) sample IV, at after-washing stage. 63
Figure 5.25 Transmission spectra of the 5CB-refilling template sample made with sample II in the visible region at 25, 30, 35, and 40 °C. 64
Figure 5.26 Reflection spectra of 5CB-refilling template sample made with sample II measured from (a) front-side and (b) rear-side at 25, 30, 35, and 40 °C. 64
Figure 5.27 Reflection spectra of the spatially tunable and hyper-reflective refilling template sample measured at x = 0 mm to x = 10 mm at 35 °C. 65

 
List of Tables
Table 4.1 Physical parameters of E7. 32
Table 4.2 Physical parameters of 5CB. 32
Table 4.3 Recipes for four various mixtures (mixtures I, II, III, and IV) for fabricating CLC polymer template. 38
Table 5.1 (a) Four CLC-monomer samples with various prescriptions. (b) Centered wavelength of reflection band of the four sample shown in (a) before and after curing, and associated blue-shift and percentage of the spectral change. 46

1P. G. de Gennes and J. Prost, The Physics of Liquid Crystals. (Clarendon Press, New York, 1993), Second ed.
2Timothy J. White, Rebecca L. Bricker, Lalgudi V. Natarajan, Vincent P. Tondiglia, Christopher Bailey, Lisa Green, Quan Li, and Timothy J. Bunning, Electromechanical and light tunable cholesteric liquid crystals, Optics Communications 283 (18), 3434-3436 (2010).
3Shin-Ying Lu and Liang-Chy Chien, A polymer-stabilized single-layer color cholesteric liquid crystal display with anisotropic reflection, Applied Physics Letters 91 (13), 131119 (2007).
4K. H. Kim, D. H. Song, Z. G. Shen, B. W. Park, K. H. Park, J. H. Lee, and T. H. Yoon, Fast switching of long-pitch cholesteric liquid crystal device, Optics Express 19 (11), 10174-10179 (2011).
5W. Russell Callen Donald C. O'Shea, William T. Rhodes, Introduction to lasers and their applications. (Addison-Wesley, 1977).
6Victor I. Kopp, Zhao-Qing Zhang, and Azriel Z. Genack, Lasing in chiral photonic structures, Progress in Quantum Electronics 27 (6), 369-416 (2003).
7H. Kogelnik and C. V. Shank, Coupled-wave theory of distributed feedback lasers, Journal of Applied Physics 43 (5), 2327-＆ (1972).
8H. Choi, J. Kim, S. Nishimura, T. Toyooka, F. Araoka, K. Ishikawa, J. W. Wu, and H. Takezoe, Broadband Cavity-Mode Lasing from Dye-Doped Nematic Liquid Crystals Sandwiched by Broadband Cholesteric Liquid Crystal Bragg Reflectors, Advanced materials 22 (24), 2680 (2010).
9M. Mitov and N. Dessaud, Going beyond the reflectance limit of cholesteric liquid crystals, Nature materials 5 (5), 361-364 (2006).
10I. Dierking, L. L. Kosbar, A. AfzaliArdakani, A. C. Lowe, and G. A. Held, Network morphology of polymer stabilized liquid crystal, Applied Physics Letters 71 (17), 2454-2456 (1997).
11Jinbao Guo, Hui Cao, Jie Wei, Duowei Zhang, Fang Liu, Guohui Pan, Dongyu Zhao, Wanli He, and Huai Yang, Polymer stabilized liquid crystal films reflecting both right- and left-circularly polarized light, Applied Physics Letters 93 (20), 201901 (2008).
12M. E. McConney, V. P. Tondiglia, J. M. Hurtubise, L. V. Natarajan, T. J. White, and T. J. Bunning, Thermally induced, multicolored hyper-reflective cholesteric liquid crystals, Advanced materials 23 (12), 1453-1457 (2011).
13M. E. McConney, V. P. Tondiglia, J. M. Hurtubise, T. J. White, and T. J. Bunning, Photoinduced hyper-reflective cholesteric liquid crystals enabled via surface initiated photopolymerization, Chemical Communications 47 (1), 505-507 (2011).
14Sivaramakrishna Chandrasekhar, Liquid Crystals. (Cambridge University Press, 1992).
15Iam-Choon Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena. (John Wiley ＆ Sons, 1994).
16P.G. De Gennes, Calcul de la distorsion d'une structure cholesterique par un champ magnetique, Solid State Communications 6 (3), 163 (1986).
17R. B. Meyer, Effects of electric and magentic fields on structrue of cholesteric liquid crystals, Applied Physics Letters 12 (9), 281-＆ (1968).
18Paul S. Drzaic, Liquid Crystal Dispersions. (World Scientific, Singapore, 1995).