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研究生： 林啟湟
Lin, Chi-Huang
論文名稱： 膽固醇手紋結構液晶薄膜及其應用之研究
Studies of Cholesteric Fingerprint Texture Films and Their Applications
指導教授： 傅永貴
Fuh, Y.G. 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： 2002
畢業學年度： 90
語文別： 英文
論文頁數： 101
中文關鍵詞： 手紋結構 、光向控制 、膽固醇相液晶
外文關鍵詞： cholesteric liquid crystals, beam steering, fingerprint texture
相關次數： 點閱：132  下載：3 
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    • 膽固醇手紋結構可經由外加一小電場(~0.3 V/mm)於膽固醇平面結構上來加以產生。在適當的d/P(厚度/螺距)比值之下，我們可以獲得均勻排列的手紋結構，因而形成所謂的膽固醇光柵。膽固醇光柵會根據其d/P比值而有兩種不同的形成過程。當1/2 £ d/P £ 1.0時，膽固醇光柵之條紋會在樣品中同時出現，且在形成過程中光柵條紋之清晰度會隨時間而增加，而我們把這一類之光柵歸類為developable-modulation 類型光柵(簡稱DM type)。當d/P³ 1.5時，膽固醇光柵之清晰的條紋會從樣品之邊緣及缺陷處慢慢產生，而在形成過程中光柵條紋會延著磨擦方向逐漸地擴展至整個樣品，因而我們把這一類之光柵歸類為growing-modulation 類型光柵(簡稱GM type)。而當樣品有著1£ d/P £ 1.5時則會生成不規則排列之手紋結構。
    我們對膽固醇光柵光向控制(beam steering)特性加以研究。在不同d/P比值的情況下，實驗的結果顯示了僅有GM type光柵其繞射光可經由外加電場及光場加以操控。換句話說，GM type膽固醇光柵之週期可以經由外加電場或光場來控制，並進而改變繞射的位置及強度。因此GM type可以應用在光向控制上，而且光向控制的過程是可逆的。
    經由簡單之加熱退火及光罩曝光之過程，可以製作出膽固醇手紋結構反射光柵(簡稱PSRFCT grating)。此類光柵之樣品是由膽固醇相液晶混合高分子前驅物(prepolymer)所製成，而膽固醇平面結構可以經由外加電壓加以改變，因而可以製作出一電壓可切換之反射式光柵。此種反射式光柵有著多重穩定(multi-stability)之效果，而且其記憶性可保持數月之久。此外反射式膽固醇手紋結構光柵亦有著高切換對比及反應快之特性。
    GM type膽固醇光柵作為光向控制及PSRFCT膽固醇手紋結構反射式光柵具有一些獨特之光電特性，甚具應用潛力。

    The fingerprint texture is obtained by applying a small electric field (~0.3 V/mm) to a cholesteric planar texture film. The applied field is parallel to the helical axis of a planar texture film. Under suitable d/P ratios, the obtained fingerprint textures are uniform, and become cholesteric gratings. The cholesteric gratings are formed in two different ways, depending on the sample’s d/P ratio. For samples with 1/2 £ d/P £ 1.0, the grating stripes simultaneously appear across the whole sample, and the contrast of the stripes increase with time during formation. We refer these as developable-modulation type (DM type). For films with d/P ³ 1.5, the clear stripes are initiated near the edges, and the defects on the substrates, and then slowly extended to the whole sample along the rubbing direction. These are referred as the growing-modulation type (GM type). For samples with 1£ d/P £ 1.5, the fingerprint texture is not uniform.
    The beam-steering applications of cholesteric gratings were studied. Films with a planar cholesteric texture and various thicknesses to pitch length ratios (d/P) were fabricated. The diffraction measurements showed that the diffracted beams could be steered either electrically or optically only for the GM type gratings. In other words, the pitch length of the GM type cholesteric gratings can be controlled electrically and optically. Thus the intensity and the angle of the diffracted beams of a GM type cholesteric grating can be changed. It is also noted that the change is reversible.
    We also studied a polymer-stabilized reflective fingerprint cholesteric texture (PSFCT) grating, fabricated using a simple process that consists of thermal annealing followed by UV irradiation through a grating photo-mask. The sample was made from a mixture of cholesteric liquid crystals and pre-polymer materials. A PSFCT film has a planar texture, which changes under an applied external field, thus forming the electrically switchable reflective grating. The multi-stability of the PSRFCT grating can be achieved by applying various voltage pulses; the memory effect of the grating’s gray-scale persists for several months. The high switching contrast ratio and fast response of the grating are also discussed.
    Due to some unique electro-optic characteristics, both the GM type grating and PSFCT reflective grating are potential for practical applications.

    Table of contents Page List of Figures VIII-XIV Chapter 1. Introduction 1 2. Properties of Cholesteric Liquid Crystals 16 2.1 Introduction to cholesteric liquid crystal materials 16 2.2 Optical properties of cholesteric liquid crystals 17 2.2.1 Bragg reflections 17 2.2.2 Transmission properties at arbitrary wavelength (normal incidence) 20 2.2.2.1 The Mauguin limit 20 2.2.2.2 Rotatory power 23 2.3 Effect of external electric field to the cholesteric liquid crystals 25 2.4 The applications of the cholesteric liquid crystals 31 2.4.1 Polymer-Stabilized Cholesteric texture 31 2.4.1.1 Normal mode 31 2.4.1.2 Reverse mode 33 2.4.1.3 Bistable color reflective mode 33 2.4.2 Broadband circular polarizers 36 3. Dynamic Pattern Formation and Beam-Steering Characteristics of Cholesteric Gratings 40 3.1 Dynamic pattern formation of cholesteric cholesteric gratings 40 3.2 Optically beam-steering characteristics of cholesteric grating 41 3.2.1 Introduction 44 3.2.2 Experiments 45 3.2.3 Result and Discussion 48 3.3 Electrically beam-steering characteristics of cholesteric gratings 57 3.3.1 Experiments 59 3.3.2 Result and Discussion 60 4. Polymer-Stabilized Reflective Fingerprint Cholesteric Texture Grating 77 4.1 Introduction 77 4.2 Experiments 78 4.3 Result and Discussion 81 5 Conclusion 94 References 99 List of Figures Figures Page 1-1: Molecular arrangement of a typical nematic liquid crystal. 2 1-2: Order vs. temperature relationship for a typical nematic liquid crystal. 4 1-3: Molecular arrangement of a typical cholesteric liquid crystal. 5 1-4: The Elastic constant of the liquid crystals. 7 1-5: The dielectric anisotropy of the liquid crystals, (a) ε∥, if n ∥E, (b) ε⊥, if n ⊥E. 8 1- 6: The electric field effect for the nematic liquid crystals with, (a) Δε> 0 and (b) Δε< 0. 10 1-7: (a) Rotation angle of director versus field in Freederickz transition. (b) The typical geometry for observing the Freederickz transition. Initially, the director n is parallel to the L axis but perpendicular to E field. When the field exceeds a certain threshold value EF, the deformation occurs. 11 1- 8: The guest-host effect in a dye-doped nematic liquid crystal. 13 1-9: The index of refraction ellipsoid of a liquid crystal. 14 1-10: The relation between neff and θ. 15 2-1: The molecular structure of CB15. 18 2-2: The common textures of the cholesteric liquid crystal films. 19 2-3: Bragg reflection from a cholesteric planar texture film. 21 2-4: (a) Polarization selectivity of a planar texture. 22 (b) Reflection spectrum of a planar texture. 22 2-5: Definition of local axes in a twisted nematic cell. 24 2-6: Effect of an external electric field applied to a cholesteric LC film for case (a) Δε<0 and (b) Δε>0. (c) Field induced pitch change in cholesteric liquid crystals. 27 2-7: Electric field effect to the planar texture. 28 2-8: Field induced phase transition of the cholesteric liquid crystals for large d/P ratios (usually ³ 3). 30 2-9: (a) Illustration of relaxation from homeotrpic texture when the bias field is zero. (b) Illustration of relaxation from homeotrpic texture when the bias field is relatively high. 32 2-10: Schematic illustration of the PSCT normal mode (a) scattering state (E=0) (b) clear state (E¹0). 34 2-11: Schematic illustration of the PSCT reverse mode (a) clear state (E=0) (b) scattering state (E¹0). 35 2-12: Schematic illustration of the PSCT color reflective bistable mode (a) bright state (E=0) (b) dark state (E¹0). 37 2-13: The illustration of a broadband circular polarizer converting the unpolarized light into linear polarized light in backlight systems. 39 3-1: Micrographs of the dynamic change of the patterns of the cholesteric gratings observed using a crossed-polarized microscope with d/p ratios of cholesteric planar textures films, (a) d/P= 1/2, (b) d/P= 8/8. The voltage applied to the sample is labeled in the figures. 42 3-1: (c) Micrographs of the dynamic change of the patterns of the cholesteric gratings observed using a crossed- polarized microscope for d/P= 8/5. The voltage applied to the sample is labeled in the figures. 43 3-2: The schematic diagram of the experimental setup for studying optically beam-steering in cholesteric gratings doped with a dichroic dye. 47 3-3: Dynamic change of the diffracted-beam patterns of the probe beam from the sample after it is pumped by an Ar+ laser with a power of 300 mW. 49 3-4: The final diffracted patterns with the sample being pumped by an Ar+ laser having power as shown in the figures. 50 3-5: The final diffraction efficiencies of the zeroth-, the first- and the second-order diffracted beams after the sample is pumped by an Ar+ laser having various powers. 51 3-6: The final diffraction angle of one of the second-order beams after the sample is pumped by an Ar+ laser having various powers. 52 3-7: Dynamic changes of the second-order diffraction angle after the pump beam (Ar+ laser) was removed. 53 3-8: Diffraction characteristic of the fingerprint texture. 56 3-9: The stripe domains of the sample observed under an optical microscope while it was pumped by an Ar+ laser having power of (a) 0 mW, (b) 50 mW, (c) 100 mW, (d) 150 mW, (e) 200 mW, (f) 250 mW, (g) 300 mW. 58 3-10: Final diffraction angle of the second-order diffracted beam as a function of the voltage applied to the cholesteric planar texture films for various pitch lengths (cell gap = 8 mm). 61 3-11: Stripe Direction of DM type (Developable Modulation type); (a) stripe direction ∥ rubbing direction for d/P~1/2, (b) stripe direction⊥ rubbing direction for d/P~1. 63 3-12: Final diffraction angle of the second-order diffracted beam after the sample is pumped by an Ar+ laser, for d/P ratios, (a) 8/16, (b) 8/8. 66 3-12: Final diffraction angle of the second-order diffracted beam after the sample is pumped by an Ar+ laser, for d/P ratios, (c) 8/5, (d)8/4. 67 3-13: The shapes (the left-hand side figures) and the dependence of the grating efficiencies (second order) on the polarization of the incident light (the right-hand side ones) of diffracted beams of the cholesteric gratings for various d/P ratios, (a) 8/16, (b) 8/14, (c) 8/13. The voltages applied to the samples are 1.5 V, 1.5V, 1.5 V for (a) ~ (c), respectively. 69 3-13: The shapes (the left-hand side figures) and the dependence of the grating efficiencies (second order) on the polarization of the incident light (the right-hand side ones) of diffracted beams of the cholesteric gratings for various d/P ratios, (d) 8/8, (e) 8/5. The voltages applied to the samples are 2 V, 2.6 V for (d), (e), respectively. 70 3-14: Schematic illustrations of the director arrangement and the light propagation in DM type gratings for d/P~1/2. The polarization of the incident light is (a) perpendicular, (b) parallel to the director axis of LCs adjacent to the substrate. 72 3-14: Schematic illustrations of the director arrangement and the light propagation in DM type gratings for d/P~1. The polarization of the incident light is (c) perpendicular, (d) parallel to the director axis of LCs adjacent to the substrate. 73 3-15: The stripe patterns of the cholesteric gratings observed using a crossed- polarized microscope for various d/P ratios, (a) 8/16, (a) 8/14, (c) 8/13, (d) 8/8, (e) 8/5. Dashed lines enclosed in (b) are the deformed stripes. The voltages applied to the sample are 1.5 V, 1.5V, 1.5 V, 2 V, 2.6 V for (a) ~ (e), respectively. 75 4-1: (a) Setup for fabricating a PSRFCT grating; (b) setup for measuring the electrooptical responses of the grating. 80 4-2: Mechanism of an electrically switchable PSRFCT grating.(a) on-state: the LC texture in the exposed regions is locked in the fingerprint texture while the unexposed regions have the planar texture.(b) off-state: applying a low voltage pulse to switch the texture in the unexposed regions into fingerprint state. 83 4-3: Microscopic textures of the PSRFCT grating prepared following a one-step process. (a) and (b) are the textures of the sample in the on and off states , respectively, observed under the transmission and reflection microscopes. (c) Diffraction pattern of the PSRFCT grating. 84 4-4: Microscopic texture of the PSRFCT grating prepared following a TSE process. (a) and (b) are textures of the sample in the on and off states observed under the transmission and reflection microscope, respectively. (c) On and off diffraction patterns of the PSRFCT grating. 86 4-5: (a) Measured diffraction intensity versus applied voltage. Each data point is the diffraction intensity from a state relaxed for 5 s after the release of the corresponding applied voltage. (b) Memory of the gray-scale in intensity of the PSRFCT grating. A and B represent the gray-scale points marked in (a). 88 4-6: Response time of a PSRFCT grating, prepared following a TSE process. (a) and (b) are the rise and fall times, respectively. (c) Response times with application of a sequential DC voltage pulse for a PSRFCT grating. 91 4-7: First-order diffraction efficiency versus the pitch length for PSRFCT gratings. The probe beam is a He-Ne laser. 93

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