Structure Of Chiral Liquid Crystals Research Articles

Sequence of Three-, Two-, and One-Dimensional Structures Formed from a Cholesteric Liquid Crystal at Change in the Chirality

In this work the formation of modulated structures in chiral liquid crystals is studied. For different chiral nematics that at low temperature form the usual (with twist in one direction) cholesteric phase, we found that at high temperature in the vicinity of the phase transition into isotropic liquid a universal sequence of structural transformations is observed. Planar cholesteric transforms at short helical pitch into the three-dimensional phases with cubic symmetry (Blue Phases), in two-dimensional structure at intermediate helical pitch and in one-dimensional structure in the plane of the sample at large pitch. The structures possess periodic orientational and translational order on scales much larger than molecular scale. Optical measurements were made on ordered structures obtained near the transition temperature to the isotropic phase. Possible reasons of formation of the structures are discussed on the basis of existing experimental data and theoretical considerations.

Optical Imaging and Analytical Design of Localized Topological Structures in Chiral Liquid Crystals

We combine numerical modeling and analytical design techniques to study several of the most common localized topological structures in frustrated chiral nematic liquid crystal cells. An energy minimization procedure is applied to the lattice model to simulate the director field distributions. These distributions are also approximated using the suitably designed analytical ansatz. We present both simulated and approximated results for optical polarizing microscopy textures and different visualizations of director field structure such as distributions of the azimuthal director angle and isolines for the normal component of the director in coordinate planes. The ansatz correctly mimicked the geometry and optical properties of the solitonic structures under consideration.

The Nematic Chiral Liquid Crystal Structure of the Cardiac Myoarchitecture: Disclinations and Topological Singularities.

This is our second article devoted to the cardiac myoarchitecture considered as a nematic chiral liquid crystal (NCLC). While the first article focused on the myoarchitecture of the left ventricle (LV), this new article extends to the whole ventricular mass and introduces the concept of disclinations and topological singularities, which characterize the differences and relationships between the left and right ventricles (RV). At the level of the ventricular apices, we constantly observed a vortex shape at the LV apex, corresponding, in the terminology of liquid crystals, to a "+1 disclination"; we never observed this at the RV apex. At the level of the interventricular septum (IVS), we identified "-1/2 disclinations" at the anterior and posterior parts. During the perinatal period, there was a significant difference in their distribution, with more "-1/2 disclinations" in the posterior part of the IVS. After birth, concomitant to major physiological changes, the number of "-1/2 disclinations" significantly decreased, both in the anterior and posterior parts of the IVS. Finally, the description of the disclinations must be considered in any attempt to segment the whole ventricular mass, in biomechanical studies, and, more generally, for the characterization of myocardial remodeling.

Dynamics of chiral liquid crystals between cylinders using Landau-de Gennes theory

ABSTRACT This study employs the Landau-de Gennes theory to investigate the behaviour of chiral liquid crystals (CLCs) between concentric and eccentric cylinders under different flow conditions. This theory was implemented in dynamic finite element simulations to solve the evolution of the microstructure of CLCs and couple it with linear momentum balance equation (modified Navier–Stokes equation) to capture the structure of CLCs. The study focused on the microstructure formation of CLCs and their performance as lubricants under various chirality strengths (θ), Deborah numbers (De), and eccentricity of eccentric cylinders. The hexagonal structure of the CLCs was found between cylinders at low De (0.001), where the chiral term predominates. Moreover, the number of hexagonal structures between cylinders was shown to be significantly correlated to the chiral strength. We showed that increasin De resulted in vanishing hexagonal pattern of CLCs and the De value at which the hexagonal pattern vanishes depends on the chiral strength. We also observed that a higher eccentricity ratio significantly increased the pressure force per length on the inner cylinders and caused more non-uniformity in the structure. Based on the findings, we concluded that the performance of CLCs as lubricants is greater at higher θ and eccentricity and lower De.

(Invited) One-Dimensionally Aligned Quantum Rods for Generation of Highly-Pure Circularly Polarized Light with High Light Intensity

Converting non-polarized visible light (natural light) into circular polarized (CP) light is currently an important issue because of the attractive properties of CP light such as acceleration of photosynthesis and enhancement of conversion efficiency of photovoltaics[1,2]. Until now, various methods for creating CP visible light, such as filtration by circular polarizer, selective reflection by chiral liquid crystal structures, and circular polarized luminescence (CPL) from chiral luminophores[3], have been reported. However, creation of highly-pure CP visible light with high light intensity is still considered as challenging issue. In this presentation, we propose an approach for creating highly-pure CP visible light with high light intensity by converting linearly polarized luminescence (LPL) using quarter-wave plate.In this work, we put spotlight on colloidal semiconductor quantum rods (QRs) because of its unique advantages described as follows: (1) Absorption coefficient and emission quantum yield are relatively high. (2) Sharp emission peaks are expected. (3) Variety of QRs with different emission colors can be excited by irradiation of single-wavelength light. We selected CdSe/CdS core/shell QRs as LPL generating 1D nanostructure. CdSe/CdS core/shell QRs were synthesized by hot-injection method. As shown in Figure 1a, the obtained suspension showed orange-colored strong emission under UV light irradiation. TEM image of the dried sample of the QRs toluene suspension indicates that the average length, average width of the obtained rod-like nanostructures were about 30 nm and 4.5 nm, respectively. After QRs/toluene suspension was mixed with poly(ethylene-co-vinyl acetate) (EVA)/toluene solution, the mixture was casted on glass slide and dried it under air. Then, the prepared composite polymer film was stretched in one direction and was attached on glass slide. When the luminescence from the QRs/EVA stretched film was observed under the linear polarizer placed in parallel direction to the stretching direction, the observed light was brighter than that of perpendicular direction (Figure 1c). By measuring the photoluminescence of parallel (I//) and perpendicular (I⊥) components under depolarized excitation at 450 nm, the QRs/EVA stretched film showed clear LPL property with constant ratio (CR) = 5.4, defined as CR = I// / I⊥ (Figure 1b). Furthermore, when the QRs/EVA stretched film was combined with a quarter-wave plate placed at 45° to the stretching direction and was observed through the right-handed (RH) or left-handed (LH) circular polarizer, brighter light was observed through LH circular polarizer (Figure 1d). By quantitative evaluation of RH- and LH-CP light components, total emission includes 82% of LH-CP light and 18% of RH-CP light (Figure 1e). These results indicate that the combination of the QRs/EVA stretched film and quarter-wave plate generate highly-pure CP light which can be distinguished by the naked eye (Figure 2d). Interestingly, the handedness of CP light can be switched by simply changing the angle between fast axis of the quarter-wave plate and stretching direction of LPL films (Figure 1f). In this presentation, we will also discuss about multiplexing of optical information by using CP light conversion.[1] W. Qin et. al., J. Phys. Chem., 2018, 122, 12566-12571. [2] B. Hu et. al., ACS Photonics, 2017, 4, 2821-2827. [3] P. Duan, M. Liu et. al., Adv. Mater. 2020, 32, 1900110. Figure 1

Voltage-controlled formation of short pitch chiral liquid crystal structures based on high-resolution surface topography.

Chiral nematic liquid crystals (CLCs) offer interesting perspectives for device applications and are fascinating materials to study because of their ability to self-assemble into complex structures. This work demonstrates that narrow lines of electron-beam resist on top of an ITO coated glass surface can dramatically influence the formation and growth of short pitch chiral superstructures in the bulk. By applying a voltage to the cell, directional growth of CLC structures along the corrugated surface can be controlled. Below the electric unwinding threshold, chiral structures start to grow along the grating lines with their helical axis parallel to the substrates. This results in a uniform lying helix-like structure at intermediate voltages and a chiral configuration with periodic undulations of the helical axis at low voltages.

Revolving supramolecular chiral structures powered by light in nanomotor-doped liquid crystals.

Molecular machines operated by light have been recently shown to be able to produce oriented motion at the molecular scale1,2 as well as do macroscopic work when embedded in supramolecular structures3-5. However, any supramolecular movement irremediably ceases as soon as the concentration of the interconverting molecular motors or switches reaches a photo-stationary state6,7. To circumvent this limitation, researchers have typically relied on establishing oscillating illumination conditions-either by modulating the source intensity8,9 or by using bespoke illumination arrangements10-13. In contrast, here we report a supramolecular system in which the emergence of oscillating patterns is encoded at the molecular level. Our system comprises chiral liquid crystal structures that revolve continuously when illuminated, under the action of embedded light-driven molecular motors. The rotation at the supramolecular level is sustained by the diffusion of the motors away from a localized illumination area. Above a critical irradiation power, we observe a spontaneous symmetry breaking that dictates the directionality of the supramolecular rotation. The interplay between the twist of the supramolecular structure and the diffusion 14 of the chiral molecular motors creates continuous, regular and unidirectional rotation of the liquid crystal structure under non-equilibrium conditions.

34.1: Invited Paper: Slowing Light in Liquid Crystals

AbstractSteep dispersion is obtained by exploiting the large optically induced refractive index change in liquid crystal media. This effect is observed either in devices made by the association of liquid crystals with a photoconductive substrate or by employing the photo‐isomerization of azo‐dye molecules hosted in a chiral liquid crystal structure. When interacting in these media, a pulse of light that is relatively long becomes delayed, hence, slow light phenomena are obtained with steep dispersion, a feature that can be exploited for improving the performances of optical sensors.

Nanoconfinement-induced structures in chiral liquid crystals.

We employ Monte Carlo simulations in a specialized isothermal-isobaric and in the grand canonical ensemble to study structure formation in chiral liquid crystals as a function of molecular chirality. Our model potential consists of a simple Lennard-Jones potential, where the attractive contribution has been modified to represent the orientation dependence of the interaction between a pair of chiral liquid-crystal molecules. The liquid crystal is confined between a pair of planar and atomically smooth substrates onto which molecules are anchored in a hybrid fashion. Hybrid anchoring allows for the formation of helical structures in the direction perpendicular to the substrate plane without exposing the helix to spurious strains. At low chirality, we observe a cholesteric phase, which is transformed into a blue phase at higher chirality. More specifically, by studying the unit cell and the spatial arrangement of disclination lines, this blue phase can be established as blue phase II. If the distance between the confining substrates and molecular chirality are chosen properly, we see a third structure, which may be thought of as a hybrid, exhibiting mixed features of a cholesteric and a blue phase.

Slow and stored light by photo-isomerization induced transparency in dye doped chiral nematics

Decelerating and stopping light is fundamental for optical processing, high performance sensor technologies and digital signal treatment, many of these applications relying on the ability of controlling the amplitude and phase of coherent light pulses. In this context, slow-light has been achieved by various methods, as coupling light into resonant media, Brillouin scattering in optical fibers, beam coupling in photorefractive and liquid crystal media or engineered dispersion in photonic crystals. Here, we present a different mechanism for slowing and storing light, which is based on photo-isomerization induced transparency of azo-dye molecules hosted in a chiral liquid crystal structure. Sharp spectral features of the medium absorption/dispersion, and the long population lifetime of the dye metastable state, enable the storage of light pulses with a significant retrieval after times much longer than the medium response time.

Structural studies on different types of ferroelectric liquid crystalline substances

Structural studies on different types of ferroelectric liquid crystalline substances Structural studies of ferroelectric liquid crystalline substances of different molecular structure have been reviewed. The discussion of the results deals mainly with the structure of chiral liquid crystals forming the smectic mesophase with ferroelectric order, as the research on ferroelectric liquid crystals, due to their high potential for application in electro-optics and photonics, has recently become one of the most attractive fields. Based on data of polarizing optical microscopy and X-ray diffraction obtained for unoriented samples we have identified the type of the mesophases, the temperature of the phase transitions and outlined the phase diagrams. An analysis of the relation between the molecular conformation and different types of phase transitions has been performed. The layer spacing and the average intermolecular distances have been determined for all studied phases from the positions of the small and the large angle diffraction peaks, respectively.

Optically generated adaptive localized structures in confined chiral liquid crystals doped with fullerene

We report the facile optical creation of switchable localized structures in chiral liquid crystals doped with fullerene. In a cholesteric cell unwound by vertical boundary conditions, the initially dispersed fullerenes are deposited from the bulk to the surface of confining glass plates by low-intensity illumination. This alters the surface boundary conditions and allows for the creation of localized particlelike structures with twist-bound defects (dubbed Torons) that are controlled by electric fields and arranged into patterns of interest for photonic and electro-optic applications.

Chiral Smectic C Structures of Virus-Based Films

Long-range ordered virus-based films were fabricated using M13 phages (viruses) which were aligned and assembled using the meniscus phenomena. Their ordered structures and morphologies were studied and characterized using polarized optical microscopy (POM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). M13 virus particles which are 880 nm in length were the basic building block of the fabricated films. Due to the unique micrometer length scale of viruses, the smectic ordering of virus particles could be easily visualized using conventional microscopy techniques and compared with a theoretical model of chiral liquid crystal structures. From the results of POM, AFM, and SEM, the viral films were determined to have a chiral smectic C structure. By a comparison of the ordering of film formation as a function of virus concentration and the formation of bundle-like domain structure found in viral thin films, a mechanism of film formation is suggested. These virus-based film structures are organized on multiple length scales, easily fabricated, and allow integration of aligned semiconductor and magnetic nanocrystals. These self-assembled hybrid materials may have applications in miniaturized self-assembled electronic devices.

Resonant X-ray diffraction study of a new brominated chiral SmCA* liquid crystal

We present a new high energy resonant x-ray scattering study of a brominated chiral liquid crystal material exhibiting antiferro, ferri and ferroelectric smectic C phases. The absorption K-edge of bromine is at much higher energy (13.49 keV) than the sulfur K-edge (2.47 keV) which should enable a wider variety of experimental geometries. A weak resonant signal is detected on free standing films in the SmCA* phase only. Interestingly, the signal is stronger in binary mixtures with a bromine free thiobenzoate compound. This observation suggests that most chiral liquid crystal structures could be studied by resonant scattering after incorporation of a molecule bearing a resonant atom as a probe. Resonant x-ray diffraction profiles are also calculated for some recently proposed distorted clock models of the 4-layer ferrielec- tric phases. Finally, the influence of defects is investigated.

Self-diffraction and relaxation in a resonant liquid crystal medium

Self-diffraction in a resonant liquid crystal medium has been investigated by the dynamic holographic technique. The light-induced anisotropy of refractive index has been determined and compared with the value of thermal nonlinearity. The multi-regime relaxation of the holographic grating has also been investigated. Doping with ketocyanine molecules has been shown to cause deformation of the chiral liquid crystal structure.

Inhomogeneous two-dimensional structures in liquid crystals

Localized axisymmetric inhomogeneous states with a continuous distribution of the director field can exist in nematics. Such structures are compressed into dense filaments under the influence of a magnetic or electric field. It is hypothesized that the given states can be achieved in filamentary nematic textures. This model is an alternative to the conventional disclination model. Two types of lattices of axial structures can exist in the entire range of existence of the modulated state. Axial structures with a kernel of finite radius can exist in cylindrical capillaries. The structure and equilibrium dimensions of the axial states are easily altered over a wide range under the influence of an applied field. The feasibility of utilizing isolated axial structures and lattices of such structures in optical data processing and imaging devices is discussed. The most promising outlook in this regard is for modulated states and axial structures in chiral liquid crystals exhibiting spontaneous polarization.

Deflection of light by phase boundaries in antiferroelectric liquid crystals

Light deflection phenomena in an antiferroelectric chiral smectic liquid crystal MHPOBC were investigated. At electric field induced phase transitions between antiferroelectric (AF) state to ferroelectric (F) one, characteristic deflected lights were observed; dependence of the angle of deflection on temperatures and on angles of incidence were quite different from those of diffraction lights due to a periodical change of refractive index caused by helical structure of chiral liquid crystals. On the basis of numerical analysis for domain walls of ferroelastic crystals, it is concluded that the light deflection is due to refraction and reflection phenomena at the phase boundaries between AF and F states coexisting at the field induced phase transition.

Electro-optic effect in an optically active nematic chiral liquid-crystal structure

A new electro-optic effect in an optically active nematic chiral structure beyond Mauguin's region has been studied. The effect is revealed in the absence of oscillations of the transmission-voltage curve, low requirements for orienting surfaces, and the ability to function at any direction of polarization of incident light. The limit relationships among wavelength, helical path, and optical anisotropy of the nematic matrix are determined. The dispersion properties of the effect are studied, and the possibility of its application for white-light modulation is shown as well. The control voltage is 6 V, and the time of switching is 50 µs.