Download or read book Optical Studies of Blue Phase III Twist bend and Bent core Nematic Liquid Crystals in High Magnetic Fields written by Pavan Kumar Challa and published by . This book was released on 2014 with total page 139 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation is mainly divided into three parts. First, the dynamic light scattering measurements on both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory, Tallahassee is discussed. In a nematic liquid crystal the molecules tend to be aligned along a constant direction, labeled by a unit vector (or "director") n. However, there are fluctuations from this average configuration. These fluctuations are very large for long wavelengths and give rise to a strong scattering of light. The magnetic field reduces the fluctuations of liquid crystal director n. Scattered light was detected at each scattering angle ranging from 0° to 40°. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 Tesla. We also observe evidence of field dependence of certain nematic material parameters. In the second part of the dissertation, magneto-optical measurements on two liquid crystals that exhibit a wide temperature-range amorphous blue phase (BPIII) are discussed. Blue phase III is one of the phases that occur between chiral nematic and isotropic liquid phases. Samples were illuminated with light from blue laser; the incident polarization direction of the light was parallel to the magnetic field. The transmitted light was passed through another polarizer oriented at 90° with respect to the first polarizer and was detected by a photo-detector. Magnetic fields up to 25Tesla are found to suppress the onset of BPIII in both materials by almost 1 degree celcius. This effect appears to increase non-linearly with the field strength. The effect of high fields on established BPIII's is also discussed, in which we find significant hysteresis and very slow dynamics. Possible explanations of these results are discussed. In the third part of the dissertation, magneto-optic measurements on two odd-numbered dimer molecules that form the recently discovered twist-bend nematic (NTB) phase, which represents a new type of 3-dimensional anisotropic fluid with about 10 nm periodicity and accompanied optical stripes are discussed. In twist-bend nematic phase the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The pitch of the oblique helocoid is in the nanometer range. Light from a red laser was passed normally through the sample placed between crossed polarizers oriented at ±45° with respect to the vertical magnetic field. Optical birefringence was measured from the transmitted light. Magnetic field of B=25T shifts downward the N-NTB phase transitions by almost 1 Celsius. We also show that the optical stripes can be unwound by a temperature and material dependent magnetic induction in the range of B=5-25T. Finally, we propose a Helfrich-Hurault type mechanism for the optical stripe formation. Based on this model we calculate the magnetic field unwinding the optical scale stripes, and find agreement with our experimental results.

Download or read book Bent Shaped Liquid Crystals written by Hideo Takezoe and published by CRC Press. This book was released on 2017-07-20 with total page 290 pages. Available in PDF, EPUB and Kindle. Book excerpt: Bent-Shaped Liquid Crystals: Structures and Physical Properties provides insight into the latest developments in the research on liquid crystals formed by bent-shaped mesogens. After a historical introduction, the expert authors discuss different kinds of mesophase structures formed by bent-shaped molecules. This book devotes the majority of its pages to physical properties such as polar switching, optics and non-linear optics, and behavior in restricted geometries. However, as chemistry is often highly relevant to the emergence of new phases, particularly with reflection symmetry breaking, it also involves a broad spectrum of interesting chemistry viewpoints.

Download or read book Biaxial Nematic Liquid Crystals written by Geoffrey R. Luckhurst and published by John Wiley & Sons. This book was released on 2015-02-23 with total page 408 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the nematic liquid crystal phase, rod-shaped molecules move randomly but remain essentially parallel to one another. Biaxial nematics, which were first predicted in 1970 by Marvin Freiser, have their molecules differentially oriented along two axes. They have the potential to create displays with fast switching times and may have applications in thin-film displays and other liquid crystal technologies. This book is the first to be concerned solely with biaxial nematic liquid crystals, both lyotropic and thermotropic, formed by low molar mass as well as polymeric systems. It opens with a general introduction to the biaxial nematic phase and covers: • Order parameters and distribution functions • Molecular field theory • Theories for hard biaxial particles • Computer simulation of biaxial nematics • Alignment of the phase • Display applications • Characterisation and identification • Lyotropic, thermotropic and colloidal systems together with material design With a consistent, coherent and pedagogical approach, this book brings together theory, simulations and experimental studies; it includes contributions from some of the leading figures in the field. It is relevant to students and researchers as well as to industry professionals working in soft matter, liquid crystals, liquid crystal devices and their applications throughout materials science, chemistry, physics, mathematics and display engineering.

Download or read book Magneto optical and Imaging Studies of Chromonic and Thermotropic Liquid Crystals written by Tanya Ostapenko and published by . This book was released on 2011 with total page 202 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation addresses three experimental questions. First, the pretransitional behavior of lyotropic chromonic liquid crystals (LCLCs) is investigated in order to gain further insight into the aggregation mechanism and structure. In order to study the pretransitional behavior of LCLCs in the isotropic phase, a high magnetic field is applied perpendicular to the light propagation direction, which induces birefringence in the material; this is called the Cotton-Mouton effect. The aggregates align with the field, which makes it possible to study how the aggregates form in the isotropic phase. The results of this study indicate that multiple optical effects can be induced, which supports the possibility of a complex aggregate structure. The second part of this dissertation explores the possibility of a biaxial nematic phase (Nb). The geometry of the liquid crystal mesogen is important and it is thought that banana-shaped liquid crystals will have an Nb phase. However, contradicting reports on different bent-core materials have not determined whether this phase exists in them. Optical techniques usually rely on a sample cell rubbing treatment to homeotropically align the main director, n, but optical misidentification of Nb could occur if the material is in a tilted uniaxial phase, which appears the same as a homeotropically-aligned biaxial phase. Using a high magnetic field to completely align n and measuring the magnetic field-induced optical phase difference perpendicular to n gives a conclusive way to determine whether a material has non-zero biaxial order. None of the materials studied appear to have an Nb phase. The last part of this dissertation examines director fluctuations in calamitic and bent-core liquid crystals using dynamic imaging analysis. Dynamic image analysis is a relatively new technique where a measurement of nematic phase fluctuations is made in direct space. These measurements are done using a polarizing microscope, heat stage and CCD camera. The advantage of this technique is that it measures small values of q, making it a complementary technique to dynamic light scattering, where large values of q are easily obtained. Basic material properties may be related to the wave vector and decay time of the fluctuations.

Download or read book Nematic and Cholesteric Liquid Crystals written by Patrick Oswald and published by CRC Press. This book was released on 2005-02-28 with total page 624 pages. Available in PDF, EPUB and Kindle. Book excerpt: Liquid crystals allow us to perform experiments that provide insight into fundamental problems of modern physics, such as phase transitions, frustration, elasticity, hydrodynamics, defects, growth phenomena, and optics (linear and non linear). This excellent volume meets the need for an up-to-date text on liquid crystals.Nematic and Cholesteric Liq

Download or read book Dynamic and Magneto optic Properties of Bent core Liquid Crystals written by Seyyed Muhammad Salili and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Polarized Light in Liquid Crystals and Polymers written by Toralf Scharf and published by John Wiley & Sons. This book was released on 2007-01-02 with total page 480 pages. Available in PDF, EPUB and Kindle. Book excerpt: Polarized Light in Liquid Crystals and Polymers deals with the linear optics of birefringent materials, such as liquid crystals and polymers, and surveys light propagation in such media with special attention to applications. It is unique in treating light propagation in micro- and nanostructured birefringent optical elements, such as lenses and gratings composed of birefringent materials, as well as the spatial varying anisotropic structures often found in miniaturized liquid crystal devices.

Download or read book Light Scattering Studies of Defects in Nematic twist bend Liquid Crystals and Layer Fluctuations in Free standing Smectic Membranes written by Shokir A. Pardaev and published by . This book was released on 2017 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This research described in this dissertation comprises three experimental topics and includes the development of an appropriate theoretical framework to understand the various observations in each. In the first part, we present results from angle-resolved second-harmonic light scattering measurements on three different classes of thermotropic nematic liquid crystals: polar and non-polar rodlike compounds, and a bent-core compound. We analyze the data in terms of the "flexoelectric" polarization induced by distortions of the nematic director field around topological defects known as inversion walls, which are analogous to Neel walls in magnetic spin systems and which often exhibit a closed loop morphology in nematic systems.The second part of this dissertation explores the possible existence of a helical polarization field in the nematic twist-bend (NTB) phase of dimeric liquid crystals, utilizing a similar nonlinear light scattering approach. The NTB phase is characterized by a heliconical winding of the local molecular long axis (director) with a remarkably short, nanoscale pitch. According to theoretical conjecture, a helical electric polarization field accompanies this director modulation, but, due to the short pitch, presents a significant challenge for experimental detection. Our study focuses on topological defects, classified as parabolic focal conics, in two achiral, NTB-forming liquid crystals. These defects generate distortions of the polarization field on sufficiently long (micron) lengths to enable a confirmation of the existence of polar structure. We analyze our results with a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a bilinear coupling between the two.The last part of the dissertation focuses on the layer dynamics of thin, free-standing membranes of a smectic-A liquid crystal, with a particular consideration of the surface (interfacial) parameters that control these dynamics. We utilize photon correlation spectroscopy to probe the contributions of distinct under- and overdamped processes to the membrane motion. According to hydrodynamic theory, the frequency and damping rate of underdamped layer motion should scale with scattering vector in a manner controlled by the relative magnitude of a surface elastic constant, which is associated with gradients in surface tension, as well as by the average surface tension. In addition, the damping in very thin films is predicted to be quite sensitive to the presence of an atmosphere surrounding the film. A distinct, overdamped mode, observable in sufficiently thick films, is also predicted to couple to the layer motion. We present results on these dynamical modes and their dispersion and demonstrate their consistency with the hydrodynamic theory subject to appropriate surface boundary conditions.

Download or read book Twist Bend and Double Twist in Liquid Crystals written by Jie Xiang and published by . This book was released on 2016 with total page 125 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chirality is ubiquitous in the natural world and plays an important role in soft condensed matter, especially liquid crystals. In this dissertation, we investigated the twist bend and double twist structures in thermotropic liquid crystals. The summary of this dissertation is as follows: 1. We demonstrated experimentally a new type of orientational nematic order in which achiral dimer molecules follow the direction of oblique helicoids. The new phase, first considered theoretically by R.B. Meyer and I. Dozov as a twist-bend nematic (Ntb), is a structural link between the well-known uniaxial nematic (N) and chiral nematic (N*) phases. The oblique helicoidal structure of the Ntb phase is evidenced by asymmetric Bouligand arches in freeze-fracture transmission electron microscopy textures. The period of oblique helicoids is 8-9 nm. The unique nanostructure leads to new macroscopic properties of Ntb, such as first-order dielectric reorientation and expulsion of twist and bend deformations of the optic axis. We measured the temperature dependence of the splay (K1 ), twist (K2 ), and bend (K3 ) elastic constants of the uniaxial nematic (N) phase formed by dimeric liquid crystal mixture with negative dielectric anisotropy. The material under study forms the nematic twist-bend (Ntb) phase when cooled down from the uniaxial nematic (N) phase. In the N phase, the bend constant K3 decreases dramatically as the temperature is lowered towards the N-Ntb phase transition. Both K1 and K2 increase as the temperature is lowered; K1 remains about 2-3 times higher than K2 near the transition to the Ntb phase. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that eventually give rise to the Ntb phase. We explored the effects of photo-isomerization on the N-Ntb phase transition using an azobenzene-based non-symmetric liquid crystal dimer. We observed an isothermal, reversible Ntb-N transition, which is attributed to a trans-cis photo-isomerization of the azobenzene fragment on UV irradiation. The cis isomers stabilize the standard nematic phase and the trans isomers stabilize the Ntb phase. 2. We observed experimentally an electrooptic effect in a cholesteric LC with a distinct oblique-helicoidal director deformation. The oblique helicoid, predicted in late 1960-ies, is demonstrated to exist in dimer materials with an anomalously small bend elastic constant. Theoretical, numerical, and experimental analysis establishes that both the pitch and the cone angle of the oblique helicoid increase as the electric field decreases. The effect can enable many applications that require dynamically controlled transmission and reflection of light. We demonstrated the electrooptic applications using the cholesteric heliconical structure, such as the electrically tunable selective reflection of light from ultraviolet to visible and infrared, full color reflective display, and electrically tunable lasing. Further work can be done to achieve the cholesteric heliconical structure with the magnetic field, light irradiation, and surface confinement, besides the electric field. The cholesteric heliconical structure can also find applications in tunable optical filter, smart windows with transparent/opaque/tunable color states. 3. We reported on a fast electro-optic switching (response time 0.1 ms) of a blue-phase-polymer templated nematic with a broad-temperature range of thermodynamic stability and hysteresis-free performance. The nematic fills a polymer template that imposes a periodic structure with cubic symmetry and submicron period. In the field-free state, the nematic in polymer template is optically isotropic. An applied electric field causes non-zero optical retardance. The approach thus combines beneficial structural and optical features of the blue phase (cubic structure with submicron periodicity) and superior thermodynamic stability and electro-optic switching ability of the nematic filler. Further work can be done using liquid crystal twist bend structure as a template, such as Ntb phase and cholesteric heliconical structure, to achieve the fast switching electrooptic performance.

Download or read book Electrooptic Effects in Liquid Crystal Materials written by L.M. Blinov and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 477 pages. Available in PDF, EPUB and Kindle. Book excerpt: Electrooptic effects provide the basis for much liquid-crystal display technology. This book, by two of the leaders in liquid-crystal research in Russia, presents a complete and accessible treatment of virtually all known phenomena occurring in liquid crystals under the influence of electric fields.

Download or read book Development and Characterization of Blue Phases Made from Bent core Liquid Crystals written by Stefanie Taushanoff and published by . This book was released on 2011 with total page 130 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Electro optical and Magneto optical Properties of Liquid Crystals written by Lev Mikhaĭlovich Blinov and published by John Wiley & Sons. This book was released on 1983 with total page 376 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Unconventional Liquid Crystals and Their Applications written by Wei Lee and published by Walter de Gruyter GmbH & Co KG. This book was released on 2021-07-19 with total page 581 pages. Available in PDF, EPUB and Kindle. Book excerpt: The work focuses on recent developments of the rapidly evolving field of Non-conventional Liquid Crystals. After a concise introduction it discusses the most promising research such as biosensing, elastomers, polymer films , photoresponsive properties and energy harvesting. Besides future applications it discusses as well potential frontiers in LC science and technology.

Download or read book Nematic Liquid Crystal Sessile Droplets In Electric And Magnetic Fields written by Zoltán Karaszi and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nematic Liquid Crystal Sessile Droplets in Electric and Magnetic Fields Abstract of Dissertation Zoltán Karaszi Materials Science Graduate Program, Kent State University Sessile droplets with uniform director structure could be used as tunable optical lenses where the focal length could be controlled by light polarization, viewing angle, and magnetic or electric fields. To achieve that, one must understand the liquid crystal director structure in various external fields. In this dissertation, I presented detailed experimental studies. I summarized the theoretical description of the director structure of uniaxial nematic liquid crystals, such as the formation and dynamics of Néel wall type metastable inversion walls, either in magnetic or electric fields or magnetic and electric fields combined. Sessile nematic droplets allow for studying the combined effect of anchoring at solid and gas interfaces. The combination of various alignments at the two surfaces and external fields results in various director distribution schemes, ranging from a defect-free, almost homogeneous state to configurations with point-, line- and wall defects. We designed a polarizing optical microscope made of non-magnetic materials that could be placed between an electromagnet's poles. The design allowed us to study the effect of various combinations of electric and magnetic fields on nematic liquid crystal sessile droplets. Additionally, a long-range microscope was used to observe the side view of the LC drop. We also built another experimental setup that enabled us to measure the focal length in response to electric fields while rotating the sample between crossed polarizers. (1) Our main experimental findings can be summarized as follows. We showed that under low magnetic fields applied along the base plane of a sessile droplet with homeotropic alignment, the director structure becomes distorted and gradually leads to a defect wall that moves toward the periphery. We explained the director field's magnetic field dependence and the defect walls' formation and motion. We have shown that at a strong enough lateral magnetic field or even at a small field that makes more than 3° with respect to the base plane, the director can be uniformly aligned along the field without the presence of the defect wall. [1] (2) Replacing the magnetic field, an AC electric field was applied along the base plane of a nematic sessile droplet with positive dielectric anisotropy; we also found a rotation of the director toward the electric field and the formation of an inversion wall perpendicular to the applied field. While at low frequencies, the direction of the wall was stationary, just as observed in magnetic fields, above the Maxwell-Wagner frequency, it turned toward the external electric field. In both cases, the defect wall was also swept toward the periphery of the drop, where it eventually disappeared. The defect wall's rotation at high frequencies resulted from the antiparallel orientation of the effective moment vector and the electric field due to the lower dielectric constant and higher electric conductivity of the defect wall than of the rest of the liquid crystal droplet. An exponential time dependence could describe the time dependence of the displacement of the electric field-induced defect wall without any fitting parameter. That, combined with the threshold for director deformation, enabled us to determine both the bend elastic constant and the rotational viscosity using much less substance than existing techniques. Uniform electric field-induced generation, rotation, and linear movement of defect walls is a unique phenomenon in soft matters.[2] (3) Multidimensional solitons and their electric field-induced movement have recently been reported in achiral and chiral nematic liquid crystals. In the presence of competing magnetic AC electric fields on a sessile droplet with positive dielectric anisotropy, we found that the inversion wall induced by a horizontal magnetic field suffers buckling at sufficiently high electric voltage applied vertically. We characterized the time and field dependence of the buckled walls' shape and motion and proposed a physical mechanism to account for the behavior. We note that, due to their spontaneous propagation, the inversion wall can also be considered as a one-dimensional soliton, i.e., a spatially localized shape preserving traveling wave packet such as observed first by J.S. Russell in 1834 in the form of a one-dimensional water wave traveling along a canal near Edinburgh. [3] (4) We also studied the behavior of nematic liquid crystal sessile droplets with negative dielectric anisotropy. We observed several new director configurations depending on the specific combination of the magnetic and electric fields. For example, at high enough voltages applied across the droplet, the radial symmetry breaks, and a spiraling deformation of the Maltese cross appears near the central defect line. This can be attributed to the twist deformation in the vicinity of the central defect line that replaces the more costly bend deformation. Applying a magnetic field perpendicular to the vertical electric field, a twisted inversion wall formed together with a vertical central defect line. When the electric field was applied parallel to the base plane of the droplet, a homeotropic central region formed along the electric field. When this field was applied together with a magnetic field in the same direction, the homeotropic central region became perpendicular to the applied field. [4,5] (5) We measured the focal length of nematic sessile droplets with positive dielectric anisotropy as a function of electric fields applied along the base plane of the lenses. It was observed that the focal length decreases during increasing fields as the effective refractive index, which is inversely proportional to the focal length, increases from "no" to "ne". The focal length of NLC droplets could also be tuned by varying the polarization direction of a linear polarizer placed in front of the lenses. At the same time, a constant AC electric field was applied along the base plane. [6] References: [1] P. Salamon, Z. Karaszi, V. Kenderesi, Buka, and A. Jákli, Liquid Crystal Spherical Caps in Magnetic Fields, Phys. Rev. Res. 2, 023261 (2020). [2] Z. Karaszi, P. Salamon, Á. Buka, and A. Jákli, Lens Shape Liquid Crystals in Electric Fields, J. Mol. Liq. 334, 116085 (2021). [3] Z. Karaszi, M. Máthé, P. Salamon, Á. Buka, and A. Jákli, Electric Field Induced Buckling of Inversion Walls in Lens-Shape Liquid Crystal Droplets, J. Mol. Liq. 365, 120177-1-7 (2022). [4] Z. Karaszi, M. Máthé, P. Salamon, Á. Buka, and A. Jákli, Lens-Shaped Nematic Liquid Crystal Droplets with Negative Dielectric Anisotropy in Electric and Magnetic Fields, Liq. Cryst. DOI: 10.1080/02678292.2022.2134594, published online (October 24, 2022). [5] Ágnes Buka, Péter Salamon, Marcell Máthé, Zoltán Karaszi, and Antal Jákli, Liquid Crystals, 2023, Nematic liquid crystals in lens shape geometry. https://doi.org/10.1080/02678292.2023.2168307 [6] Z. Karaszi, K. Perera, and A. Jákli, Tuning the Focal Length of Sessile Nematic Droplets by Electric Fields, under preparation, (2023).

Download or read book Statistical Mechanics of Polar Biaxial and Chiral Order in Liquid Crystals written by Subas Dhakal and published by . This book was released on 2010 with total page 118 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation addresses three distinct problems in liquid crystal: phase transitions among different SmCP phases of bent-core liquid crystal, flexoelectric effect in nematic liquid crystal and interplay between chirality and biaxiality in cholesteric liquid crystal. These problems are studied with computer simulations and analytical calculations. The first part of this dissertation is about chiral switching in smectic phases of bent-core liquid crystals. The phenomenon is investigated with a lattice Monte Carlo simulation and a phenomenological theory. Both methods show that the antiferroelectric (AFE) ground state switches into a ferroelectric state (FE) at sufficiently large electric field. In this transition, the chirality of the phase could change or not. In the case of weak inter-layer chiral coupling, anticlinic order of molecules from layer to layer could remain through the transition -thus changing the chirality of the phase. However, for a large value of chiral coupling, the anticlinic state could change to a synclinic state in the transition -thus preserving the chirality of the phase. Another area of focus is on flexoelectric effect in nematic liquid crystal. Recent experiments have shown that bent-core liquid crystals have a surprisingly large bend flexoelectric coefficient, up to 35 nC/m, roughly three orders of magnitude larger than the typical value. With this large bend flexoelectric coefficient, bent-core liquid crystals may be practical materials for converting mechanical into electrical energy. One of the focal points of this dissertation is to explain the large flexoelectric effect found in bent-core liquid crystals. Theoretical models of splay and bend flexoelectric effect are developed that incorporates the coupling between the director distortion (either splay or bend) and polarization. Through these models the flexoelectric response as a function of interaction parameters, temperature, and applied electric field is studied. Simulations and analytical calculations show that there is a large flexoelectric effect in the nematic phase, which diverges as the system approaches the transition to the polar phase. This observation demonstrated that flexoelectricity is not just a molecular effect but could be a statistical phenomenon associated with the response of correlated volumes of molecules, which increases as one approaches the polar phase. Finally, we investigate the statistical mechanics of chirality and biaxiality in liquid crystals through a variety of theoretical approaches, including Monte Carlo simulations, lattice mean-field theory, and Landau theory. All of these calculations show that there is an important interaction between cholesteric twist and biaxial order: The twist acts as a field on the biaxial order, and conversely, the biaxial order increases the twist, i.e. reduces the pitch. We model the behavior of chiral biaxial liquid crystals as a function of temperature, and discuss how the predictions can be tested in experiments.

Download or read book Optics And Nonlinear Optics Of Liquid Crystals written by Iam-choon Khoo and published by World Scientific. This book was released on 1993-04-28 with total page 440 pages. Available in PDF, EPUB and Kindle. Book excerpt: This is a monograph/text devoted to a detailed treatment of the optical, electro-optical and nonlinear optical properties of all the mesophases of liquid crystals and related processes, phenomena and application principles. Quantitative data on material and optical parameters spanning the ultraviolet, visible, infrared as well as the microwave regimes are presented along with detailed theoretical treatments of basic liquid crystal physics, material properties and nonlinear optics.Starting with a discussion on the basic building blocks of liquid crystalline molecules, the authors proceed to present in a pedagogical manner current theories, experiments, and applications of these unique and important optical properties of liquid crystals. Numerous tables of hard-to-find liquid crystalline parameters, a self-contained chapter on general nonlinear optics, and comprehensive literature review are also included.

Download or read book Nonlinear Optical Responses of Blue Phase Liquid Crystals and Their Applications written by Tsung Jui Ho and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Are they called blue phases because they are in blue color? This is the most common question asked by many people. The answer is yes but no. The name was given by the scientist who first observed the special liquid crystalline phase reflecting the color blue, however, the reflection color does not represent the phases. The major difference of blue-phase liquid crystal from other phases is their self-assembly three-dimensional photonic crystalline structure with a lattice constant typically around hundreds of nanometers, and their ability to show Bragg reflection over the visible spectrum.At first, blue-phases were not popular because they are optically isotropic, i.e. no birefringence. However, they attracted wide attention over academic or industrial studies owing to their unique optical properties, such as a fast response time, being polarization- free, possessing no restriction on thickness, and having a self-assembly 3-D structure. Nowadays, scientists in chemistry are able to expand the temperature range from 1 K to almost 100 K and cover the room temperature. Therefore, blue phases are shown to be a promising material to overcome the disadvantage of typical liquid crystal materials.In this dissertation, I will present a comprehensive study of nonlinear optical properties in blue-phase liquid crystal as well as their applications. First, ultrafast optical response of BPLC will be introduced. The results showed that pure transparent BPLC is capable of significantly attenuating a picosecond laser pulse with very high intensity. The mechanism is Maxwell stress-induced flow-reorientation effect. Owing to its tightly bounded structure, BPLC is a promising material for optical switching or optical limiter devices.Furthermore, a detailed study of all-optical image processing with dye-doped BPLC will be revealed. Unlike typical nematic liquid crystals, blue phases are not restricted to sample thickness, and, therefore, Bragg grating condition can be applied to BPLC. The results showed that the diffraction efficiency of BPLC under holographic setup can provide results nearly one hundred times greater than that of the case with nematic liquid crystal. The high diffraction efficiency makes BPLC capable of many different applications, such as hologram reconstruction, phase conjugation, or photorefractive effect. Moreover, an interesting and simple way to prolong the grating memory is discovered, and the detaildynamical studies are shown in the dissertation.