Tilt Angle Of Liquid Crystal Research Articles

Polymer sustained alignment for vertical aligned liquid crystal devices with a fast response and high contrast

ABSTRACT We demonstrate fast response and high contrast vertical alignment (VA) liquid crystal devices (LCDs) with polymer sustained alignment (PSA). In general, the rise time of a liquid crystal (LC) is a trade-off relationship with the decay time. In the present study, an appropriate tilt angle of LCs was anchored by polymerisation using UV irradiation with the application of a threshold voltage and optimisation of the reactive mesogen (RM) concentration. The PSA VA-LCDs showed a fast optical response to the applied electric field because the backflow (optical bounce) was suppressed by anchoring with the polymer. In addition, the fast response was maintained until a high voltage was applied. The transmittance of these PSA VA-LCDs at 0 V was identical to that of VA-LCDs without a polymer and the opaque phenomena at the applied voltage were eliminated because alignment of LCs and RMs was improved. The findings of this work are expected to enable the development of displays and phase modulators with improved performance.

Using Liquid Crystals for In Situ Optical Mapping of Interfacial Mobility and Surfactant Concentrations at Flowing Aqueous-Oil Interfaces.

Flow-induced states of fluid interfaces decorated with amphiphiles underlie phenomena such as emulsification, foaming, and spreading. While past studies have shown that interfacial mass transfer, the kinetics of surfactant adsorption and desorption, interfacial mobility, and surfactant reorganization regulate the dynamic properties of surfactant-laden interfaces, few simple methods permit simultaneous monitoring of this interplay. Here, we explore the optical responses of micrometer-thick films of oils (4-cyano-4'-pentylbiphenyl, 5CB) with a liquid crystalline order in contact with flowing aqueous phases of soluble [e.g., sodium dodecyl sulfate (SDS)] or insoluble (e.g., 1,2-dilauroyl-sn-glycero-3-phosphocholine) amphiphiles. We observe the onset of flow of 0.5 mM SDS solutions within a millifluidic channel (area-average velocity of 200 mm/s) to transform a liquid crystal (LC) film with an alignment along the interface normal into a bright birefringent state (average LC tilt angle of 30°), consistent with an initially mobile interface that shears and thus tilts the LC along the flow direction. Subsequently, we observed the LC film to evolve to a steady state (over ∼10 s) with position-dependent optical retardance controlled by gradients in surfactant concentration and thus Marangoni stresses. For 0.5 mM SDS solutions, by using particle tracking and a simple hydrodynamic model, we reveal that the dominant role of the flow-induced interfacial surfactant concentration gradient is to change the mobility of the interface (and thus shear rate of LC) and not to change the easy axis (equilibrium orientation) or anchoring energy (orientation-dependent interfacial energy) of the LC. At lower surfactant concentrations (0.015 mM SDS), however, we show that the LC directly maps flow-induced interfacial surfactant concentration gradients via a change in the local easy axis of the LC. When combined with additional measurements obtained with simple salts and insoluble amphiphiles, these results hint that LC oils may offer the basis of general and facile methods that permit mapping of both interfacial mobilities and surfactant distributions at flowing interfaces.

Soft and broadband infrared metamaterial absorber based on gold nanorod/liquid crystal hybrid with tunable total absorption.

We design a soft infrared metamaterial absorber based on gold nanorods dispersed in liquid crystal (LC) placed on a gold film and theoretically investigate its total absorption character. Because the nanorods align with the LC molecule, the gold nanorods/LC hybrid exhibits different permittivity as a function of tilt angle of LC. At a certain tilt angle, the absorber shows an omnidirectional total absorption effect. By changing the tilt angle of LC by an external electric field, the total absorption character can be adjusted. The total absorption character also depends on the concentration, geometric dimension of nanorods, and defect of nanorod arrangement in LC. When the LC contains different size of gold nanorods, a broadband absorption can be easily realized. The characteristics including flexibility, omnidirectional, broadband and tunablility make the infrared metamaterial absorber possess potential use in smart metamaterial devices.

Alignment Properties of Liquid Crystal Molecules under AC and DC Voltage Application Studied by Surface Plasmon Resonance and Optical Waveguide Spectroscopy

In this study, we report the evaluation of the tilt angle of liquid crystal (LC) molecules near the surface of SiO2 alignment layers and in the whole cell when AC and/or DC voltages are applied. The LC molecule 4-cyano-4'-n-pentylbiphenyl was used. The LC cell consisted of a Au (50 nm)/SiO2 (30 nm)/LC (3 µm)/SiO2 (30 nm)/Au (100 nm) system. Surface plasmon resonance (SPR) spectroscopy and optical waveguide spectroscopy (OWS) in an attenuated total-reflection configuration were used to monitor the alignment properties of LC molecules. From the SPR/OWS angular scan properties, the profile of the tilt angle from the surface to inside the cell was obtained. Furthermore, the effect of space-charge polarization on the LC alignment properties was observed from the SPR kinetic properties. As demonstrated in this report, this technique should provide useful information for understanding the interfacial phenomena in LC displays.

Control and managing of localized states in two-dimensional systems with periodic forcing

We study the formation of localized structures in two-dimensional systems with periodic forcing, showing that these types of systems provide an adequate framework for the study and control of localized structures. Theoretically, we introduce a dissipative ϕ 4 model as a prototype for a bistable spatially forced system, and we show that with different spatial forcings of small amplitudes, such as square or hexagonal grids, this model exhibits a family of localized structures. By changing the forcing parameters, we control the bistability between the various induced patterns. Experimentally, based on an optical feedback with spatially amplitude-modulated beam, we set-up a two-dimensional forced experiment in a nematic liquid crystal cell. By changing the forcing parameters, the system exhibits a family of localized structures that are confirmed by numerical simulations for the average liquid crystal tilt angle.

Driven Front Propagation in 1D Spatially Periodic Media

We study front propagation in one-dimensional spatially periodic media. Based on an optical feedback with a spatially amplitude modulated beam, we set up a one-dimensional forced experiment in a nematic liquid crystal cell. By changing the forcing parameters, the front exhibits a pinning effect and oscillatory motion, which are confirmed by numerical simulations for the average liquid crystal tilt angle. A spatially forced dissipative varphi;{4} model, derived at the onset of bistability, accounts qualitatively for the observed dynamics.

Viewing Angle Control of a Hybrid-Aligned Liquid Crystal Display

A method for viewing angle control of a hybrid-aligned liquid crystal (LC) cell is reported. The viewing angle of proposed structure can be controlled by using vertical switching, by which the tilt angle of LC is controlled, while the optical transmittance can be controlled by using horizontal switching, by which the optic axis (OA) of LC can be rotated. A 3-terminal electrode structure composed of the top common electrode, the bottom grid electrodes, and the bottom common electrode is employed for both the vertical switching and the horizontal one.

Nonlinear dynamics of the petal-like patterns in a liquid crystal light valve with rotational optical feedback

The dynamics of the petal-like patterns observed in a liquid crystal light valve with optical feedback are investigated both experimentally and numerically. Based on a phenomenological model, the linear stability analysis predicts the bistability of the liquid crystal tilt angle for several voltage ranges that agrees with the experimental results. The static and dynamical features of the petal patterns obtained in the numerical simulation are very close to the experimental observations. When increasing the voltage applied to the light valve, the numerical simulations reveal a scenario of bifurcation from the static to a fluctuating petal pattern, which is confirmed by the experimental observations.

Control of reverse twist domain near a pixel edge using strong vertical electric field in the fringe-field switching liquid crystal device

In the fringe-field switching (FFS) liquid crystal (LC) device, competition between two opposite forces to twist LCs exists near the pixel edge. The authors proposed an advanced FFS structure with the strong vertical field near the pixel edge which was induced between Cr black matrix (BM) and pixel electrode due to electrical connection between the Cr BM and the common electrode. The vertical field induces a tilt angle of LCs which increases with increasing field intensity. As a result, the degree of twist in the reverse direction decreases with increasing the pixel voltage, and thus voltage-dependent LC dynamics becomes very stable even at high applied voltages.

Effect of polyimide layer surfaces on pretilt angles and polar anchoring energy of liquid crystals

Considering the surface distribution of polyimide chains, the properties of the unidirectional out-of-plane nematic liquid crystal tilt angle are investigated in view of the van der Waals interaction between nematic liquid crystal molecules and two polyimide layers: the anisotropic upper layer and the isotropic lower layer. The dependence of the pretilt angle upon the thickness of the upper polyimide layer oriented by rubbing is estimated from experiment and theory. Another experimental result shows that the relation between the anchoring energy and the thickness of the polyimide layer could be dependent on lower material. This was also explained in view of the nonretarded van der Waals interaction.

폴리이미드 표면에서의 네마틱 액정의 틸트 제어를 이용한 High Tilted OCB(HTOCB) 모드

In this paper, we have improved a novel high tilted optically compensated bend (OCB) (HTOCB) mode by using high tilt angle that was generated by the unique baking condition on the homeotropic alignment layer. The high tilt angle of liquid crystal (LC) was generated by new alignment process that tilt angle changed homeotropic state to homeogenous state using Hot-plate equipment; we obtained about <TEX>$40\~50^{\circ}$</TEX> tilt angle with negative and positive dielectric anisotropy on the homeotropic polyimide (PI), and then LC tilt angle decreased as increasing baking temperature and time. At last, we obtained about <TEX>$10^{\circ}$</TEX> with positive type NLC <TEX>$({\Delta}n>0)$</TEX>. Also, the LC tilt angle of positive type NLC <TEX>$({\Delta}n>0)$</TEX> decreased as increasing rubbing strength at the same baking temperature and time. The novel LC operating mode (HTOCB) that used the high tilt angle by the new alignment method was improved. The response time of the novel HTOCB cell was faster than that of conventional OCB cell. We suggest that the developed the novel HTOCB cell using control of tilt angle on the homeotropic surface is a promising technique for the achievement of a fast response time and a high contrast ratio.

EO characteristics for a photoaligned VA‐1/4 π cell on a photopolymer layer

Abstract—The generation of a liquid‐crystal (LC) tilt angle on a copolymer with chalconyl and cholesteryl moiety characteristics was performed, and the electro‐optical (EO) performance of the photoaligned vertical‐alignment (VA) 1/4 π cell by polarized UV exposure on a homeotropic photopolymer surfaces was studied. The LC tilt angles decreased as UV exposure time increased on the copolymer surfaces. A tilt angle of 87° in NLC was observed with an UV exposure of 3 min on the photoalignment‐2 surface. The LC tilt angle is attributed to increased chalcone moiety with increasing UV exposure time. Excellent voltage‐transmittance (V‐T) curves of the photoaligned VA 1/4 π cell by polarized UV exposure on the photopolymer surface for 3 min containing a cholesteryl moiety of 8% were obtained. The V‐T and response‐time characteristics can be improved by the presence of a cholesteryl moiety in the photopolymer.

Accurate optical determination of liquid crystal tilt angle by the half-leaky guided mode technique

The first resonant guided mode of P→P reflection in Half-leaky Guided Mode (HLGM) technique is very sensitive to the tilt angle of uniaxial liquid crystals. A formula is presented to describe the relationship between the tilt angle and the incident angle of the first resonant valley point of P→P reflectivity, R pp. This method is suitable to measure tilt angle of any degree, especially 3–85°, with high sensitivity. The great variations of LC layer thickness, LC near-surface sublayers, PI layers, or ITO layers have few effects on the position of the first resonant valley point of R pp. The method can be used to measure tilt angles with high sensitivity, without other influences, and exempt from the time consuming numerical fitting with a complex model, which is needed in other methods by Guided Modes.

Influence of the Microtopography of the SiO Covered by PVA Layers on the Tilt Angle of Liquid Crystals

An evaluation of the tilt angle variation observed recently when unrubbed polyvinylic alcohol is deposited on obliquely evaporated SiO layers is presented. The method of calculation supposes that t...

Mechanism of nematic molecular alignment based on friction charges and surface topology by rubbing

Abstract A new and simple electric charge interaction mechanism has been proposed to explain the behaviour of the tilt angle of liquid crystals by rubbing. The mechanism of the molecular alignment is elucidated based on the effect of a static electric charge on a substrate surface treated by rubbing. An electric field which is dependent on the topology of a substrate surface allows a planar orientation of molecules with positive dielectric anisotropy. With increasing rubbing strength, the tilt angle varies slowly. The director of molecules with negative dielectric anisotropy is uniform and has a tilt angle determined by the molecular permanent dipole direction. The air-liquid crystal interface case is also considered. This model enables us to give a unified picture of the molecular alignment mechanism.

Temperature dependence of liquid crystal tilt angles

In a homogeneous cell the origin of the steep decrease, which is always observed in the liquid crystal tilt angle on an SiO coating as a function of temperature in the range close to Tc, is traced back to the growth of twist domains. It appears that a magnetic null method is unsuited for accurate tilt angle measurements in the relevant temperature region, because it is based on the assumption of a homogeneous director pattern in the sample cell.