水晶体のようにレンズを動かすことなく，その焦点距離の制御を行うため、本論文では複屈折性をもつ液晶材料を利用した小型かつ薄型で耐環境性の高い可変焦点レンズ（液晶レンズ）の開発を目指している．特に、従来のように液晶の分子配向制御に電界を用いず、超音波振動により分子配向を静的に制御する手法とそのメカニズムの解明を目的とする。 / Variable-focus ultrasound liqui dcrystal lenses combining ultrasonic waves and liquid crystal materials were developed and investigated, focusing on the multifunctional properties that were not possible with conventional optical lenses. In particular, control of the effective lens aperture, control of the focal point in the radial direction for image stabilizationt, and expansion of the focal length variation range, which had been considered a problem from a practical standpoint, were investigated. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

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Liquid crystal (LC)-based adaptive optics are important for information processing, optical interconnections, photonics, integrated optics, and optical communications due to their tunable optical properties. In this dissertation, we describe novel liquid crystal photonic devices and their fabrication methods. The devices presented include inhomogeneous polymer-dispersed liquid crystal (PDLC), polymer network liquid crystals (PNLC) and phase-separated composite film (PSCOF). Liquid crystal/polymer composites could exist in different forms depending on the fabrication conditions. In Chap. 3, we demonstrate a novel nanoscale PDLC device that has inhomogeneous droplet size distribution. In such a PDLC, the inhomogeneous droplet size distribution is obtained by exposing the LC/monomer with a non-uniform ultraviolet (UV) light. An electrically tunable-efficiency Fresnel lens is devised for the first time using nanoscale PDLC. The tunable Fresnel lens is very desirable to eliminate the need of external spatial light modulator. Different gradient profiles are obtained by using different photomasks. The nanoscale LC droplets are randomly distributed within the polymer matrix, so that the devices are polarization independent and exhibit a fast response time. Because of the small droplet sizes, the operating voltage is higher than 100 Vrms. To lower the driving voltage, in Chap. 2 and Chap. 3, we have investigated a polymer-network liquid crystal (PNLC) using a rod-like monomer structure. Since the monomer concentration is only about 5%, the operating voltage is below 10 Vrms. The PNLC devices are polarization dependent. To overcome this shortcoming, stacking two cells with orthogonal alignment directions is a possibility. In Chap. 3, another approach to lower the operating voltage is to use phase-separated composite film (PSCOF) where the LC and polymer are separated completely to form two layers. Without multi-domain formed in the LC cell, PSCOF is free from light scattering. Using PNLC and PSCOF, we also demonstrated LC blazed grating and Fresnel lens. The diffraction efficiency of these devices is continuously controlled by the electric field. Besides Fresnel lens, another critical need for imaging and display is to develop a system with continuously tunable focal length. A conventional zooming system controls the lens distance by mechanical motion along the optical axis. This mechanical zooming system is bulky and power hungry. To overcome the bulkiness, in Chap. 4 we developed an electrically tunable-focus flat LC spherical lens which consists of a spherical electrode imbedded in the top flat substrates while a planar electrode on the bottom substrate. The electric field from the spherical and planar electrodes induces a centrosymmetric gradient refractive index distribution within the LC layer which, in turn, causes the focusing effect. The focal length is tunable by the applied voltage. A tunable range from 0.6 m to infinity is achieved. Microlens array is an attractive device for optical communications and projection displays. In Chap. 5, we describe a LC microlens array whose focal length can be switched from positive to negative or vise versa by the applied voltage. The top spherical electrode glass substrate is flattened with a polymer layer. The top convex substrate and LC layer work together like a zoom lens. By tuning the refractive index profile of the LC layer, the focal length of the microlens array can be switched from positive to negative or vise versa. The tunable LC microlens array would be a great replacement of a conventional microlens array which can be moved by mechanical elements. The fast response time feature of our LC microlens array will be very helpful in developing 3-D animated images. A special feature for LC/polymer composites is light scattering. The concept is analogous to the light scattering of clouds which consist of water droplets. In Chap. 6, we demonstrate polymer network liquid crystals for switchable polarizers and optical shutters. The PNLC can present anisotropic or isotropic light scattering behavior depending on the fabrication methods. The use of dual-frequency liquid crystal and special driving scheme leads to a sub-millisecond response time. The applications for display, light shutters, and switchable windows are emphasized. Although polymer networks help to reduce liquid crystal response time, they tend to scatter light. In Chap. 7, for the first time, we demonstrate a fast-response and scattering-free homogeneously-aligned PNLC light modulator. Light scattering in the near-infrared region is suppressed by optimizing the polymer concentration such that the network domain sizes are smaller than the wavelength. As a result, the PNLC response time is ~300X faster than that of a pure LC mixture except that the threshold voltage is increased by ~25X. The PNLC cell also holds promise for mid and long infrared applications where response time is a critical issue.

In this dissertation, the response time issue of the liquid crystal (LC) devices is investigated in meeting the challenges for display and photonic applications. The correlation between the LC director response time and the optical response time is derived theoretically and confirmed experimentally. A major contribution of this thesis is that, based on the small angle approximation, we derive rigorous analytical solutions for correlating the LC director response time to its consequent optical response times (both rise and decay) of a vertical-aligned nematic LC cell. This work successfully fills the gap in the literature of LCD switching dynamics. An important effect related to response time, backflow is analyzed using a homogeneous LC cell in an infrared wavelength. The Leslie viscosity coefficients can hardly be found in the literature. A new effective approach to estimate the Leslie coefficients of LC mixtures based on MBBA data is proposed in this dissertation. Using this method, the Leslie coefficients of the LC material under study can be extracted based on its order parameters. The simulation results agree with the experimental data very well. This method provides a useful tool for analyzing the dynamic response including backflow. Cell gap is an important factor affecting the LC response time. Usually a thinner cell gap is chosen to achieve faster response time, since normally both rise and decay times are known to be proportional to d2. However, they are valid only in the region. In the large voltage region where , the optical decay time is independent of d. In this thesis, we find that between these two extremes the response time is basically linearly proportional to d. Our analytical derivation is validated by experimental results. Therefore, in the whole voltage region, the physical picture of the optical response time as a function of the cell gap is completed. This analysis is useful for understanding the grayscale switching behaviors of the LC phase modulators. In conclusion, this dissertation has solved some important issues related to LC optical response time and supplied valuable tools for scientists and engineers to numerically analyze the LC dynamics.

Liquid crystal (LC) devices can be operated as amplitude modulators and phase modulators. LC amplitude modulation is commonly used in liquid crystal display (LCD) while phase-only modulation is useful for laser beam steering, tunable grating, prism, lens, and other photonic devices. Most LC devices are polarization dependent and require at least one polarizer. As a result, the optical efficiency is low. To enhance display brightness, a power hungry backlight has to be used leading to a high power consumption and short battery life. In a LC phase modulator, the polarization dependent property complicates the laser beam steering system. It is highly desirable to develop new operating mechanisms that are independent of the incident light polarization. In this dissertation, we have developed eight polarization-independent liquid crystal operation principles: three of them are aimed for displays and the other five are for phase modulators. For amplitude modulations, a new polymer-dispersed liquid crystal (PDLC) and two new dye-doped LC gels are polarizer-free by combining light scattering with dye-absorption effects. In phase modulation, we explore five device concepts: PDLC and Polymer-Stabilized Cholesteric Texture (PSCT), homeotropic LC gels, thin polymer film separated double-layered structure, and double-layered LC gels. In the low voltage regime, both PDLC and PSCT have a strong light scattering. However, as the voltage exceeds a certain level, the phase modulation is scattering-free and is independent of polarization. The homeotropic LC gels do not require any biased voltage and the response time is still fast. Although the remaining phase in these devices is small, they are still useful for micro-photonic device applications. To increase the phase change, thin polymer film separated double-layered structure is a solution. The orthogonal arrangement of top and bottom LC directors results in polarization independence. However, the response time is slow. Similarly, double-layered LC gels are not only polarization independent but also fast response due to the established polymer network.

Liquid crystals have been a major subject of research for the past decades. Aside from the variety of structures they can form, they exhibit a vast range of optical phenomena. Many of these phenomena found applications in technology and became an essential part of it. In this dissertation thesis we continue the line to propose a number of new applications of optical effects in liquid crystals and develop their theoretical framework. One such application is the possibility of beam combining using Orientational Stimulated Scattering in a nematic liquid crystal cell. Our numerical study of the OSS process shows that normally this possibility does not exist. However, we found that if a number of special conditions is satisfied efficient beam combining with OSS can be done. These conditions require a combination of special geometric arrangement of incident beams, their profiles, nematic material, and more. When these conditions are fulfilled, power of the beamlets can be coherently combined into a single beam, with high conversion efficiency while the shape and wave-front of the output beam are still of good quality. We also studied the dynamics of the OSS process itself and observed (in a numerical model) a number of notorious instabilities caused by effects of back-conversion iv process. Additionally, there was found a numerical solitary-wave solution associated with this back-conversion process. As a liquid crystal display application, we consider a nematic liquid crystal layer with the anisotropy axis modulated at a fixed rate in the transverse direction with respect to light propagation direction. If the layer locally constitutes a half-wave plate, then the thinscreen approximation predicts 100% -efficient diffraction of normal incident wave. If this diffracted light is blocked by an aperture only transmitting the zero-th order, the cell is in dark state. If now the periodic structure is washed out by applying voltage across the cell and light passes through the cell undiffracted, the light will pass through the aperture as well and the cell will be in its bright state. Such properties of this periodically aligned nematic layer suggest it as a candidate element in projection display cells. We studied the possibility to implement such layer through anchoring at both surfaces of the cell. It was found that each cell has a thickness threshold for which the periodic structure can exist. The anchored periodic structure cannot exist if thickness of the cell exceeds this threshold. For the case when the periodic structure exists, we found the structure distortion in comparison with the preferable ideal sinusoidal profile. To complete description of the electromechanical properties of the periodic cell, we studied its behavior at Freedericksz transition. Optical performance was successfully described with the coupled-mode theory. While influence of director distortion is shown to be negligibly small, the walk-off effects appear to be larger. In summary, there are good prospects for use of this periodically v aligned cell as a pixel in projection displays but experimental study and optimization need to be performed. In the next part we discuss another modulated liquid crystal structure in which the director periodically swings in the direction of light propagation. The main characteristic of such structure is the presence of bandgap. Cholesteric liquid crystals are known to possess bandgap for one of two circular polarizations of light. However, unlike the cholesterics the bandgap of the proposed structure is independent of polarization of normally incident light. This means that no preparation of light is needed in order for the structure to work in, for example, liquid crystal displays. The polarization universality comes at the cost of bandgap size, whose maximum possible value ∆ωPTN compared to that of cholesterics ∆ωCh is approximately twice smaller: ∆ωPTN ≈ 0.58∆ωCh if modulation profile is sinusoidal, and ∆ωPTN ≈ 0.64∆ωCh if it is rectangular. This structure has not yet been experimentally demonstrated, and we discuss possible ways to make it.

Liquid crystal (LC) materials have been widely applied in electro-optical devices, among which display is the most successful playground and numerous new applications in photonic areas (such as laser beam steering devices) are also emerging. To well guide the device design for optimum performance, accurate modeling is of prior and practical importance. Generally, the modeling of LC devices includes two parts in sequence: accurate LC molecule deformation extraction under external electric fields and optical calculation thereafter for the corresponding electro-optical behaviors. In this dissertation, first, hybrid finite element method and finite difference method are developed to minimize the free energy of the LC systems. In this part of study, with computer-aided derivation, the full forms of the LC free energy equations without any simplification can be obtained. Besides, Galerkin's method and weak form technique are further introduced to successfully degrade the high order nonlinear derivative terms associated with the free energy equations into ones that can be treated by first order interpolation functions for high accuracy. The developed modeling methods for LC deformation are further employed to study display structures, such as 2D and 3D in-plane switching LC cells, and provides accurate results. Followed is the optical modeling using extended Jones matrix and beam propagation method to calculate the electro-optical performances of different devices, according to their amplitude modulation property or diffractive one. The developed methods are further taken to assist the understanding, development, and optimization of the display and photonic devices. For their application in the display area, sunlight readable transflective LCDs for mobile devices and the related optical films for wide viewing angle are developed and studied. New cell structure using vertically aligned liquid crystal mode is developed and studied to obtain a single cell gap, high light efficiency transflective LCD that can be driven by one gray scale control circuit for both transmissive and reflective modes. And employing an internal wire grid polarizer into a fringe field switching cell produces a single cell gap and wide viewing angle display with workable reflective mode under merely two linear polarizers. To solve the limited viewing angle of conventional circular polarizers, Poincare sphere as an effective tool is taken to trace and understand the polarization change of the incident light throughout the whole LC system. This study further guides the design of high performance circular polarizers that can consist of purely uniaxial plates or a combination of uniaxial and biaxial plates. The developed circular polarizers greatly enhance the viewing angle of transflective LCDs. Especially, the circular polarizer design using a biaxial film can even provide comparable wide viewing angle performance for the same vertically aligned cell as it is used between merely two linear polarizers, while using circular polarizers can greatly boost the display brightness. As for the beam steering device modeling, the developed LC deformation method is taken to accurately calculate the associated LC director distribution in the spatial light modulator, while beam propagation method and Fourier transformation technique are combined to calculate the near and far fields from such devices. The modeling helps to better understand the origins and formations of the disclinations associated with the fringe fields, which further result in reduced steering efficiency and output asymmetric polarizations between positive and negative diffractions. Optimization in both voltage profile and driving methods is conducted to well tune the LC deformation under strong fringe fields and improve the light efficiency.

This dissertation covers some important topics on the liquid crystal-substrate surface effects, including theoretical derivations and confirming experimental results. The research work is expected to make important impacts on liquid crystal device designs and to open new doors for further research along these topics. In this dissertation, a novel high-electric-field technique is developed to characterize the anchoring energy of vertically-aligned liquid crystal cells. Both theoretical analyses and confirming experimental results are presented. Vertically-aligned liquid crystal cells with buffed polyimide alignment layers are used to validate the measurement techniques. Based on the voltage-dependent transmittance of the liquid crystal cells, a linear fitting can be obtained, which leads to a precise determination of the anchoring energy. If some specific liquid crystal material parameters are known, then the traditional cell capacitance measurements can be avoided. Anchoring energy and cell gap effects on liquid crystal response time is theoretically analyzed and experimentally investigated. A novel theory on the liquid crystal dynamics is developed. In this part, two different theoretical approaches are discussed: one is surface dynamic equation method and the other is effective cell gap method. These two different approaches lead to consistent results, which are also confirmed by our experimental results. This work opens a new door for LCD industry to optimize liquid crystal response time, and it is especially critical for liquid crystal cells with thin cell gap, which is a promising approach for fast response time liquid crystal display. Pretilt angle effects on liquid crystal dynamics are analyzed theoretically and validated experimentally. Analytical expressions are derived to describe liquid crystal response time under nonzero pretilt angle conditions. The theoretical analysis is confirmed experimentally using vertically-aligned liquid crystal cells. These results quantitatively correlate pretilt angles with liquid crystal response time, which is important for optimizing liquid crystal response time.

Conjugated polymers are semiconducting materials that are currently being researched for numerous applications from chemical and biological sensors to electronic devices, including photovoltaics and transistors. Much of the novel research on conjugated polymers is performed in academic settings, where scientists are working to prepare conjugated polymers for commercially viable applications. By offering numerous advantages, inherent in macromolecular materials, conjugated polymers may hold the key to cheap and environmentally friendly manufacturing of future electronic devices. Mechanical flexibility, and solvent-based coating processes are two commonly cited advantages. Transitions in the backbone conformation of polythiophenes (PT) in organic solvents have been widely observed to influence thin-film morphology. However, conformational transitions of water-soluble PT derivatives, with respect to their intramolecular versus intermolecular origin, remain largely obscure. Here, conformational transitions of a water- soluble polythiophene in aqueous ionic surfactants are investigated by means of Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), polarizing optical microscopy (POM), ultraviolet-visible (UV-Vis) absorption and fluorescence spectroscopy, and various X-ray scattering techniques. As-prepared complexes exist as stable hydrogels. Upon dilution, a significant time-dependent chromism occurs spontaneously. A coil-to-rod conformational transition is identified in this mechanism and verified using small-angle x-ray scattering (SAXS). Study into the corresponding kinetics demonstrates an inverse first-order rate law. It is found that the conformational transition is thermally reversible and concentration-independent. The critical transition temperature is largely dependent on the surfactant formulation. A theoretical model is presented to explain this new phenomenon and the mechanisms behind its influence on the optoelectronic and solid-state morphological properties. A relationship between the dilute-solution processing with surfactants and the final properties of the system is substantiated.