Texture Of Cholesteric Liquid Crystal Research Articles

Enhancing the Efficiency of Color Conversion Micro-LED Display with a Patterned Cholesteric Liquid Crystal Polymer Film.

Color-converted micro-light-emitting diode (micro-LED) displays with wide color gamut, high ambient contrast ratio, and fast response time are emerging as a potentially disruptive technology. However, due to limited optical density and thickness of the color-conversion film, the blue light leakage and low color-conversion efficiency still hinder their widespread applications. In this paper, we demonstrate a patterned cholesteric liquid crystal (CLC) polymer film with two special optical functionalities. On the green and red sub-pixels, the corresponding planar CLC texture acts as a distributed Bragg reflector for the blue light, which in turn improves the color conversion efficiency and expands the color gamut. On the blue sub-pixels, the corresponding focal-conic CLC texture acts as light scattering medium, which helps to reduce the angular color shift. Further analysis reveals that the patterned CLC film can alleviate the crosstalk between green and blue color filters. Therefore, compared to the display system without such a CLC film, our proposed device structure increases the color conversion efficiency by 143% (at ~90% Rec. 2020) and reduces average angular color shift Δu’v’ from 0.03 to 0.018 at the viewing angle with the most severe color shift. Such a patterned CLC film is applicable to all kinds of color-conversion display systems, including organic and inorganic phosphors.

Side-Chain Liquid Crystal Co-Polymers for Angular Photochromic Anti-Counterfeiting Powder and Fiber

Anti-counterfeiting technologies with the features of easy distinguishability, high cost performance, and good processability are needed to meet the demands of a market during the consumption upgrading moment. A series of side-chain liquid crystal co-polymers (SCLCPs) are designed, synthesized, and blended, and the preparation of a series of angular photochromic materials that have different center reflection wavelengths in the visible and near infra-red region is reported in this article. Differential scanning calorimetry and polarized optical microscopy were utilized to characterize the phase transition behaviors and self-assembling structures of the SCLCPs. The selective reflection properties were characterized with a UV/VIS/IR spectrum study and further verified by scanning electron microscopy. The results showed that the SCLCPs had the desired reflection wavelengths and thermal stability. The SCLCPs could easily form a planar texture of cholesteric liquid crystal and, depending on the good processability, anti-counterfeiting powders and fibers with angular photochromic features were prepared and characterized to prove the potential applications of the SCLCPs in anti-counterfeiting labels.

Effects of pretilt angle and anchoring energy on alignment of uniformly lying helix mode

ABSTRACTWe investigated the effects of pretilt angle and anchoring energy on the formation of a uniformly lying helix (ULH) texture of cholesteric liquid crystals (CLCs). Pretilt angle was controlled by the thickness of a vertical alignment layer coated onto a planar alignment layer. In the given pretilt angle, the anchoring energy was enhanced by introducing reactive mesogen to the vertical alignment material. To characterise quantitatively the formation of the ULH texture we introduced reflectance, governed by areas of the ULH region and the planar-aligned CLC region. We found that the ULH texture was more widely formed under the condition of higher pretilt angle and weaker anchoring energy. Also, a more uniform alignment of the ULH texture was achieved with the higher pretilt angle even under the same anchoring energy condition.

Photoswitchable and dye-doped bubble domain texture of cholesteric liquid crystals.

We demonstrate control of the transmittance of the naturally formed bubble domain (BD) texture of cholesteric liquid crystals (CLC) with negative dielectric anisotropy confined into a cell with homeotropic surface anchoring. By using a photosensitive chiral dopant with variable helical twisting power under light irradiation, control of packing density of bubbles, spatial patterning, and all optical switching between bistable states with different optical densities is achieved. By introducing dichroic dye into the CLC mixture, a bistable and switchable by applied electric field guest-host system is obtained. The light dimming properties of dye-doped BD CLC systems may lead to development of a wide range of applications.

Transmittance and reflectance of cholesteric and blue phase liquid crystal

Blue phase liquid crystal display (BPLCD) is emerging as next-generation display, because of its fast response speed and very wide viewing angle. However, it has some weak points to be improved. The light leakage at the dark state affects the contrast ratio, and needs to be analyzed and improved. Considering the double-twist structure of blue phase liquid crystal (BPLC) and the simple twist structure of cholesteric liquid crystal (ChLC), the two twist structures are similar. The transmittances and reflectances of planar and focal conic texture of cholesteric liquid crystal and blue phase II liquid crystal are simulated by finite-difference time domain (FDTD) method. The FDTD method is based on the Maxwell’s equation, and can calculate the optical rotatory power directly. The effective optical rotatory powers of the three liquid crystal states are proposed and compared using the light leakages at the cell with crossed and parallel polarizers. The results show that the transmittance of BPLC with crossed polarizers is lower than that of planar texture and larger than that of focal conic texture of ChLC. The optical rotation of BPLC is not the same at any point in one periodic cross section in the light path because the liquid crystal arrangement is complex, the effective optical rotatory power is defined as the average value of the optical rotatory powers at all points. Comparing with the optical rotatory powers of planar and focal conic textures of ChLC, the optical rotatory power of BPLC is less than that of planar texture and larger than that of focal conic texture. Moreover, the Bragg reflections are also simulated, the results show that blue phase liquid crystal is similar to planar state cholesteric liquid crystal, only the reflection intensity is smaller, and no obvious Bragg reflection appears in focal conic state cholesteric liquid crystal. Considering the optical rotation and Bragg reflection, the light leakage and reflective light of BPLCD cannot be ignored if the helix pitch is not less enough, however, these of focal conic texture of ChLC are very small compared with those of BPLC, as a result, the focal conic texture of ChLC can replace blue phase liquid crystal to obtain the good dark state and high contrast ratio.

Diffractive Liquid Crystal Pressure Detector

In this work we present the study of reconfigurable by an applied electric field and sensitive to mechanical deformations bubble domain (BD) texture of cholesteric liquid crystal confined into a cell with homeotropic alignment conditions. We discuss the phenomenon of metastability of the BD texture, optimal conditions for obtaining and reconfiguring of the BD texture with applied electric field and pressure. We present and analyze time response curves to an applied electric field with the threshold pressure and suggest the application of the BD texture as a pressure detector.

Cholesteric liquid crystal–carbon nanotube composites with photo-settable reversible and memory electro-optic modes

The photoresponsive electro-optical composites based on cholesteric liquid crystal (CLC) with optically controlled chirality and a minute amount of carbon nanotubes (CNTs) are studied. In cells with homeotropic anchoring, these composites exhibit a transition from fingerprint texture to homeotropic nematic texture in the course of photoinduced unwinding of the cholesteric helix. Compared with the CLC counterpart, this transition is much delayed, because of the stabilization of cholesteric filamentary domains by CNTs. The CLC-CNT composites demonstrate dual-mode operation with optical switching between reversible and memory mode. It is found that the memory response is associated with the elastic network of filamentary cholesteric domains that stabilizes the planar CLC texture reached in an electric field. In turn, the reversible mode corresponds to the unwound cholesteric state. Potential applications of this effect are discussed.

P.99: Alignment Peculiarities of Cholesteric Liquid Crystals on the Surfaces Processed by Plasma Beam

AbstractIt is demonstrated that plasma beam alignment process allows one to combine uniformity of planar texture and stability of focal conic texture of cholesteric liquid crystals (CLC) and thus obtain highly contrast and truly bistable samples. It provides suitable anchoring conditions for ULH texture. It can also be readily applied for the alignment patterning that opens new prospects for CLC devices.

Reverse-Mode Polymer-Stabilized Dual-Frequency Cholesteric Texture Cell for Dual Mode Operations

We demonstrated a polymer-stabilized dual-frequency cholesteric texture (PSDFCT) cell that functions smoothly in both dynamic switch and steady bistable usage, dual mode operation. The PSDFCT cell is a reverse-mode texture device containing a cholesteric liquid crystal (ChLC) and polymeric mixture. As polymer concentration increases, the PSDFCT cell exhibits increasing operational voltage and diminishing contrast ratio, which results from the increasing polymer network anchoring force to firmly stabilize the ChLC texture. Switching from planar texture to focal-conic texture is triggered by applying a threshold alternating current (AC) bias at low frequency (60 Hz). The opposite change, from focal-conic to planar texture, is achieved by varying the AC bias frequency from low (60 Hz) to high (35 kHz). The dual dielectric anisotropies of ChLC allow efficient dynamic switching by varying the bias frequency. Under bistable operation, a larger AC bias voltage generates polymer network distortion, directly affecting the ChLC mixture. The reciprocal switch, between planar texture and focal-conic texture, is performed at the greater AC bias by varying bias frequency.

胆甾相液晶盒贝纳德效应实验

The conception of Bernard effect and cholesteric liquid crystal texture are introduced.The sample of cholesteric liquid crystal in planar texture is prepared,the texture change with temperature field by polarizing microscope is observed.The Bernard effect or pane grate effect under the cleaning temperature is appeared.The experimental phenomena show that the temperature field induced the liquid crystal molecule convection,the birefringence interference is occurred,and liquid crystal director distribution is periodical variation,so that the microcosmic Bernard effect pattern formed.The experimental phenomena wait for explanation rightly,and have some significance for liquid crystal physics basic research.

Effect of Optical Properties of Planar Texture on Some Lasing Characteristics Dye-Doped Cholesteric Liquid Crystals

Effect of optical properties of a planar texture and its thickness on lasing spectrums and thresholds in dye-doped cholesteric liquid crystal (CLC) of steroid type is explored. Transition from the qualitative planar texture to the poor texture quality is accompanied by change of characteristic mode structures and by shift of barycentre in the long-wave side and the considerable growth of the lasing threshold. It is found that in the CLC texture created by substrates with perpendicular directions of orientation the stable single-mode lasing takes place. The obtained results show that in steroid CLC, unlike induced one, lasing spectrums could be described by the coupled wave model.

Anisotropy of the local field of the light wave in cholesteric liquid crystals

The experimental values of the Lj components of the Lorentz tensor have been obtained for the first time for the quasinematic layer of the cholesteric phase and in the smectic phase A for homologs of cholesteric fatty ethers using the dispersion dependences of the refraction indices for the planar texture of cholesteric liquid crystals (CLCs). The dependence of the Lj components on the homolog number, mesophase temperature, the magnitude of birefringence, and the change in the orientational ordering of molecules in the cholesteric phase and at the cholesteric-smectic A phase transition was determined. Isotropization of the Lorentz tensor L and the local field tensor f was found for CLCs when the birefringence of LCs and the anisotropy of molecular polarizability decreased simultaneously. The anisotropy Δf was found to be negative for the quasinematic layer of CLCs and the smectic phase in the visible range of the spectrum. The values of Lj, obtained with known local field models for CLCs and smectics A, gave positive Δf irrespective of the chemical structure of molecules, optical anisotropy of LCs, and the spectral region, which contradicts to the experiment.

Photo-patterning micro-mirror devices using azo dye-doped cholesteric liquid crystals

A simple method for fabricating patternable micro-mirror devices by photo-induced alignment of dye-doped cholesteric liquid crystal (CLC) is demonstrated. The CLC texture can be changed from random distribution to nearly perfect planar by the photo-excited adsorbed dyes. This structure transformation leads to a substantial reflectivity increase. Using this photo-patterning technique, one- and two-dimensional micro-mirror arrays which function as gratings are also demonstrated.

Effect of small liquid crystal molecules on the driving voltage of cholesteric liquid crystal displays

In order to achieve a lower driving voltage of a cholesteric liquid crystal display (Ch-LCD) we formulated new nematic liquid crystal mixtures with dopants. The interesting thing is that the driving voltage is decreased more by adding the low molecular weight of liquid crystal materials in the cholesteric mixtures without decreasing its reflectivity. The cholesteric mixture is optimized by adding 20 wt% of CN-007 enough to drive the display, necessary for using a commercial STN driver. Homogeneous alignment layers provide a higher reflective panel because of stabilizing the planar texture of cholesteric liquid crystal, while homeotropic alignment layers provide a higher contrast ratio because of stabilizing focal-conic structure. The rubbing condition depending on the driving scheme also affects the panel properties. The combination of the panel structure and the process condition of cholesteric mixtures make the Ch-LCD high reflectivity and contrast ratio. Two 5.7 in. VGA monochromic cholesteric reflective liquid crystal displays with a high resolution like this folding type are demonstrated using a STN driver.

Mechanism of Relaxation from Electric Field Induced Homeotropic to Planar Texture in Cholesteric Liquid Crystals

We have investigated the relaxation process from a homeotropic structure to the planar one in a cholesteric liquid crystal. The relaxation occurs after switching off the electric field. We have developed a new experimental photography technique with a time resolution of about 20μs and applied it to the investigation of textural changes during the transition. After the liquid crystal goes through a defect-free transient planar structure, the development of the cholesteric helices occurs in two stages. Direct observations have shown that formation of the final equilibrium state involves a nucleation process. We used optical retroreflection spectroscopy and measurement of light scattering to observe changes of the cholesteric pitch and orientation of helices as well as the size of domains in time for various treatments of the glass substrates. We have also proposed a theoretical model that describes the experimental data satisfactorily.

Aqueous dispersions of cholesteric liquid crystals and their optical properties

This paper reports on a theoretical and experimental investigation of light transmission by a layer of aqueous suspension of polymer dispersed cholesteric liquid crystals (CLC) with a helical molecular structure. The transmission spectra and the spectral order parameter of the supramolecular texture of CLC confirm the high degree of ordering of CLC molecules in the spherical cells. Cell morphology and the spectral dependences of light transmission by the plane-parallel layer of CLC aqueous dispersion are considered.

Monodomain of the Focal Conic Texture in Cholesteric Liquid Crystals

The growth process of isolated focal conics is investigated under orthogonal electric and magnetic fields. It is found that isolated focal conics elongate parallel to the magnetic field. This results in the monodomain of densely packed focal conics having the helical axis perpendicular to the orthogonal fields.

Forming Condition of Striped and Scrolled Texture in Cholesteric Liquid Crystals

In a wedge-shaped cell with homeotropic boundary conditions, the forming process of cholesteric textures is observed under a magnetic field. It is found that the type of cholesteric texture depends on the difference between the induced pitch in the cell and the equilibrium pitch of the sample.

The field-induced instability in the planar texture of cholesteric liquid crystals

A more accurate calculation is given of both the striped perturbation pattern and the square grid perturbation pattern that are formed in the planar cholesteric layer as soon as the applied field reaches a threshold value. Notably, it is shown that the distance between the boundaries must exceed a critical value in order to allow such a distortion. The values of the critical field and the wavelength of the distortion are compared with existing calculations.

Bistability behavior of texture in cholesteric liquid crystals in an electric field

With the homeotropic boundary condition, the field-induced cholesteric-to-nematic texture transition can show a large hysteresis effect such that at the same field strength either the cholesteric or the nematic texture is stable.