Properties Of Cholesteric Liquid Crystals Research Articles

Magnetically induced tunable exceptional and Dirac points

The photonic properties of cholesteric liquid crystals (CLCs) in an external magnetic field parallel to the helix axis are theoretically investigated for the first time in the short-wavelength band of the spectrum, where exceptional points (EPs) are found. It was shown that at the EP the real parts of two of four wave numbers touch each other or intersect at the absence of the dielectric anisotropy, and their imaginary parts intersect. At the EP the reflection, transmission and absorption show the same magnitudes for two eigenmodes of CLC layer. At anisotropic absorption near the EP there appear points where the magnetically induced transmittance/absorption are observed. The tunability of the EP was shown, i.e. we can change its wavelength by changing the magnitude of the external magnetic field. It was demonstrated the resonant enhancement of the chirality near the EP. Then it was shown that near the EP the two eigenmodes of CLC layer become strongly non-orthogonal.

Effect of different surface alignment layers and temperature changes on bandwidth of Bragg reflection in chiral nematic liquid crystal

In recent years, the practical properties of cholesteric liquid crystals (CLCs) have been widely studied due to their unique feature of selective Bragg reflection. In this study, we investigated the following aspects: (i) the effect of surface alignment using polyvinyl alcohol, polyamide, and polyimide as covering substrate, (ii) the impact of temperature changes on the reflection bandwidth and, consequently, variations in the cholesteric pitch. Furthermore, we extended Li’s four-parameter model to the cholesteric environment using Haller’s assumption and Vuk’s equations for nematic liquid crystals (NLCs) and Fergason’s theory for CLCs. The fit of the experimental data with this model demonstrated an excellent agreement. The experimental data revealed that the S5011 chiral dopant, with left-handedness, used in the NLC environment of the host, exhibits a significant helical twist power (HTP). This leads to the narrowing of the reflection band width with increasing temperature, without causing a noticeable change in the wavelength of the central reflection. This feature highlights the high potential of these types of chiral materials as thermally stable materials for creating selective-reflective optical filters that remain stable with temperature changes, particularly away from the cholesteric to isotropic transition point.

Exceptional points and lines and Dirac points and lines in magnetoactive cholesteric liquid crystals

We investigated the properties of cholesteric liquid crystals (CLCs) being in external static magnetic field directed along the helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, and in the presence of absorption new features appear in the optics of CLCs. We have shown that in this case new photonic band gaps (PBGs) appear. This new PBG is sensitive to the polarization of the incident light. But if the chirality sign of the polarization of the incident diffracting light for the basic PBG (which exist also at the absence of external magnetic field) is determined only by the chirality sign of the CLC helix, then for the second one it is determined by the external magnetic field direction (i.e., on whether the directions of the external magnetic field and the incident light are parallel, or they are antiparallel). We have shown that in this case besides Dirac points there appear also Dirac lines as well as exceptional points and exceptional lines. And moreover, near some of these points and lines the narrow bands or wide bands of magnetically induced transparency, and magnetically induced absorption appear, and near the others a wide coherent perfect absorption band appears that is insensitive to incident light polarization. And finally at some wavelength the same reflection, transmission and absorption takes place for any polarization of incident light, although it is not Dirac point nor exceptional point. This system can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.

Dirac points, new photonic band gaps, and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength-dependent magneto-optical activity parameter.

We investigated the properties of cholesteric liquid crystals (CLCs) being in an external static magnetic field directed along the helix axis. We considered a dichroic CLC, that is, CLC with parameters ReΔ=Reɛ_{1}-Reɛ_{2}/2=0 and ImΔ=Imɛ_{1}-Imɛ_{2}/2≠0, where ɛ_{1,2} are the principal values of the local dielectric permittivity tensor. We have shown that in the case of the wavelength dependence of the magneto-optic activity parameter, new features appear in the optics of dichroic CLCs, in particular, in this case new Dirac points appear. Dirac points are points where there is an intersection of any two wave vector curves (they degenerate) and a linear law of the wave vector dependence on the frequency near these points. And moreover, at some Dirac points photonic band gaps (PBGs) appear; at others, lines of magnetically induced transparency (MIT), that is, a full transmission band appears, in an absorbing medium. In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field, three PBGs of different nature appear: a transmittance band, two narrow lines of MIT, and at others a broadband MIT, etc. This system is nonreciprocal, and the nonreciprocity changes over a wide range. It is observed both for reflection and transmittance and for absorption. The soft-matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can be applied as tunable narrow-band or broadband filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and in other devices.

Optics of helical periodical media with giant magneto-optical activity

The features of the optical properties of cholesteric liquid crystals (CLC) with enormous magneto-optical activity were investigated. It is shown that the system under consideration has the property of nonreciprocal reflection / transmission, absorption and light energy localization. It was shown that this system can work as an almost ideal tunable polarization plane rotator. An external magnetic field leads to a change (tune) of the localized light energy in the CLC layer over a wide range and this property can also find important practical applications.

Nonreciprocal localization of light in magnetoactive cholesteric liquid crystals in an external magnetic field

The optical properties of cholesteric liquid crystals (CLCs) exhibiting magneto-optical activity in the presence of an external static magnetic field have been investigated. The boundary-value problem of light transmission through a finite layer of a magnetoactive CLC at normal incidence of light is considered in the limiting case of the absence of local linear birefringence in the medium. That is a CLC with parameters Re∆=0,Im∆≠0, where ∆=ε1−ε2/2, andε1,ε2 are the principal values of the local dielectric tensor, is considered. In this case, in the absence of an external magnetic field, all linearly polarized incident waves undergo practically the same reflection in the photonic band gap (PBG). An external magnetic field removes this degeneracy. For the first time, the features of the localization of light in a layer of a magnetoactive CLC are investigated, and it is shown that an external magnetic field leads to a change in the localized light energy in the CLC layer and nonreciprocal accumulation of light energy takes place.

Viscoelastic Properties of Cholesteric Liquid Crystals from Hydroxypropyl Cellulose Derivatives

Viscoelastic Properties of Cholesteric Liquid Crystals from Hydroxypropyl Cellulose Derivatives

Effect of Ferroelectric Nanopowder on Electrical and Acoustical Properties of Cholesteric Liquid Crysta

Ferroelectric nano-materials are very sensitive to several external stimuli and have attracted great deal of attention due to their property of improving various properties such as photoluminescence, higher polarization, fast response time, low operating voltage and improved conductivity. For enhancing the physical properties, a proper selection of nano-materials for liquid crystals depends upon various factors such as size, shape, preparation methods, surfactant concentration and amount of doping materials. In the present study an attempt is made to study electrical and acoustical properties of cholesteric liquid crystal after dispersing ferroelectric nano-powder of Barium Titanate (BaTiO3). In addition with this particle size and surface area of pure and nono-particle dispersed liquid crystal were also measured. Our investigation shows increase in Rao’s constant or molar sound velocity, which indicates increase in molecular density indicating a close packing of the material. The measurement of dielectric relaxation at different frequencies gives information about the dynamics of polar groups and molecular motion.

Review on laser properties of cholesteric liquid crystals with multiple-emulsion microstructures

Review on laser properties of cholesteric liquid crystals with multiple-emulsion microstructures

Conductivity and dielectric properties of cholesteryl tridecylate with nanosized fragments of fluorinated graphene

In order to improve the performance properties of electro-optical devices based on mesogenic materials, a cholesteric liquid crystal (cholesteryl tridecylate, X-20) was doped with nanoscale fragments of multilayer fluorinated graphene (GrF). A stable dispersion of X-20 with GrF was obtained. It was shown that the addition of 0.02 wt% of the nanosized GrF slightly affects phase transition temperatures of the mesogen, insignificantly reducing the mesophase temperature range due to increasing of Cr–SmA and decreasing N*–Iso transition temperatures. The dielectric properties of the pure X-20 and X-20/GrF composition were investigated in the frequency range from 20 Hz to 5 MHz and within the temperature range from 303 to 373 K. It was established that the introduction of the dopant is accompanied by an increase of the absolute value of the dielectric permittivity of the composition. At the same time, for the X-20/GrF composition a significant increase of the specific ionic conductivity was observed in the range of all studied phases due to an increase of free charge carriers. At the low frequencies of the electric field, the main contribution to the loss of dielectric permittivity makes the ionic conductivity. For the pure X-20, with the frequency increase, the contribution of the ionic conductivity decreases rapidly. This leads to a serious decrease of the imaginary part of the dielectric permittivity values and at the frequency of around 20 kHz it becomes negligible. For the X-20/GrF composition, this effect can be observed at the frequency of approximately 500 kHz and the imaginary part of the dielectric permittivity increases almost 10 times. The obtained results show the real possibility to modify conductive and dielectric properties of cholesteric liquid crystal by doping it with carbon nanomaterials. This approach opens up new opportunities in creating and improving of microantennas, frequency filters and photonic crystals based on liquid crystals.

Augmenting Bragg Reflection with Polymer-sustained Conical Helix

There has been a recent surge of interest in smart materials and devices with stimuli-responsive properties for optical modulations. Cholesteric liquid crystals (CLCs) are a unique class of light-manipulating materials, and strongly interact with light and other electromagnetic (EM) waves. Because of their intricate helical structure, new properties of CLC have emerged revealing unique optical behavior that has resulted in rewriting Braggs’ law for how light interacts with soft materials. The aim of this work is to push the limits of spectral tuning with a new method of augmenting light-cholesteric interactions using a polymer-sustained conical helix (PSCH) structure. We experimentally explore the reversibility of reflective wavelength modulation and validate the mechanism enhanced by a polymer-sustained helicoidal structure via theoretical analyses. The conical helix structure of a CLC, formed by low-field-induced oblique orientation of cholesteric helices, is comprised of a chiral dopant, a conventional nematic, and bimesogenic and trimesogenic nematics. Polymerizing a small amount of a reactive mesogen in the CLC with an applied electric field produces a templated helical polymer network that enables three switched optical states, including light-scattering and transparent states as well as color reflection in response to an applied increasing or decreasing electric field. An electro-activated PSCH optical film covers a wide color space, which is appropriate for tunable color device applications. We envisage that this PSCH material will lead to new avenues for controlling EM waves in imaging and thermal control, smart windows and electronic papers.

Rheological properties of cholesteric liquid crystals as lubricant additives

Rheological properties of Cholesteryl n-valerate, Cholesteryl decanoate and Cholesteryl myristate which are esters of cholesterol have been studied. Phase transition temperatures and rheological parameters such as viscosity, elastic modulus G[Formula: see text], loss modulus G[Formula: see text] as functions of temperature, shear rate and time are investigated. In frequency sweep test, a higher transition crossover region has occurred for Cholesteryl myristate, whereas for Cholesteryl n-valerate a frequency-independent plateau prevailed for both the moduli. The occurrence of blue phase in Cholesteryl decanoate during temperature sweep measurements is an indication for the rheological support. The results for steady state have informed that cholesteric esters are having non-Newtonian flow behavior in their respective cholesteric phases. The power-law model has explained well the shear rate dependence of shear stress. A few practical applications of these esters as lubricant additives are discussed, too.

Fano resonance in a cholesteric liquid crystal with dye

In this paper, we consider the optical properties of cholesteric liquid crystals (CLCs) with a dye. We investigate the reflection, transmission, and absorption spectra peculiarities. We show that in the presence of dye molecules, in certain conditions, the photonic band gap (PBG) splits into two, three, etc., PBGs. A simple geometric method for determination of the Bragg frequencies and the widths of forbidden bands is proposed. It is shown that we can change the Bragg frequencies and the widths of forbidden bands by changing the parameters of dye molecules. We investigate the light localization peculiarities in the CLC layer in the presence of dye molecules. The peculiarities of the spectra of the photonic density of states and the spectra of light energy density in the CLC layer are also investigated. We show that in the presence of two types of resonances the observation of Fano resonance is possible. The peculiarities of Fano resonance in the CLC with dyes are investigated. We show that this system can be used for achieving a narrow bandwidth spectral response in transmission or reflection.

Facet-dependent electro-optical properties of cholesteric liquid crystals doped with Cu2O nanocrystals

Excellent electro-optical (E-O) performances are essential for high-quality reflective cholesteric liquid crystal (LC) displays, but are often limited by the high driving voltages required by these displays. Dispersing functional nanomaterials into the LCs has emerged as a promising approach to achieve outstanding E-O properties. In this work, we report the facet-controlled E-O properties of a chiral nematic LC (N*LC) doped with cubic, octahedral, and rhombic dodecahedral Cu2O. The outstanding E-O properties of the doped systems are related to the interaction between the liquid crystals and Cu2O dopants with different exposed crystal planes. Doping with octahedral and rhombic dodecahedral Cu2O reduces the stability of the planar state, as a result of both the surface abundance of active Cu atoms that interact with the polarized LC molecules, and the large amounts of vertexes and edges on the crystal surfaces, which accelerate the transition from the planar to the focal conic state under an applied electric field. Rhombic Cu2O is the most effective dopant for improving the E-O properties of the present LCs, resulting in a 65.31% reduction of the threshold voltage. The facet and morphology effects highlighted in this work provide a new pathway to develop excellent energy-saving meso-materials with exposed high-reactivity facets, improving their potential applications in electro-optical technologies and information displays.

Active thermal fine laser tuning in a broad spectral range and optical properties of cholesteric liquid crystal.

In this study, we achieved active fine laser tuning in a broad spectral range with dye-doped cholesteric liquid crystal wedge-type cells through temperature control. The spatial pitch gradient of each position of the wedge cell at room temperature was almost maintained after developing a temperature gradient. To achieve the maximum tuning range, the chiral dopant concentration, thickness, thickness gradient, and temperature gradient on the wedge cell should be matched properly. In order to understand the laser tuning mechanism for temperature change, we studied the temperature dependence of optical properties of the photonic bandgap of cholesteric liquid crystals. In our cholesteric liquid crystal samples, when temperature was increased, photonic bandgaps were shifted toward blue, while the width of the photonic bandgap was decreased, regardless of whether the helicity was left-handed or right-handed. This is mainly due to the combination of decreased refractive indices, higher molecular anisotropy of chiral molecules, and increased chiral molecular solubility. We envisage that this kind of study will prove useful in the development of practical active tunable CLC laser devices.

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

ABSTRACTPhotooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.

Wetting of cholesteric liquid crystals.

We investigate theoretically the wetting properties of cholesteric liquid crystals at a planar substrate. If the properties of substrate and of the interface are such that the cholesteric layers are not distorted, the wetting properties are similar to those of a nematic liquid crystal. If, on the other hand, the anchoring conditions force the distortion of the liquid crystal layers the wetting properties are altered, the free cholesteric-isotropic interface is non-planar and there is a layer of topological defects close to the substrate. These deformations can either promote or hinder the wetting of the substrate by a cholesteric, depending on the properties of the cholesteric liquid crystal.

Effects of Molecular Configuration and Driving Architecture on Electro-Optical Properties of Cholesteric Liquid Crystal Displays

The effects of molecular configuration and driving architecture on the electro-optical properties of cholesteric liquid crystal (ChLC) displays are demonstrated. Two kinds of ChLC cells, composed by mixing the chiral dopant with the positive nematic LC (P-ChLC cell) and with the negative nematic LC (N-ChLC cell), are fabricated in order to make a detailed investigation. Compared with the conventional P-ChLC cell driven by the uniform vertical electric fields, the N-ChLC cell driven by the in-plane switching electric fields shows a higher output light transmittance and a more stable helical molecular configuration, which is attributed to the fact that the focal conic configuration does not exist in the N-ChLC cell. In addition, via the three-terminal-electrode driving architecture, a faster switching response (5.2 ms) of the N-ChLC cell at a low driving voltage is successfully achieved. This study not only exhibits the difference in the electro-optical properties of the P-ChLC and N-ChLC cells, but also provides a driving method for the N-ChLC cell to enhance its electro-optical performance, which is beneficial to the development of fast dynamic optical devices.

Dynamics of cholesteric liquid crystals in the presence of random magnetic fields

Based on dynamic renormalization group techniques, this letter analyzes the effects of external stochastic perturbations on the dynamical properties of cholesteric liquid crystals, studied in the presence of a random magnetic field. Our analysis quantifies the nature of the temperature dependence of the dynamics; the results also highlight a hitherto unexplored regime in cholesteric liquid-crystal dynamics. We show that stochastic fluctuations drive the system to a second-ordered Kosterlitz-Thouless phase transition point, eventually leading to a Kardar-Parisi-Zhang (KPZ) universality class. The results go beyond quasi–first-order mean-field theories, and provide the first theoretical understanding of a KPZ phase in distorted nematic liquid-crystal dynamics.

Light polarization states of a cholesteric liquid crystal probed with optical ellipsometry

Herein a useful methodology to study optical properties of cholesteric liquid crystals (Ch-LC) is proposed by using the Fourier decomposition ellipsometry technique to calculate the Stokes parameters of transmitted and reflected light in the UV–Vis spectral range. Combining Bragg reflection and optical activity we were able to obtain ∼100% of linear or circular light polarization from the Ch-LC sample using achromatic and non-polarized light source. The photonic bandgap and the polarization components can be controlled with the temperature as a result of alterations in the helix pitch of the cholesteric phase. Finally, it is demonstrated the correlation between the dissymmetry factor (g) calculated via the Stokes parameter S3 and the reflection spectrum. The data revealed that the maximum value of S3 is not coincident with the peak of maximum reflection. The reflected or transmitted light analysis via Stokes parameters obtained by ellipsometry showed an alternative and low cost method for optical characterization in Ch-LC.