PHD, Kent State University, 2023, College of Arts and Sciences / Materials Science Graduate Program

Waveguide liquid crystal display (WLCD) is a newly developed transparent display technology. Since polarizers and color filters are not necessary for the WLCD, high transparency is easily reached. A light-emitting diode (LED) is installed on the edge of the display and the produced light is coupled into the display. When no voltage is applied, the liquid crystal is uniformly aligned and is transparent. The incident light propagates through the display in waveguide mode due to the total internal reflection at the interface between the substrate and air, and no light comes out of the viewing side of the display. The display appears transparent. When a voltage is applied, the liquid crystal is switched to a micrometer-sized polydomain state and becomes scattering. The incident light is scattered out of the waveguide mode and comes out of the viewing side of the display. We developed a few methods to improve the performance of the waveguide display. First, by using patterned photo-polymerization or patterned ITO electrode, the scattering efficiency of the liquid crystal in the voltage-on state is significantly enhanced. Second, the spatial uniformity of the light intensity of the display is significantly improved by the light waveguide plate. Third, we achieved 8 inch full color transparent light waveguide LCD prototype that utilizes field sequential color (FSC) scheme to display full color images. Fourth, we developed a light waveguide LCD based on the flexoelectric effect using dimer, which exhibits high contrast ratio. Lastly, based on the flexoelectric effect we developed a reconfigurable liquid crystal diffraction grating whose diffraction angle and efficiency can be controlled by the applied voltage. The light waveguide liquid crystal transparent display has the merits of high contrast ratio, suitable driving voltage, and a sub-milli second ultrafast response time. It does not use polarizers and color filter as in conventional LCDs. It also has an ultrahigh tra (open full item for complete abstract)

Committee: Dengke Yang (Advisor); Philip J. Bos (Committee Member); Songping Huang (Committee Member); Sam Sprunt (Committee Member); Hiroshi Yokoyama (Committee Member) Subjects: Materials Science; Optics; Physics

PHD, Kent State University, 2016, College of Arts and Sciences / Chemical Physics

In this work, we describe dynamic behavior of free-standing bent-core liquid crystal filaments under dilative and axial compressive stresses in the B7 phase. We found that such filaments demonstrate very complex structures depending on the filament's temperature relative to the isotropic phase, initial filament thickness, and velocity at which the filament is pulled or compressed. We also present our experimental methods, results and analysis of the rupture and recoil properties of several bent-core liquid crystal filaments, anticipating that they may serve as a model system for complex biological fibers. After that, we systematically describe rheological measurements for dimeric liquid crystal compounds. We studied the shear-induced alignment properties, measured the viscoelastic properties as a function of temperature, shear rate, stress and frequency, and compared the results with the rheological properties of conventional chiral nematic and smectic phases. Then we present results of chiral nematic liquid crystals composed of flexible dimer molecules subject to large DC magnetic fields between 0 and 31T. We observe that these fields lead to selective reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-C band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation in accordance with early theoretical predictions. Finally, we report an unprecedented magnetic field-induced shifts of the isotropic-nematic phase transition temperature observed in liquid crystal dimers where two rigid linear mesogens are linked by flexible chains of either even- or odd-numbered hydrocarbon groups. This effect is explained in terms of quenching of the thermal fluctuations and decrease of the average bend angle of molecules in the odd-numbered dimers.

Committee: Antal Jakli PhD (Advisor); John West PhD (Committee Member); Samuel Sprunt PhD (Committee Member) Subjects: Chemistry; Physics

PHD, Kent State University, 2020, College of Arts and Sciences / Chemical Physics

Liquid crystals are widely used in flat panel displays and smart windows. In flat panel display application, one way to improve the efficiency is to decrease the driving frequency when static images are displayed. As the driving frequency is decreased, the transmittance of the display may vary with time, a phenomenon known as flickering. We carried out both experimental and simulation studies to investigate the origins that cause the flickering problem. Our results show that flexoelectric effect and ions in the liquid crystal are the main factors responsible for the flickering. We quantitatively analyzed the flickering caused by the two factors. The ionic effect can be eliminated by using the fluorinated liquid crystals with high resistivity. The flexoelectric effect is attributed to the intrinsic flexoelectric coefficient of the liquid crystal and nonuniformity of the liquid crystal director configurations. We demonstrated that polymer stabilization can smooth the spatial variation of the liquid crystal orientation, while doping a liquid crystal dimer can reduce the flexoelectric coefficient of the liquid crystal. Using these methods we are able to reduce the flickering significantly. Radiant energy-flow control and privacy control are two important features for smart windows (or glass). Current smart window technologies can, however, only control one of them: radiant energy flow or privacy. Therefore, a dual-mode smart window is highly desirable. We developed a dual-mode switchable liquid-crystal window that can control both radiant energy flow and privacy. The switchable liquid-crystal window makes use of dielectric and flexoelectric effects. In the absence of an applied voltage, the window is clear and transparent, and radiant energy can flow through it and the scenery behind the window can be seen. When a low-frequency (50 Hz) voltage is applied, the window is switched to an optical scattering and absorbing state by a flexoelectric effect, and thus, privacy i (open full item for complete abstract)

Committee: Dengke Yang (Advisor); Philip J. Bos (Committee Member); Robin Selinger (Committee Member); Elizabeth K. Mann (Committee Member); Xiaoyu Zheng (Committee Member); James Gleeson (Committee Member) Subjects: Materials Science; Optics; Physics

PHD, Kent State University, 2024, College of Arts and Sciences / Department of Physics

The study of Organic Electrochemical Transistors (OECTs) has been growing since its discovery in the early 1980s. OECTs have garnered considerable interest for chemical and biological sensing and bioelectronics due to their low cost, the tunability of the organic molecules, low temperature processing, the ease of scaling, high sensitivity, and their stability in an aqueous environment. Due to their low gate voltage operating, they are considered as efficient switches and powerful amplifiers. Present OECTs mainly use liquid electrolytes in OECTs, although there is a growing need for solid electrolytes since they can be easily integrated into wearable devices. Recently our group developed ionic liquid crystal elastomers (iLCEs) and demonstrated that they can be used as solid electrolytes of OECTs This will broaden the range of applications toward medical health monitoring and soft robotics. In my dissertation, I further studied iLCEs as solid electrolytes for OECTs. I describe the role of various ionic liquids in the performance of the iLCE using the same LCE. It is found that the ionic liquid that phase separates from the reactive mesogenic monomers and forms micron size ionic channels performs better than the less phase separating ionic liquid. The switching time of the order of a second was found to be possible using the ionic liquid with smaller ions and larger channels. These results provide many exciting opportunities in various areas and impact future applications, although they are far from optimized. Fine tuning of the high-performance iLCEs requires an intricate compatibility between the LCE and the electrodes and appears to be laborious. Secondly, I demonstrated that iLCE based OECTs can function as highly sensitive bending sensor. The transfer curve of the iLCE-based OECT shows opposite variation with upward and downward bending representing a directional sensitive bending sensor. The change in the drain current is 4 orders of magnitude larger tha (open full item for complete abstract)

Committee: Dr. Antal Jákli (Advisor); Dr. Björn Lüssem (Committee Member); Dr. Elizabeth Mann (Committee Member); Dr. Marianne Prévôt (Committee Member); Dr. Elda Hegmann (Committee Member) Subjects: Physics

PHD, Kent State University, 2016, College of Arts and Sciences / Chemical Physics

Photoalignment is a well-established technique for surface alignment of the liquid crystal director. Previously, chrome masks were necessary for patterned photoalignment but were difficult to use, costly, and inflexible. To extend the capabilities of photoalignment we built an automated maskless multi-domain photoalignment device based on a DMD (digital multimirror device) projection system. The device is capable of creating arbitrary photoalignment patterns with micron-sized features. Pancharatnam-Berry phase (PB-phase) is a geometric phase that arises from cyclic change of polarization state. By varying the azimuthal anchoring angle in a hybrid-aligned liquid crystal cell we can control the spatial variation of the PB-phase shift. Using our automated photoalignment device to align the liquid crystal arbitrary wave front manipulations are possible. The PB-phase shift effect is maximized when the cell is tuned to have a half-wave retardation and disappears at full-wave retardation, so the cell can be switched on and off by applying a voltage. Two wavefront controlled devices developed using this technique will be discussed: A switchable liquid crystal phase shift mask for creating sub-diffraction sized photolithographic features, and a transparent diffractive display that utilizes a switchable liquid crystal diffraction grating.

Committee: Hiroshi Yokoyama (Committee Chair); Phil Bos (Committee Member); Deng Ke Yang (Committee Member); Elizabeth Mann (Committee Member); Alexander Seed (Committee Member) Subjects: Chemical Engineering; Chemistry; Physics

PHD, Kent State University, 2007, College of Arts and Sciences / Chemical Physics

We investigate the optical properties of liquid crystal devices whose dark states are optically compensated to give minimum transmittance. Firstly, the effect of multiple internal reflections on the extinction ratio of perfectly compensated liquid crystal devices is studied. We find the previously unexplained wavelength dependence of light leakage in an ideally compensated device is caused by the interference of the internally reflected ordinary and extraordinary waves. While these effects have not been previously made clear, they can place a limit on the performance of liquid crystal devices used as displays, optical switches and optical attenuators. Secondly, we study the off-axis light transmission characteristics of the bright state of common liquid crystal device modes. Our research shows there is an unexpected universal shape of the off-axis light transmittance in its bright state, regardless of what liquid crystal mode is used. To understand this surprising fact, we consider simple dark and bright state models in terms of phase retardation and transmittance. We also investigate the luminance and color properties of bright state common LCDs such as ECB, VA, Pi-cell, and TN modes. According to the results, the universality of the optical properties of a bright state is maintained not only in the transmittance but also in the luminance and color properties. Thirdly, we have studied the off-axis light transmission properties of the bright state in Pi-cell devices as a function of the white state director configuration. Above a critical pretilt angle or white state voltage, the light transmittance is a much stronger function of the incident angle of light. To understand the facts, we develop a model that explains this result and provides a description of the basic issues affecting the optics of these types of devices. Finally, we explore the phase compensation of the dark states in LCDs, and find that there is a limitation for the compensation of a uniaxial layer (open full item for complete abstract)

Committee: Philip Bos (Advisor) Subjects: Physics, Optics

PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

In part because of its anticipated application for faster and lower power-consuming electro-optic devices, the biaxial phase of nematic liquid crystals, characterized by two optic axes, has long been sought after. We have investigated the existence of thermotropic nematic biaxiality in a relatively new system, liquid crystalline tetrapodes, through dynamic light scattering. Liquid crystalline tetrapodes are formed by the attachment of four mesogenic molecules to a single silicon or germanium atom through flexible siloxane chains. Our results, obtained in various scattering geometries and tested against available theory, strongly support the existence of a biaxial nematic phase. The temperature dependent slowing down of the biaxial order parameter fluctuations indicates that the uni – biaxial transition is weakly first order in a 4-ring tetrapode and second order in a 3-ring homolog. The temperature dependence of the relaxation rates of the biaxial order parameter mode and of the scattered intensity associated with biaxial optic axis fluctuations is explained by a Landau-deGennes model of the free energy. In particular, we have confirmed that the intensity exhibits the expected scaling with the uniaxial and biaxial order parameter magnitudes, for several distinct geometries, in the biaxial phase.Another relatively new class of nematics is the lyotropic chromonic liquid crystals (LCLCs) formed from concentrated mixtures of disc-like dye molecules in water. Because of their potential applications in dye based polarizing and compensating films and in biological sensing, and because of recent analogies drawn between LCLCs and the liquid crystalline phases of DNA solutions, LCLCs are interesting systems to study. Using dynamic light scattering on well aligned samples, we have explored particularly the temperature dependence of the elastic constants and orientational viscosities of nematic Disodium cromoglycate LCLCs. These parameters show a significant anisotropy. In part (open full item for complete abstract)

Committee: Samuel Sprunt PhD (Advisor) Subjects: Physics

PHD, Kent State University, 2005, College of Arts and Sciences / Department of Physics

Dynamical properties of the nematic and isotropic phase of several relatively new bent-core liquid crystals, as contrasted to conventional straight-core (“calamitic”) liquid crystals, have been systematically studied by dynamic light scattering. Objective: Comparison of properties of bent-core nematics with straight-core nematics and search for a biaxial nematic phase due to the bent-core shape Observations: Nematic phases are rather uncommon in bent-core compounds because nematic structure occurs only if the molecules can rotate relatively freely around their long axis, the condition apparently met in only a fraction of bent-core materials synthesized so far. The obtained results show that the elasticity to viscosity ratio for uniaxial bent-core nematics is typically two orders of magnitude lower than for straight-core nematics, due evidently to the large viscosity associated with optic axis distortions in the bent-core case. This result is independent of the normal mode of the director fluctuations probed. In one compound polarized light scattering data reveal fluctuations associated with the biaxial order parameter, occurring as a pretransitional effect on approach to the biaxial smectic-C phase. However, results on a homeotropically-aligned sample of another compound provide preliminary light scattering evidence for nematic biaxiality. Objective: To determine nature of isotropic to nematic phase transition and to measure dielectric fluctuations in the isotropic phase near the transition. Observations: The orientational order parameter fluctuations in the isotropic phase have been studied for the first time in bent-core compounds. Analogous to classic light scattering experiments performed on calamitic liquid crystals, fluctuations in nematic order in bent-core compounds exhibit a mean-field-like critical slowing down on approach to the isotropic-nematic transition from above. The fluctuations are intrinsically several orders of magnitude slower than for typical (open full item for complete abstract)

Committee: Samuel Sprunt (Advisor) Subjects:

PHD, Kent State University, 2024, College of Arts and Sciences / Materials Science Graduate Program

Stimuli-responsive functional soft materials have been the focus of attention and been widely applied in advanced devices. Chiral nematic liquid crystals (also called cholesteric liquid crystals, CLCs), which possess intrinsic self-organized helical superstructures, are good candidates to create diffraction gratings (DGs) for optical devices, due to the characteristics of adjustable pitch under external stimuli like light, temperature, electric field, and so forth. Here, we develop two novel photoresponsive CLCs, which are enabled by adding two novel axially chiral molecular switches into the nematic LC host, respectively. Those chiral molecular switches exhibit superior compatibility, high helical twisting power (HTP), and a big HTP change under photoisomerization. Accordingly, electro- and photo-driven orthogonal switching of CLC diffraction gratings, and visible light, temperature, and electric field-driven in-plane rotation of CLC diffraction gratings are demonstrated, which exhibit great potential application in beam steering, spectrum scanning, and beyond. Like CLCs, twist-bend nematic liquid crystals (NTB LCs) also possess an intrinsic heliconical structure although the constituent molecules are achiral, but the molecular director is tilted with a cone angle around the conic helical axis and the heliconical pitch is very small. We study the structure and optical properties of NTB LCs when the chiral dopant is introduced. We show that adding chiral dopant does not induce a twisting of the heliconical axis, but increases the cone angle. Then, based on this fundamental study, we develop a novel thermally switchable smart window enabled by phase transition from NTB phase to chiral nematic phase. Such smart window is energy-saving and exhibits great potential in applications for buildings, vehicles, and beyond. Moreover, we develop a novel switching mechanism between planar and focal conic states in bistable reflective display. The CLC is switched from the foca (open full item for complete abstract)

Committee: Deng-Ke Yang (Advisor); Quan Li (Advisor); Barry Dunietz (Committee Member); Xiaoyu Zheng (Committee Member); Philip J. Bos (Committee Member) Subjects: Chemistry; Materials Science; Physical Chemistry; Physics

PHD, Kent State University, 2023, College of Arts and Sciences / Materials Science Graduate Program

In imaging system, de-focus and astigmatism are the most common optical aberrations, and finding non-mechanical approach of correcting these aberrations is of great interest. Although non-mechanical correction of de-focus has been widely studied, astigmatism correction remains relatively unexplored. Motivated by this gap, this Ph.D. thesis focuses on the development of a new type of gradient refractive index (GRIN) liquid crystal (LC) lenses capable of non-mechanical correction of both astigmatism and de-focus. The proposed device consists of a stack of three electrically tunable cylindrical lenses that implement a concentric stripe electrode and segmented phase profile design. This system offers several advantages, including a simple, low-cost structure, a large aperture size (50 mm), low voltage drive, and a compact design. Compared to conventional mechanical approaches, this non-mechanical solution has significant potential for various applications such as wavefront correction in large telescopes, microscopy, augmented reality/virtual reality, and prescription eyeglasses. In the second part of the thesis, challenges associated with concentric electrode-based large aperture (50 mm) LC lenses with segmented phase profile designs are investigated, including haze-related and diffraction-related issues. Effective solutions are provided to enhance the optical quality of these lenses (reduction of fringing field effect with an insulator layer and inclusion of black mask). By addressing these challenges, a 50 mm aperture size electrically focus tunable LC spherical lens with enhanced optical quality is developed. The proposed tunable lenses exhibit lightweight (<2 g), slim (<2 mm), and compact flat designs with fast switching speeds (<750 ms) and low driving voltages (<5 V), making them suitable for important near-to-eye applications such as accommodation-convergence mismatch correction in augmented reality (AR)/virtual reality (VR) head-mounted displays (HMDs) and pr (open full item for complete abstract)

Committee: Philip Bos (Committee Chair); Liang-Chy Chien (Committee Member); Syed Shihab (Committee Member); John Portman (Committee Member); Deng-Ke Yang (Committee Member) Subjects: Chemical Engineering; Materials Science; Optics

PHD, Kent State University, 2021, College of Arts and Sciences / Materials Science Graduate Program

In this dissertation, a novel approach to a liquid crystal electro-optical device for light beam steering is examined. The proposed apparatus, so called "FFS-PPD", with fast switching variable phase and tunability is successfully developed for the first time. The device was first designed, analyzed and optimized with the help of modeling. It was then fabricated and the proof of concept was demonstrated by characterization.Other types of liquid crystal optical devices such as Pancharatnam-Berry phase lens (PPL) and segmented phase profile (SPP) variable focal lens and their emerging applications in AR/VR/3D systems are also discussed.

Committee: Philip Bos (Advisor); Hiroshi Yokoyama (Committee Member); Yang Deng-Ke (Committee Member) Subjects: Chemical Engineering; Materials Science; Optics; Physics

PHD, Kent State University, 2018, College of Arts and Sciences / Chemical Physics

Our research uses a dynamic cell to measure azimuthal anchoring energy and disclination line tension in a nematic liquid crystal. The dynamic cell is a cell which has controllable twist angle and cell thickness. By increasing the twist angle to an angle greater than 90 degrees, the cell becomes super twisted. In this state, if the cell thickness is decreased to a critical thickness, the surface anchoring breaks. The azimuthal anchoring energy can be calculated from the twist elastic constant, the twist angle, and the cell thickness. When the anchoring breaks, a disclination line is formed separating regions of opposite handed twist. By balancing the forces due to the disclination line tension and the twist distortion, a stable disclination line can be achieved. We can calculate the line tension of the disclination from the radius of curvature of the disclination line at equilibrium. In order to generate disclination lines more easily, we designed a disclination line nucleation site by photopatterned surface alignment. We found that the line tension increases with the cell thickness. By measuring the line tension with respect to cell thickness and temperature, we investigate the core of the disclination line.

Committee: Hiroshi Yokoyama PhD (Advisor); Peter Palffy-Muhoray PhD (Committee Member); Elizabeth Mann PhD (Committee Member); Antal Jakli PhD (Committee Member); Sam Sprunt PhD (Committee Member) Subjects: Condensed Matter Physics; Physics

PHD, Kent State University, 2017, College of Arts and Sciences / Chemical Physics

Chirality is a central scientific concept but its superstructural and nanoscale manifestation are not well understood, severely limiting the potential of materials in a range of emerging applications. In the first part of this dissertation, we examine a lyotropic chromonic liquid crystal (LCLC) forming a nematic phase for its more intense response to gold nanoparticles capped with chiral L-cysteine ligand molecules than to their free molecular counterparts. We show that such nanoparticles induce a tighter chiral twist within, and similar among, the stacks of nematic LCLCs but with several orders of magnitude fewer chiral molecules. The second part of this dissertation focuses on the SmCP phase and the development of an optically isotropic antiferroelectric liquid crystal (OI-AFLC) display mode. This report describes in detail the synthesis and characterization of a series bent-core liquid crystal (LC) systems. Multi-component mixtures of these compounds achieved room temperature switching between optically isotropic and birefringent states; several mixtures displayed a modulated SmCaPA phase extending from below -40 °C up to about 100 °C. Requiring high fields and showing slow switching speeds, their utility is found in non-video-rate switching applications requiring high contrast.

Committee: Torsten Hegmann Dr. (Advisor); Antal Jákli Dr (Committee Member); Oleg Lavrentovich Dr. (Committee Member); Mann Elizabeth Dr. (Committee Member); Paul Sampson Dr. (Committee Member) Subjects: Chemistry; Materials Science; Physics

PHD, Kent State University, 0, College of Arts and Sciences / Chemical Physics

Long range orientational order of nematic liquid crystals has been an inspiration for both theoretical works and practical applications. Using a combination of analytic calculations and numerical simulations, we investigate the interplay between orientational order and geometric constraints. In the first part of this dissertation, we concentrate on nematic order in liquid membranes, where the curvature induces non-uniformity and vice versa. Variations in nematic order, especially near topological defects, play an essential role in the interaction with curvature. In the second part, we consider the combination of the anisotropy of nematic liquid crystals with the large reversible deformations of elastomers as a mechanism for programmable deformations in soft materials.

Committee: Jonathan Selinger (Advisor); Robin Selinger (Committee Member); Antal Jakli (Committee Member); John Portman (Committee Member); Arden Ruttan (Committee Member) Subjects: Physics

PHD, Kent State University, 2013, College of Arts and Sciences / Department of Physics

Liquid crystal (LC) elastomers encompass a unique combination of the anisotropic order of liquid crystals and the tendency of the polymeric network to disorder and maximize entropy by adopting isotropic configuration. As a result, they exhibit unconventional elastic behavior, namely, soft-elasticity and the shape memory effect, which is otherwise absent in a classical elastomer network. In this work, two main-chain smectic-C liquid crystal elastomers have been investigated with synchrotron x-ray diffraction to understand the evolution of liquid crystal microstructure under applied strain and its relationship to soft-elasticity and the shape memory effect. The elastomers were subjected to uniaxial strain, allowed to relax at constant strains, allowed to recover after removal of the external stress, and finally annealed by heating above their clearing temperatures while changes in their molecular organization were measured and analyzed. The experiments reveal the presence of two different relaxation mechanisms in these systems at low and high strains. At low strains, the system's behavior is elastic and the smectic layers are reoriented with layer-normals distributed in a plane perpendicular to the stretch direction. The system relaxes relatively slowly (time constant ~ 45 minutes) which is attributed to the flow properties of the liquid crystal layers embedded in the elastomer network. A different relaxation mechanism dominates at high strains and appears to have its origin in the polymer component of the system. The equilibration time (~ 4 - 8 minutes) conforms to an order of magnitude faster relaxation. Due to misaligned microdomains at small strains, the value of global orientational order parameter S for the mesogenic parts is initially small (~ 0.15). With increasing strain, the local domain-directors, the mesogens, and the polymer chains, all tend to align parallel to the stretch direction giving rise to a higher measured value of S ~ 0.83 at a strain of ~ 4.0. (open full item for complete abstract)

Committee: Satyendra Kumar Dr. (Advisor) Subjects: Condensed Matter Physics; Physics

PHD, Kent State University, 2011, College of Arts and Sciences / Chemical Physics

Blue phases (BP) are phases of chiral liquid crystals formed by lattices of double-twist cylinders with corresponding lattices of disclination lines. The conditions under which such an arrangement is energetically favorable generally occur in only a narrow band of temperature ranges, commonly less than 2C. Several methods have been devised to expand this range, notably polymer stabilization or nanoparticle stabilization. In addition, there is interest in using unconventionally shaped liquid crystals to form blue phases as molecular biaxiality is expected to help stabilize the phase. Here, we explore the potential of bent-core liquid crystals in creating stable blue phase materials with broad temperature ranges. It is known that the presence of achiral bent-core molecules in a chiral nematic material can induce blue phases where none were previously observed, and that liquid crystal dimers with a flexible center can create broad-temperature blue phases. Here we develop broad-temperature blue phase materials using bent-core nematic liquid crystals and chiral dopants, resulting in the first known example of a broad-range BPIII. Additional methods of phase stabilization are utilized in an attempt to increase the temperature range of the phase to encompass room temperatures.

Committee: Antal Jakli PhD (Advisor); Hiroshi Yokoyama PhD (Committee Member); Liang-Chy Chien PhD (Committee Member); Robert Twieg PhD (Committee Member); E.C. Gartland PhD (Committee Member) Subjects: Materials Science

PHD, Kent State University, 2008, College of Arts and Sciences / Chemical Physics

Wavelength tunable devices have generated great interest in basic science, applied physics, and technology and have found applications in Lidar detection, spectral imaging and optical telecommunication. This thesis focuses on the physics, technology and application of several wavelength tunable devices based on liquid crystal technology, especially on Holographic Polymer Dispersed Liquid Crystals (HPDLC). HPDLCs are formed through the photo-induced polymerization process of photopolymerizable monomers, and self-diffusion process and phase separation process of the mixture of liquid crystals and monomers, when the mixtures of liquid crystals and monomers are exposed to the interfering monochromatic light beams. The infomation from the interfering pattern is recorded into the holographic liquid crystal/polymer composites, which are switchable or tunable upon external electric fields. Based on the electrically controllable beam steering capability of transmission HPDLCs, novel switchable circular to point converter (SCPC) devices are demonstrated for selecting and routing the wavelength channels discriminated by a Fabry-Perot interferometer, with application in Lidar detection, spectral imaging and optical telecommunication. SCPC devices working in both visible and near infrared (NIR) wavelength ranges are demonstrated. A random optical switch can be created by integrating a Fabry-Perot interferometer with a stack of SCPC units. Liquid crystal Fabry-Perot (LCFP) Products have been analyzed, fabricated and characterized for application in both spectral imaging and optical telecommunication. Both single-etalon system and twin-etalon system are fabricated. Finesse of more than 10 in visible wavelength range and finesse in more than 30 in NIR are achieved for the tunable LCFP product. The materials, fabrication and characterization of lasing emission of dye doped HPDLCs are discussed. Lasing from different modes of HPDLCs is studied and both the switching and tunabilit (open full item for complete abstract)

Committee: Jack Kelly (Advisor); Gregory Crawford (Committee Member); Deng-Ke Yang (Committee Member); Eugene Gartland (Committee Member); Qi-Huo Wei (Committee Member); Donald White (Other) Subjects: Materials Science; Optics; Physics; Polymers

PHD, Kent State University, 2005, College of Arts and Sciences / Chemical Physics

Four types of unconventional liquid crystal systems - amphotropic glycolipids; novel bent-core liquid crystals, bent-core liquid crystal and glycolipid mixtures, and colloidal crystal-liquid crystal systems - were studied and characterized by polarizing microscopy, electrical current, digital scanning calorimetry, and dielectric spectroscopy. - Thermotropic properties of glycolipids show a number of unusual properties, most notably high (60-120) relative dielectric constants mainly proportional to the number of polar sugar heads. The relaxation of this dielectric mode is found to be governed by the hydrogen bonding between sugar heads. - Studies on novel bent-core liquid crystals reveal a new optically isotropic ferroelectric phase, molecular chirality-induced polarity, and transitions between molecular chirality and polarity driven phases. - Mixtures of several bent-core substances with nematic, polar SmA and SmC phases, and a simple amphiphilic sugar lipid with SmA mesophase found to obey the well known miscibility rules, i.e. the sugar lipid mixes best with the polar SmA bent-core material. In addition, the chiral sugar lipid was found to induce tilt to the non-tilted polar SmA phase, which represents a new direction among the chirality – polarity – tilt relations. - The effects of the surface properties and electric fields were studied on various colloid particles – liquid crystal systems. It is found that the surface properties (hydrophobicity, roughness, rubbing) of the substrates are important in determining the size and symmetry of colloidal crystals. The director field of the liquid crystal infiltrated in the colloid crystals can be rendered both random and uniform along one of the crystallographic axis. We present the first observations of DC electric-field-induced rotational and translational motion of finite particles in liquid crystals. The electrorotation is essentially identical to the well - known Quincke rotation, which in liquid crystals triggers a (open full item for complete abstract)

Committee: Antal J&#225;kli (Advisor) Subjects: Physics, General

PHD, Kent State University, 2004, College of Arts and Sciences / Chemical Physics

The main objective of the dissertation is to develop polymer stabilized liquid crystals applications with the aim of fast switching liquid crystal devices. We have scrutinized the design of the experimental setup and the optimization of the polymerization condition. We evaluate and discuss various PSLC systems, including the study of morphology of polymer networks and the electro-optical characteristics. Optically compensated bend (OCB) nematic or pi-cell is known to exhibit a fast response time and wide viewing angle with compensated films. But this bend mode has a splay-bend transition problem. These splay and bend states are topologically distinct and for real display modes it should be operated with bend mode without splay state. We introduced a polymer stabilization of the bend nematic mode in which we used polymer network acts as the volume surfaces to stabilize the bend state of the nematic liquid crystal. The switching time of a polymer-stabilized bend nematic display shows a fast rise time of 1.2ms and a 8.8ms decay time, which is sufficiently fast for video display applications. A liquid crystal blazed grating having a prismatic polymer microstructure has been developed using polymer-stabilized optical pattern forming state of a cholesteric liquid crystal. The prismatic polymer structure is formed by photo-induced localization and polymerization of a small concentration of monomer onto one substrate nearest to the UV The light incident at different angles from the normal. Using these method periodical one-dimensional patterns with a prismatic shape of polymer can be structured at the surface. The optical diffraction properties of the gratings were evaluated by the application of electric field and light incident angles, corroborate the blazed grating of asymmetric reflective index modulation. The dissertation continues on the exploration of an application of the short pitch cholesteric liquid crystals. Uniformly lying helix (ULH) of a cholesteric liquid cr (open full item for complete abstract)

Committee: Liang-Chy Chien (Advisor) Subjects: Physics, Condensed Matter

Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

Spatial light modulators are a critical technology in optical systems. Liquid crystal on silicon (LCoS) devices are a particular type spatial light modulators used in numerous applications including wavelength selective switches for optical communications, display systems including projectors, 3-D holographic displays, heads up displays in cars, and virtual reality and augmented reality (VR/AR) headsets, and adaptive optics such as optical beam shaping, beam steering, and wavefront correction. Compared to other spatial light modulators, LCoS technology features high resolution and low cost. This work focuses on developing phase-only LCoS modulators for wavelength selective switches, free space optical communications, and holographic display. Traditional LCoS phase modulators suffer from polarization-dependent phase modulation, so development of polarization-independent LCoS phase modulators is an important research topic. This dissertation explores the design, fabrication, and testing of a novel polarization-independent LCoS (PI-LCoS) phase modulator and explores its use in a free space optical transceiver application. The design of the device consists of a silicon backplane, a thin-film polymer quarter-wave plate, and an electro-optically active liquid crystal layer. The inclusion of a polymer waveplate allows for polarization conversion and polarization independent modulation. The silicon backplane uses a specialized pixel circuit to increase driving voltages and provide stable phase modulation. The fabrication process consists of spin coating thin films onto the silicon backplane to form the quarter-wave plate and liquid crystal alignment layers, then a glass cover is used to contain the liquid crystal. Polarization-independent phase modulation was demonstrated through beam steering experiments, a critical function for applications in wavelength selective switches. Applications of the LCoS phase modulator were explored by demonstrating wavefront correction f (open full item for complete abstract)

Committee: Chongchang Mao (Advisor); Kiryung Lee (Committee Member); Shamsul Arafin (Committee Member); Sanjay Krishna (Committee Member) Subjects: Electrical Engineering; Optics