Twisted Nematic Research Articles

Electrical liquid crystal integration with VCSEL arrays for tunable orthogonal polarization laser

Vertical-cavity surface-emitting laser (VCSEL) arrays that can emit orthogonally polarized light have shown broad application potential and market demand in the fields of optical communication, optical sensors, and biomedicine. Two kinds of polarization-switching VCSEL arrays based on anti-parallel (AP) oriented liquid crystal (LC) external cavities and twisted nematic (TN) oriented LC external cavities are proposed in this paper. The light is independently guided by the LC electrically, achieving uniform lasing of linearly polarized light with mutually orthogonal polarization modes and equal power. The uniformity and polarization tunable ability of the lasing orthogonally polarized light power is guaranteed by the integration of the LC external cavity and the VCSEL array.

Four-bit input linear optical quantum computing with liquid crystal devices

Four-bit input linear quantum computing with liquid crystal (LC) devices was studied using the Deutsch–Jozsa algorithm. A laser beam is split into four paths and passed through a configuration of twisted nematic (TN) LC device/(λ/2 waveplate)/TN LC device for computation. In order to minimize the effect of photon loss in the light path, a 4 × 4 matrix analysis was done for optimization. We fabricated a prototype for four optical-path linear quantum computing. As a result, we obtained changes in the number of photons counting according to Deutsch–Jozsa’s algorithm under the extremely weak light state.

Optical measurements of the twist constant and angle in nematic liquid crystal cells

We present a reliable optical method for measuring the twist elastic constant K2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {K}_2$$\\end{document} and for assessing the total twist angle in a standard nematic twist cell. The method relies on the use of a non-standard configuration of crossed polarisers and a twist cell, which allows us to measure accurately the twist-cell parameters by reducing the degeneracy between them. Grid patching and an efficient beam propagation method are utilised in the numerical models used for fitting the experimental data. The modelling shows that the polarisation dynamics in a twist cell is non-trivial and much more complex than in a planar cell. The twist elastic constant of three commonly used liquid crystals (5CB, 6CHBT and E7) was successfully extracted from cross-polarised intensity measurements.

Optimization of retarder parameters for switchable privacy mode automotive displays under asymmetric viewing conditions

A switchable privacy mode (SPM) display should not be seen by a driver during driving (privacy mode), but it needs to be seen by the driver and a co-driver when a car is stopped (share mode). In this work, a SPM display was designed using a bottom twisted-nematic (TN) liquid crystal display (LCD) and a top in-plane switching (IPS) LCD where a half-wave plate (HWP) is located between the two LCD panels. The SPM display in front of the co-driver was considered, which is seen under asymmetric viewing conditions. The orientation of the slow axis, retardation dispersion, and biaxiality of the retarder were varied to maximize the SPM’s optical performance in the driver’s and co-driver’s viewing directions. Transmittance at the driver’s viewing direction can be reduced from 4.22 to 0.03% in privacy mode by replacing the positive dispersion retarder with a negative dispersion retarder.

Polarization‐independent liquid crystal device with microdomain twisted structure

AbstractThe polarization‐independent liquid crystal (LC) phase modulators can greatly improve the efficiency and reduce the complexity of the optical system. However, it is difficult for the LC phase modulator to achieve both good low‐loss modulation and polarization‐independent properties. A LC phase modulator of single layer microdomain twisted structure has been proposed to achieve good polarization independence and low loss in phase modulation. However, the multi‐exposure in fabrication is difficult to realize in production process. To simplify the fabrication process, an alternate LC (ALC) mask is proposed with the alternate arrangement of twist‐nematic (TN) LC and nematic LC. The fabrication process is optimized to two steps exposure with the usage of ALC and is suitable for large‐amount‐production. The measurement results showed that the single layer microdomain twisted LC phase modulator may get very low polarization dependence, about 3.52% under 2π phase modulation depth, with low loss. The device shows good potential application in optical communication, wearable device, and display.

Quantum optical control experiment using Mach–Zehnder interferometer and twisted nematic liquid crystals

A Mach–Zehnder interferometer with an optical phase control by a twisted nematic (TN) liquid crystal (LC) device was studied. In a typical TN LC device, a change in the optical rotation state of π/2 can be obtained by applying a voltage of about 5 V with a cell thickness of several-micron. The TN cell is inserted into one optical path in the Mach–Zehnder interferometer. We obtained a pattern change when voltage was applied to the LC cell due to the change of the optical interference state. As a result, a change in the number of photons under weak light conditions is observed.

Rigid-flex stamped indium tin oxide film for convertible self-alignment

The alignment layer process, which is a key element for liquid crystal (LC) displays, was investigated for efficiency improvement and for suggesting a new mode. To produce alignment films that can be used in both twisted-nematic (TN) and vertical alignment (VA) configurations, InO solution is doped with SnO. Then, the pattern was transferred by attaching a one-dimensional soft-imprint mold, and pattern and orientation analyses were performed in accordance with the SnO ratio. The horizontal orientation performance improved as the SnO ratio increased, and the best orientation performance was observed for a film doped with 70% SnO. Further, the thermal stability improved and anchoring energy increased, which was measured to be very close to that of the rubbed polyimide film. The low transmittance of InO was overcome by doping with SnO, which has high transmittance. In the film doped with 80% SnO, the LC molecules were aligned in a vertical direction and were suitable for VA mode operation. By simple adjustment of the doping ratio, differences are observed with respect to the existing films in that the performance of the alignment layer is improved and the pretilt angles of the LC molecules can be changed. These convertible characteristics are expected to play a revolutionary role in the development of new LC driving modes.

Influence of dichroism on the optical properties of a twisted nematic liquid crystal cell in the terahertz region.

Polarization control is essential in terahertz (THz) imaging. Liquid crystals (LCs) have the potential to functionalize tunable polarization-control devices. Here, a twisted nematic (TN) cell using a hydrogen-bonded LC is fabricated, and the influence of dichroism in the THz region is discussed. Our results indicate that the polarization state in the Gooch-Tarry minimum condition is affected by the LC dichroism; a nondichroic LC is required for complete linearly polarized output. The output intensity of the dichroic LC-TN cell changed when electrically switched or when the incident THz wave polarization direction was rotated 90°. These intensity variations disappeared when using the nondichroic hydrogen-bonded LC.

Tracing chirality from molecular organization to triply-periodic network assemblies: Threading biaxial twist through block copolymer gyroids

Chirality transfer from the level of molecular structure up to mesoscopic lengthscales of supramolecular morphologies is a broad and persistent theme in self-assembled soft materials, from biological to synthetic matter. Here, we analyze the mechanism of chirality transfer in a prototypical self-assembly system, block copolymers (BCPs), in particular, its impact on one of the most complex and functionally vital phases: the cubic, triply-periodic, gyroid network. Motivated by recent experimental studies, we consider a self-consistent field model of ABC* triblock copolymers possessing an end-block of chiral chain chemistry and examine the interplay between chirality at the scale of networks in alternating double network phases and the patterns of segmental order within tubular network domains. We show that while segments in gyroids exhibit twist in both polar and nematic segmental order parameters, the magnitude of net nematic twist is generically much larger than polar twist, and more surprising, reverses handedness relative to the sense of polar order as well as the sense of dihedral twist of the network. Careful analysis of the intra-domain nematic order reveals that this unique chirality transfer mechanism relies on the strongly biaxial nature of segmental order in BCP networks and relates the biaxial twist to complex patterns of frame rotation of the principal directors in the intra-domain texture. Finally, we show that this mechanism of twist reversal leads to chirality selection of alternating gyroid networks in ABC* triblocks, in the limit of very weak chirality.

P‐12.4: Low Voltage Switchable Bistable Twist Nematic Display

The most common electronic paper technology is E‐ink, which enables many novel applications such as e‐readers, watches and electronic shelf labels. However, due to the TFT backplane driving and complex production processes, the cost of this technology is high, which limits its popularity. Recently, a low‐cost π‐bistable twist nematic (π‐BTN) liquid crystal display with a high pretilt angle is proposed. It has the same structure and production method as an ordinary liquid crystal display. Passive driving enables the manufacture of high‐resolution bistable displays without the use of thin‐film transistors. Compared with the current bistable twist nematic display technology, the high pretilt angles structure helps to achieve a lowvoltage and fast‐respond bistable display. This wide viewing angle, high contrast and TFT‐free display is a promising candidate for flexible displays.

ВЗАИМОДЕЙСТВИЕ И ДИНАМИКА ТОПОЛОГИЧЕСКИХ ДЕФЕКТОВ В ДОМЕННОЙ СТРУКТУРЕ ЗАКРУЧЕННОГО НЕМАТИКА

The dynamics and interaction of topological defects in the domain structure of π/2 twisted nematic liquid crystal are studied. A feature of Williams domains in twisted nematics is that hydrodynamic flows in them, along with the tangential velocity component, also have an axial component, the direction of which is opposite in neighboring domains. The axial velocity component arises as a result of the strong coupling between the initial twisted director orientation n and the hydrodynamic flow velocity. In domain structures of π/2 twisted nematics, as well as in planar oriented ones, edge dislocations with topological charges S =±1 arise. With increasing applied voltage density of dislocations grows. Dislocations move both along the Williams domains ( climb ) and perpendicular to them ( glide ). At a certain rate of the increasing applied voltage U > U c, distortions of the domain structure begin to propagate on both sides of the defect core. This leads to the formation of a special topological defects - dislocations with a “diffused” core, or linear defects, which are oriented perpendicular to the Williams domains and move mainly along them. The arising linear defect has the same topological charge as the initial dislocation. It is shown that their interaction is qualitatively well described by the perturbed sine-Gordon equation.

Twist-bend nematic drops as colloidal particles: Structural features.

The mesogen CB7CB [1″,7″-bis(4-cyanobiphenyl-4'-yl)heptane], mixed with a small quantity of a long chain amphiphile, is examined for the structural features of twist-bend nematic (N_{TB}) drops acting as colloidal inclusions in the isotropic and nematic environments. In the isotropic phase, the drops nucleating in the radial (splay) geometry develop toward escaped radial, off-centered structures, involving both splay and bend distortions. With further growth, they transform into low-birefringence (near-homeotropic) objects, within which remarkably well-organized networks of parabolic focal conic defects evolve in time. In electrically reoriented near-homeotropic N_{TB} drops, the pseudolayers develop an undulatory boundary possibly attributable to saddle-splay elasticity. In the matrix of the planar nematic phase, N_{TB} droplets appearing as radial hedgehogs attain stability in the dipolar geometry, through their association with hyperbolic hedgehogs. With growth, on transformation of the hyperbolic defect into its topologically equivalent Saturn ring around the N_{TB} drop, the geometry turns quadrupolar. Significantly, dipoles are stable in smaller drops, while quadrupoles are stable in larger ones. The dipole-quadrupole transformation is reversible, but is hysteretic with respect to drop size. Importantly, this transformation is often mediated by nucleation of two loop disclinations, one appearing at a marginally lower temperature than the other. The existence of a metastable state with partial formation of a Saturn ring and persistence of the hyperbolic hedgehog raises a question relating to the conservation of topological charge. In twisted nematics, this state features in the formation of a giant unknot that binds all N_{TB} drops together.

Electro-Optical Characteristics of Quasi-Homogeneous Cell in Twisted Nematic Mode

A liquid crystal (LC) director distribution was numerically analyzed in 90-degree twisted nematic (TN) LC cells with a symmetric and an asymmetric azimuthal anchoring strength of the alignment substrate and the influence of anchoring strength on the electro-optical property of the TN cell was evaluated. The twist angle decreased with decreasing azimuthal anchoring strength and the LC orientation changed to a homogeneous orientation with the twist angle of 0 degrees in the LC cell with asymmetric azimuthal anchoring strength, specifically with the strong anchoring substrate and the weak anchoring substrate below a critical strength. The asymmetric anchoring LC cell was fabricated by using a poly (vinyl cinnamate) alignment substrate as the weak anchoring surface and a polyimide alignment substrate as the strong anchoring surface. The LC cell performed the dark–bright–dark switching of the transmittance in the crossed polarizers, since the homogeneous LC orientation changed to the TN orientation again with increasing the applied voltage. Therefore, it was experimentally confirmed that LC molecules rotated at 90 degrees in the plane on the alignment surface by the electric field perpendicular to the weak anchoring substrate.

Temperature-switched orientation properties through physicochemically constructed aluminum zirconium oxide films

In general, it has one of the characteristics of homeotropic alignment and homogeneous alignment depending on the material and manufacturing process of the liquid crystal alignment layer. However, in this study, we propose a new type of alignment film that has the characteristics of changing the liquid crystal alignment direction according to the curing temperature. The AlZrO alignment film produced by the solution process and brush coating method showed homeotropic alignment characteristics when cured at low temperatures and homogeneous alignment characteristics when cured at high temperatures. These switching characteristics according to the curing temperature showed stable LC alignment, respectively, and the threshold voltages were 0.81 V and 0.857 V in vertical alignment (VA) mode and twisted nematic (TN) mode, respectively, demonstrating excellent electro-optical properties. The mechanism of these switching characteristics was analyzed through surface modification analysis. This study introduces new insights into liquid crystal alignment films that satisfy all of the high-efficiency fabrication process, various application possibilities due to their switching properties, and excellent electro-optical properties.

Polarization-insensitive liquid crystal Fresnel lens based on self-assembly polymer gravels and chiral dopant.

A polarization-insensitive liquid crystal (LC) Fresnel lens is developed with binary LC configurations of 90°-twisted nematic (TN) and vertically-aligned (VA) domains in the adjacent zones. A LC mixture comprised of nematic host, photopolymer and chiral material is initially filled into the VA cell with orthogonal rubbing treatment. After the ultraviolet irradiation on the filled LC cell through a photomask with Fresnel zone plate pattern, the interactions among orthogonal rubbing treatment, self-assembly polymer gravels, and chiral material induce the 90°-TN structure in the odd zones, whereas the initial VA structures are maintained in the even zones. The fabricated LC Fresnel lens with binary configuration emerges a maximum diffraction efficiency of around 35% at a voltage of 2.3 V, close to the theoretical diffraction limit of around 41%. The diffractive focus of the LC Fresnel lens is polarization-insensitive at the voltage above 2 V. When the voltage reaches 10 V, the diffractive focus vanishes. The numerical calculation confirms that the polarization-insensitive property appears in the primary focus of the LC Fresnel lens. This work reports a simple method to develop a highly efficient, polarization-insensitive, and electrically tunable LC Fresnel lens which is favorable for imaging system.

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

56.2: Invited Paper: Novel Bistable Liquid Crystal Materials for AR/VR Technology, Vehicle and Flexible Display

The object of study is chiral-nematic and smectic A liquid crystals. The aim of the work is to develop new bistable materials with improved physico-chemical and electro-optical properties for various applications. Chiral liquid-crystalline compounds and compounds characterized by the presence of the smectic A phase over a wide temperature range were synthesized during the studies. The influence of the viscoelastic properties of liquid crystals, the anchoring energy of the alignment material, and the helical twisting power of chiral dopants on the memory effect in chiral-nematic liquid crystals was investigated. In order to clarify the possible mechanism of bistability in highly twisted nematics, we performed a numerical simulation of the reorientation of liquid crystal molecules in an electric field. It was also necessary to find out which parameters of the liquid crystal material have the greatest effect on the bistability. Based on the synthesized smectic compounds, mixtures with different values of optical anisotropy and viscosity were developed. The influence of the compounds structure (bridge fragments, polar groups, alkyl chains) on mesomorphic and basic physico-chemical characteristics was determined. Electrically and electrochemically stable ion dopants were also synthesized with a number of organic cations and organic/inorganic anions. The composition of mixtures has been optimized for long-term operation, especially in the upper operating temperature range.

How Do Intermolecular Interactions Evolve at the Nematic to Twist-Bent Phase Transition?

Polarized beam infrared (IR) spectroscopy provides valuable information on changes in the orientation of samples in nematic phases, especially on the role of intermolecular interactions in forming the periodically modulated twist–bent phase. Infrared absorbance measurements and quantum chemistry calculations based on the density functional theory (DFT) were performed to investigate the structure and how the molecules interact in the nematic (N) and twist–bend (NTB) phases of thioether dimers. The nematic twist–bend phase observed significant changes in the mean IR absorbance. On cooling, the transition from the N phase to the NTB phase was found to be accompanied by a marked decrease in absorbance for longitudinal dipoles. Then, with further cooling, the absorbance of the transverse dipoles increased, indicating that transverse dipoles became correlated in parallel. To investigate the influence of the closest neighbors, DFT calculations were performed. As a result of the optimization of the molecular cores system, we observed changes in the square of the transition dipoles, which well corresponds to absorbance changes observed in the IR spectra. Interactions of molecules dominated by pairing were observed, as well as the axial shift of the core to each other.

Mueller matrix polarimetric analysis applied to characterize the physical parameters of a twisted-nematic liquid–crystal modulator

This work presents a complete polarimetric study of a twisted-nematic (TN) liquid–crystal (LC) cell. We review the physical models that describe the cell and analyze the different modulation regimes. We extend the usual Jones matrix approach, where these microscopic physical models were developed, to the corresponding Mueller matrix approach. This polarimetric analysis is then used to obtain the effective linear and circular retardance components of the cell and to characterize its physical parameters like the twist angle, the orientation of the LC director axis and the maximum retardance. The technique simplifies previous approaches with the advantage of employing a single wavelength. Noteworthy, it also resolves the ambiguities in the determination of the physical parameters. Experimental evidence of the effectiveness in predicting the optical modulation is shown for a single-pixel TN-LC cell. Finally, a simplified procedure is presented under the assumption that the TN-LC cell is a pure retarder component, which is useful to perform a rapid calibration of the device.

Tunable Liquid Crystal Alignment and Driving Mode on Lanthanum Aluminum Zirconium Zinc‐Oxide Thin Film Achieved by Convenient Brush‐Coating Method

AbstractTunable liquid crystal (LC) alignment with twisted nematic (TN) and vertical alignment (VA) driving modes is achieved by a convenient brush‐coating method. The brush‐coating process combines film deposition and alignment treatment processes, and lanthanum aluminum zirconium zinc oxide (LaAlZrZnO) is used for the LC alignment layer. X‐ray photoelectron spectroscopy results confirm LaAlZrZnO film formation with fine optical transparency. Atomic force microscopy and line profile data indicate nano/micro surface structure with dense or directional morphologies induced by shear stresses according to film curing temperature. Low‐temperature‐cured films exhibit hydrophobic properties with low surface energy, whereas high‐temperature‐cured films show hydrophilicity with high surface energy. The geometric constraints and chemical affinities of the films induce homeotropic and homogeneous LC alignments on the surface according to the film curing temperature, respectively. The LC alignment state is verified by polarized optical microscopy and pretilt angle analysis. The brush‐coated LaAlZrZnO film demonstrates high thermal budget for advanced LC systems. The VA‐mode and TN‐mode LC cells based on the LaAlZrZnO film enable high electro‐optical performances and low power consumption. Therefore, the brush‐coating method is expected to be adoptable for next‐generation LC device applications.