Cholesteric Liquid Crystal Sample Research Articles

Green synthesized TiO2 nanoparticles dispersed cholesteric liquid crystal systems for enhanced optical and dielectric properties

Green synthesis of nanoparticles (NPs) is a simple, low-toxic, and cost-effective way to make NPs. The decreased optical band gap attributable to oxygen vacancies in green synthesized TiO2 NPs is responsible for the unique features for photocatalytic applications as well as increased optical properties. To explore optical and dielectric characteristics, green synthesized TiO2 NPs were dispersed in the cholesteric liquid crystal (CLC). By adding 27 wt% R-811 chiral dopant to E7 pure nematic liquid crystal, CLC was formed. The reflection color in the red region was observed in the polarizing optical microscope (POM) at ambient temperature, and the alignment was improved as the dispersion concentration was increased. Pure and TiO2 NPs dispersed CLC samples were subjected to UV–Visible absorbance, FE-SEM, EDAX, XRD, TEM, photoluminescence (PL), and FTIR analysis. As compared to pure CLC, the PL intensity increased for the NPs dispersed CLC samples without any substantial change in the emission peak position and the UV absorbance decreased as the NPs concentration increased. Furthermore, when compared to pure CLC, reduction in the optical band gap for TiO2 NPs dispersed CLC was observed. FTIR analysis revealed the impact of nanoparticle dispersion on CLC molecular dynamics. Dielectric spectroscopy results show a decrease in the relaxation frequency for TiO2 dispersed CLC system which is a positive outcome of this investigation. The findings reveal that TiO2 NPs when dispersed in CLC had significantly improved optical and dielectric properties. As a result, the optimal TiO2 NPs concentration in CLCs may have a variety of credential applications.

Influence of UV light intensity on dielectric behaviours of pure and dye-doped cholesteric liquid crystals

This study aims to explore the ultraviolet (UV) light effects on dielectric properties of azo dye methyl red (MR)-doped cholesteric liquid crystal (CLC) composite. The CLC composite formed in the study contains 5CB nematic liquid crystal (LC) and S811 chiral additive. To obtain CLC/MR composite, CLC was dispersed with 2% wt/wt MR azo dye. Dielectric measurements of the CLC and CLC/MR samples were performed using dielectric spectroscopy technique in wide frequency range in the absence of UV light and at various UV light intensities (30, 60 and 90 mW/cm2) for 0 and 40 V. It was observed that the dielectric constant increased with UV light at low-frequency values. Dielectric loss data were utilized to extract relaxation frequency which was observed to increase with UV light exposure. Dielectric anisotropy of CLC sample exhibited typical transition from p-type to n-type, but this behaviour disappears for CLC/MR composite above 60 mW/cm2 UV light intensity. Cole–Cole plots were used for extraction of dielectric relaxation data and obtained results showed that CLC sample shows Debye type relaxation only at 0 V whereas CLC/MR composite exhibits non-Debye type relaxation both at 0 V and 40 V. Moreover, conductivity properties of the samples were also investigated, and dc conductivity values were extracted from experimental ac conductivity values. It was found that MR incorporation increased dc conductivity, also it was significantly increased by UV light exposure and thus similar effect was also observed in current–voltage characteristics of the samples. The results show that azo dye MR molecules are suitable for CLCs since they lead to some enhancements in dielectric and electrical properties.

Steady state solutions in a model of a cholesteric liquid crystal sample

Motivated by recent mathematical studies of Fréedericksz transitions in twist cells and helix unwinding in cholesteric liquid crystal cells [(da Costa et al. in Eur J Appl Math 20:269–287, 2009), (da Costa et al. in Eur J Appl Math 28:243–260, 2017), (McKay in J Eng Math 87:19–28, 2014), (Millar and McKay in Mol Cryst Liq Cryst 435:277/[937]–286/[946], 2005)], we consider a model for the director configuration obtained within the framework of the Frank-Oseen theory and consisting of a nonlinear ordinary differential equation in a bounded interval with non-homogeneous mixed boundary conditions (Dirichlet at one end of the interval, Neumann at the other). We study the structure of the solution set using the depth of the sample as a bifurcation parameter. Employing phase space analysis techniques, time maps, and asymptotic methods to estimate integrals, together with appropriate numerical evidence, we obtain the corresponding novel bifurcation diagram and discuss its implications for liquid crystal display technology. Numerical simulations of the corresponding dynamic problem also provide suggestive evidence about stability of some solution branches, pointing to a promising avenue of further analytical, numerical, and experimental studies.

Chirality-Enhanced Periodic Self-Focusing of Light in Soft Birefringent Media.

We experimentally and numerically show that chirality can play a major role in the nonlinear optical response of soft birefringent materials, by studying the nonlinear propagation of laser beams in frustrated cholesteric liquid crystal samples. Such beams exhibit a periodic nonlinear response associated with a bouncing pattern for the optical fields, as well as a self-focusing effect enhanced by the chirality of the birefringent material. Our results open new possible designs of nonlinear optical devices with low power consumption and tunable interactions with localized topological solitons.

Learning physical properties of liquid crystals with deep convolutional neural networks

Machine learning algorithms have been available since the 1990s, but it is much more recently that they have come into use also in the physical sciences. While these algorithms have already proven to be useful in uncovering new properties of materials and in simplifying experimental protocols, their usage in liquid crystals research is still limited. This is surprising because optical imaging techniques are often applied in this line of research, and it is precisely with images that machine learning algorithms have achieved major breakthroughs in recent years. Here we use convolutional neural networks to probe several properties of liquid crystals directly from their optical images and without using manual feature engineering. By optimizing simple architectures, we find that convolutional neural networks can predict physical properties of liquid crystals with exceptional accuracy. We show that these deep neural networks identify liquid crystal phases and predict the order parameter of simulated nematic liquid crystals almost perfectly. We also show that convolutional neural networks identify the pitch length of simulated samples of cholesteric liquid crystals and the sample temperature of an experimental liquid crystal with very high precision.

Polen Tespiti için Sıvı Kristal Esaslı Yenilikçi Sensör Geliştirilmesi

In this study, we have reported liquid crystal based-sensor for the detection of pollen concentration which triggers an allergy. Cholesteric liquid crystal which is mesophase of thermotropic liquid crystals was used a sensing element in the pollen sensor. The cholesteric liquid crystal samples operating at room temperature were formulated using nematic liquid crystal and chiral dopants in different compositions. The pollen concentration which is embedded in the cholesteric liquid crystal was determined as a function of change in color of cholesteric liquid crystal in the prototype. We demonstrated that the variation of color from green to blue with the increasing pollen concentration using Ocean Optics spectrophotometer. Therefore, the rapid and easy determination of pollen concentration have been became possible by using this prototype. Our sensor will provide simplicity and self-test as a screen of pollen concentration by using the optical advantage of cholesteric liquid crystal as a sensing element.

Small footprint cholesteric liquid crystal laser.

We demonstrate a small optical bench footprint laser assembly based on the small pulsed Nd:YAG laser head SSY-1 for pumping cholesteric liquid crystal (CLC) lasers and illustrate its performance using low molecular weight CLC samples doped with the fluorescent dye PM597. A low lasing threshold, narrow laser line, and far-field interference patterns of the CLC laser were observed using the SSY-1-based laser assembly as the pump. The emission characteristics of the CLC laser are similar to those observed with comparable CLC materials pumped by an order of magnitude physically larger and many orders of magnitude more expensive commercial Nd:YAG laser systems. The small footprint CLC laser demonstrated in this work provides an opportunity for significant size and cost reduction of CLC lasers and fostering their practical applications.

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

ABSTRACTAn anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.

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.

用偏光显微术研究聚合物分散胆甾相液晶微滴形貌

Polymer dispersed cholesteric liquid crystal samples with large droplets were made,and the droplets had strange"gem"texture by polarizing microscope.The new vertical helix and parallel helix configuration in the polymer disperse cholesteric liquid crystal droplets were proposed.Considering the bipolar and radiation configuration of polymer disperse nematic liquid crystal,the"gem"texture may correspond with degeneracy coexistence of multi domain texture of vertical helix and parallel helix configuration.This experiment had basic research significance for liquid crystal optics and reference value to design the new liquid crystal device.

Direction switching and beam steering of cholesteric liquid crystal gratings

This work proposes two mechanisms for switching the direction of stripes in cholesteric liquid crystal (CLC) gratings. The stripe direction depends on the ratio of cell gap to the natural pitch length (d/P0) of a CLC sample. Electrical switching is based on the different pitches at the planar and the transient planar states. Optical switching, however, changes pitch by using the photo-isomerization effect of the azobenzene doped in a CLC sample. Using the two mechanisms, we can switch the stripe directions in two orthogonal directions. Furthermore, the beam-steering capability of CLC gratings also remains effective after switching directions.

Characteristics of wide-band reflection of polymer-stabilised cholesteric liquid crystal cell prepared from an unsticking technique

An unsticking technique was applied for the fabrication of a polymer-stabilised cholesteric liquid crystal (PSCLC) cell. The template of polymer matrix in the planarly aligned cholesteric liquid crystal (Ch-LC) was perfectly preserved by lifting off the hydrophilic antisticking substrate. The desired PSCLC cell could then be obtained by injecting a third Ch-LC sample between two designed templates of PSCLC layer. By adjusting the pitch distribution in the two stacking templates, a reflectivity of nearly 50% incident light could be yielded and the bandwidth of the reflection spectrum could be controlled accurately in the PSCLC cell. Thus, a method to modulate the wide-band reflection of 50% visible light flux range was offered.

Resonant gratings in planar Grandjean cholesteric composite liquid crystals

We propose to induce a two-dimensional (2D) periodic modulation structure into a planar Grandjean cholesteric liquid crystal (CLC) to demonstrate the intrinsic 2D photonic crystal properties of such materials. The structure combines a thin transmission grating and a Bragg reflective grating. One advantage of using CLC is the intrinsic high quality Bragg structure, which can be modulated by an electric field: shifting the wavelength band edge by changing the applied field. Another interesting property is the polarization dependence. The main difference between using CLC Bragg instead of a linear grating is the need to operate with a circularly polarized light, because the CLC modes are circular in such a regime. We present preliminary results obtained with what we believe to be the first switchable photonic CLC (PCLC) sample, made up of a polymer CLC gel.

Lasing in Dye–Doped Photonic Liquid Crystal Devices

We have investigated the effects of pumping laser's polarization and incident angles, dye concentration, cell gap, and lasing wavelength on the lasing efficiency of the dye doped cholesteric liquid crystal (CLC) devices. These factors impact the lasing efficiency due to the existence of the photon bleaching and quench in the dye molecules, the competition of the optical gain and loss in the media, and the helical layers dependent amplification provided by the distributed feedback. The maximum lasing efficiency (∼ 0.5%) was obtained at λ ∼ 608 nm from a 10 µm CLC sample doped with 1 wt% DCM when the pump laser beam is left-handed circularly polarized and the incident angle is normal to the sample.

투과 및 반사 스펙트럼을 이용한 콜레스테릭 액정 필름의 광학 상수 결정

이론적으로 계산된 투과 및 반사 스펙트럼을 실험적으로 측정된 결과에 대해 곡선 맞춤(curve fitting)을 수행하여 콜레스테릭 액정 필름(CLC, cholesteric liquid crystal)의 광학 상수를 결정하였다. 이론적인 투과 및 반사 스펙트럼 계산을 위해 Bewemn의 4<TEX>${\times}$</TEX>4행렬법이 사용되었다. 서로 다른 반사밴드를 갖고 있는 세 종류의 샘플에 대해 이 방법을 적용하였다 CLC샘플에 대한 곡선 맞춤 결과로부터 얻은 굴절률 값을 수평 배향된 LC샘플에 대해 측정된 값과 비교하였는데, 두 결과는 세 CLC 샘플에 대한 곡선 맞춤 결과 오차 범위 내에서 일치하였다. We have obtained optical constants for cholesteric liquid crystal (CLC) films by fitting theoretical spectra of transmittance and reflectance with measured ones. Berreman's 4<TEX>${\times}$</TEX>4 matrix approach was used for the calculation of theoretical spectra. The fitting method was applied to three samples with different reflection bands and fitted results of refractive indices for CLC samples were compared with measured ones for a parallel-aligned LC sample. It was found that the two results were in fairly good agreement within the error of the fitted results for the three CLC samples.

High-frequency acoustic domains in liquid crystals

A mechanism for the formation of one-and two-dimensional domains in planar nematic and cholesteric liquid crystal samples in an ultrasonic field is discussed. A new approach to the analysis of this phenomenon based on the concepts of nonequilibrium hydrodynamics and taking into account the relaxation properties of the mesophase is experimentally substantiated.

A simple experimental technique for investigating phase transitions using the DC Kerr effect

Order-disorder transitions may be detected by the discontinuity in the Kerr constant at the transition point. A simple method of investigating the temperature dependence of the Kerr effect is described and results for the isotropic and blue phases of a cholesteric liquid crystal sample are presented.

Crawling and spiraling of cholesteric fingers in electric field

We show that two types of fingers exist in homeotropic samples of cholesteric liquid crystals of positive dielectric anisotropy : fingers of a first species in which the director field is continuous, and fingers of a second species which are topologically singular and of the same nature as spherulites (also called cholesteric bubbles). When the former are subjected to a low frequency AC electric field, they crawl slowly along their axes whereas the latter drift perpendicularly to their axes and form spirals when one of their ends is pinned on a defect. This work supplements spirals recently observed in Nice by Kamaye and Gilli [8] and by Mitov and Sixou [9] in similar systems.

Chiral Ionbinding in Amphiphilic Cholesteric Liquid Crystals

Abstract The chirality in three amphiphilic cholesteric liquid crystal samples, prepared from three different detergents each synthesised from a chiral precursor with the S configuration is investigated. Diastereoisomers, resulting from the interaction of the D- and L-alanine enantiomers with the chiral micelle surface have been visualized using NMR. The sign of the optical rotation in each sample is determined using a polarizing microscope. Laser diffraction twist measurements are also made. Inversions in the headgroup and in the alanine enantiomers configuration in respect to chirality are discussed.

Amphiphilic cholesteric liquid crystals prepared from the quaternary ammonium surfactantS-(−)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide

Abstract We describe the first successful attempt to produce amphiphilic cholesteric liquid crystals using a chiral quaternary ammonium surfactant, namely S-(−)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide. Only amphiphilic cholesteric liquid crystal samples were made where the micelle structure is related to disc shaped micelles in the achiral ND phase. The surfactant concentration dependence of the twist and the temperature dependence of the twist were made using laser diffraction. The twist in the amphiphilic cholesteric liquid crystal samples was too small to produce total iridescence.