A liquid crystal cell with double-side protrusion electrodes for fast response and low-voltage operation

We propose a homogeneously aligned liquid crystal (LC) cell with double-side protrusion electrodes for fast response and low-voltage operation. In the proposed device, both the bottom and top substrates have pixel electrodes to generate the fringe electric field. Because the penetration depth of the electric field is increased owing to the protrusion electrodes, the operating voltage is very low and the turn-on time is dramatically reduced compared with the conventional in-plane switching (IPS) mode. Moreover, LC molecules anchored strongly to the penetrated protrusion electrodes on both substrates exert a strong restoring force, resulting in a fast turn-off time. We found that the total response time of the LC cell with the proposed structure is three times faster than that of the conventional IPS mode.     

The in-plane switching of homogeneously aligned nematic liquid crystals

We have investigated the electro-optical effects and physical switching principle of homogeneously aligned nematic liquid crystals when applying an in-plane electric field with interdigital electrodes. By using the in-plane switching (IPS) of the liquid crystals which is achieved by the in-plane electric field, the viewing angle characteristics of the electro-optical effects were confirmed to be far superior to those of the conventional twisted nematic mode in which the electric field is applied along the direction perpendicular to the substrates. The non-reversal region of grey scales was extremely wide in which a high contrast ratio was kept, even along quite an oblique direction in the IPS mode. In order to clarify the switching principle of the liquid crystals in the IPS mode, a simplified expression describing the threshold behaviour of the device was derived with the assumption that a uniform in-plane electric field was applied along a direction perpendicular to the director and parallel to the homogeneously aligned nematic slab, and found to be sufficiently able to explain the experimental results. First, a critical field at which the liquid crystals just began to twist, was found to be proportional to the reciprocal of the cell gap. Second, it was the electric field and not the voltage that drives the liquid crystals. This relationship was due to the independence of the electric field regarding the liquid crystal layer normal direction. So the threshold voltage in the IPS mode was strongly dependent on the variation of the cell gap. For the dynamical response mechanism of the liquid crystals to the in-plane electric field, the switching on and off processes of the liquid crystals were analysed quantitatively. The relaxation time of the liquid crystals when removing the electric field could be described as proportional to the square of the cell gap. A thinner cell gap also proved to be effective in obtaining a fast response time in the IPS mode. In contrast, the switching on time when applying the in-plane electric field was found to be inversely proportional to the difference between the square of the electric field strength and the square of the critical electric field strength at which the liquid crystals began to deform.     

Mixed-field-switching liquid crystal mode using in-plane and fringe fields self-adjusted by bottom floating electrode for transmittance enhancement

We demonstrate a liquid crystal (LC) mode switched by mixed electric fields of in-plane and fringe fields, which are self-adjusted by adopting a bottom floating electrode for enhanced electro-optical properties. In our LC mode structure, conventional in-plane switching (IPS) electrodes are formed as pixel electrodes and common electrodes on an insulating layer and floating electrodes that are patterned per the sub-pixels. When the areas of the pixel and common electrodes are identical, the voltage of the bottom floating electrode is spontaneously determined to be half the value of the pixel voltage, which ideally generates symmetric fringe fields with both pixel and common electrodes. Due to the in-plane fields additionally generated between the pixel and common electrodes, the proposed LC structure operates by mixed-field switching (MFS), which shows higher transmittance than fringe-field switching (FFS) and IPS LC modes. Transmittance of the conventional FFS and IPS LC modes is highly sensitive to the in-plane electrode’s width (w) and spacing (l) condition, but the proposed MFS LC mode shows good transmittance without degradation with large variations of the in-plane electrode’s spacing-to-width ratio (l/w).     

Viewing angle characteristics of a homogeneously aligned two-domain liquid crystal cell with an inter-digitated electrode

The in-plane switching (IPS) mode in liquid crystal displays is known to exhibit a wide viewing angle. However, since the LC director rotates in one direction in the plane, devices with a single domain exhibit both a colour shift depending on the viewing angle, and greyscale inversion at specific angles especially at low grey levels. This has been improved by wedge shaped electrodes so that fields in two directions exist inside a pixel, causing the LC molecules to rotate in opposite directions to compensate each other; this acts as a virtual two domains structure. Nevertheless, the colour shift still exists to some extent, especially at low grey levels. In this paper, we propose a realistic two-domain IPS mode that exhibits a minimized colour shift at all grey levels on changing the viewing direction. In this device, the LC molecules are initially aligned in two directions orthogonal to each other, and two field directions exist perpendicular to each other. We have performed device simulations with respect to viewing angle characteristics, and found that IPS devices with a real two-domain structure reduce the variation of the retardation more effectively, when the viewing direction changes.     

Slow relaxation processes in nematic liquid crystals at weak surface anchoring

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV-activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong 'in-plane' electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

Grey behaviour analysis of in-plane switching mode liquid crystal displays with weak-anchoring effects

In this paper we calculate the relative transmittance variation at grey level voltage with cell gap variation, and the different surface anchoring strengths at both interface. The threshold voltage of an IPS mode LCD with weak anchoring strength was investigated by linear stability analysis; it was strongly dependent on the variation of cell gap. We analyse the transmittance variation at grey level voltage from the threshold behaviour of an IPS mode LCD with symmetric and asymmetric weak anchoring surface conditions. The ratio of relative transmittance variation of cells with weak anchoring and strong anchoring surfaces at the grey level voltage, ΔT weak grey / ΔT str grey , depends linearly on V weak c / V str c .     

Cell gap‐dependent transmission characteristics of a fringe‐electric field‐driven homogeneously aligned liquid crystal cell,for a liquid crystal with negative dielectric anisotropy

Transmittance characteristics were studied as a function of cell gap for a homogeneously aligned liquid crystal (LC) cell driven by a fringe‐electric field—named fringe‐field switching (FFS) mode. The light efficiency of a conventional LC cell using in‐plane switching and twisted nematic modes, where the LC director is determined by competition between elastic energy and electrical energy, does not depend on cell gap as long as the cell retardation value remains the same; i.e. only dielectric torque contributes to the deformation of the LC director. However, the transmittance of the FFS mode is dependent on the cell gap such that it decreases as the cell gap decreases, although the cell retardation value remains the same. This unusual behaviour (unlike that of conventional LC cells) arises because in the device the elastic and dielectric torques have the role of determining the LC director, such that the driving voltage giving rise to maximum transmittance becomes strongly dependent on the electrode position when the cell gap is as small as 2?µm. In addition, the LCs at the centre of the pixel and common electrodes are not sufficiently twisted because of a competition between the two elastic forces, which tries to twist the LCs in plane and hold them in their initial state by surface anchoring.     

The effect of replacing a benzene ring with a saturated six-membered ring on the mesomorphic properties of 4,4′-disubstituted diphenyldiacetylenes

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV‐activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong ‘in‐plane’ electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

Advantageous voltage-holding ratio characteristics induced by in-plane electric fields, and the optimization concept of liquid crystals for an in-plane switching electro-optical effect

In-plane switching (IPS) of liquid crystals showed advantageous voltage-holding ratio (VHR) characteristics so that liquid crystals with low resistivity could provide higher VHRs compared with the twisted nematic effect. This experimental result was obtained when electric fields were applied approximately parallel to the substrate plane using the IPS electro-optical effect. We found that the in-plane electric field generates supplementary capacities which support retention of an externally applied voltage over the liquid crystal layer during non-selected periods of the active matrix driving scheme, because the liquid crystal layer can be connected with an insulating layer, an orientation layer and even a substrate in parallel. Based on these advantageous VHR characteristics, liquid crystal materials suitable for the IPS effect were appropriately optimized. We propose evaluation parameters, derived from the physical switching principles of the liquid crystals, to obtain lower driving voltage and faster response speeds. These parameters are effective in optimizing the physical properties of liquid crystals without variation of the cell gap. We use the proposed evaluation parameters and the advantageous VHR characteristics to demonstrate the optimization approach and we suggest a novel possible use of liquid crystal materials with low resistivity which cannot be implemented conventionally. Finally, we prove that liquid crystals with low resistivity generate the Ir internal potential by the drift of ionic species.     

Azimuthal director gliding at a strongly anchoring interface of polyimide

A gliding of the director at the interface between a nematic liquid crystal and a solid medium is generally observed at many interfaces giving weak or moderately strong anchoring. This phenomenon is characterized by strongly non-linear dynamics and very long relaxation times (hours-days). The gliding of the director has also been observed very recently at the interface between a rubbed polyimide layer and a nematic liquid crystal which gives strong azimuthal anchoring. However, due to the weak nature of the experimental signals that characterizes the strong anchoring, this latter measurement was appreciably affected by thermal drift. In this paper, we develop a new experimental reflectometric method whereby the thermal drift is appreciably reduced. The method allows us to obtain more accurate signals and to investigate their time dependence. It is shown that the director gliding is well represented by a stretched exponential, as well as in the case of weak anchoring substrates. These measurements confirm that the gliding of the director is a universal phenomenon characterizing any kind of substrate with either weak and strong anchoring.     

Effect of crosslinking polymer networks on the molecular reorientation and electro‐optical performance of in‐plane switching vertically aligned liquid crystal devices

The effects of crosslinking polymer networks (PNs) on the molecular reorientation and electro‐optical properties of vertically aligned (VA) liquid crystal (LC) devices are investigated by applying an in‐plane switching (IPS) electric field. Through the polymerization process, crosslinking PNs are developed on the substrate surface, effectively increasing the anchoring energy and governing the LC molecular reorientation. With its stronger anchoring effect, the PNs cell shows good light transmittance and excellent vertical alignment quality, as compared to the pure LC cell. Furthermore, the alignment transformation and transmittance bounce resulting from the transient process of LC molecular reorientation are eliminated when the cell is operated at high voltages. The rising‐time (t_r) and falling‐time (t_f) responses of the PNs cell are significantly improved, and around 36% improvement in the optical switching response is obtained. In addition, the dynamic gray‐level t_r and t_f responses of the PNs cell are enhanced by around 55% and 42%, respectively, at a low driving voltage (～12 V). This developed VA‐IPS LC/PNs cell benefits not only the LC molecular alignment but also the electro‐optical performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1123–1130     

Crucial influence of polar alignment on the dynamics of in-plane switching cells

We investigated theoretically the dynamics of in-plane switching (IPS) cells with small pretilt angle and found that the liquid crystal director variation causes optical bounce after switching on an applied voltage. We analysed the behaviour of the director by computer simulation and found that the optical bounce occurs during the rising period with the normal twist and tilt angles of the directors in the IPS cell in the absence of the field-induced backflow effect. Pretilt angle is the source of this optical bounce.     

Crucial influence of polar alignment on the dynamics of in-plane switching cells

We investigated theoretically the dynamics of in-plane switching (IPS) cells with small pretilt angle and found that the liquid crystal director variation causes optical bounce after switching on an applied voltage. We analysed the behaviour of the director by computer simulation and found that the optical bounce occurs during the rising period with the normal twist and tilt angles of the directors in the IPS cell in the absence of the field-induced backflow effect. Pretilt angle is the source of this optical bounce.     

The role of the anchoring conditions in the electrorheological behaviour of a nematic constrained by two coaxial cylinders and submitted by a pressure drop

We study a nematic liquid crystal (NLC) filling the region between two coaxial cylinders subjected to the simultaneous action of both, a pressure gradient applied parallel to the axis of the cylinders and a radial low-frequency electric field. For the LC 4′-n-pentyl-4-cyanobiphenyl (5CB), we consider strong and weak anchoring conditions (WAC) to obtain the configuration of the director and the velocity profile for non-slip boundary conditions. Finally, we calculate the apparent viscosity for the nematic.     

Fast switching of vertically aligned negative liquid crystals by optically hidden relaxation

We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.     

Photo-reversible liquid crystal alignment using azobenzene-based self-assembled monolayers: comparison of the bare monolayer and liquid crystal reorientation dynamics

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.     

Quantification of the azimuthal anchoring of a homogeneously aligned nematic liquid crystal using fully-leaky guided modes

A novel optical guided mode technique, the fully-leaky guided mode technique, has been used to investigate the director distortion under the application of an in-plane electric field of a homogeneously aligned conventional cell filled with the nematic liquid crystal E7. The liquid crystal is aligned using polyimide rubbed along the direction of the gold electrode edges. A weak field is applied across a 3 mm gap between the gold electrodes to induce small changes in the twist angle of the director. These distortions are determined by fitting to the angledependent reflectivity and transmissivity data and are compared with continuum theory. From careful analysis of the results, both the twist elastic constant, k22, and the azimuthal anchoring strength, Wa, of the system are obtained. At 23.5 C for E7 on rubbed polyimide we find that k22=(6.50 +/- 0.05)x10-12N and Wa=(2.9 +/- 0.2)x10-5 J m-2.     

Advanced wide viewing angle technology in liquid crystal display modes

This paper reviews the advanced optical configurations for in-plane switching (IPS) and vertical alignment (VA) liquid crystal (LC) cells with wide viewing angle in a visible wavelength range. Optical compensation and optimisation to eliminate off-axis light leakage in the dark state is performed on a Poincaré sphere using the trigonometric and the Muller matrix method. By optimising the wavelength dispersion of used optical retardation films, we could achieve wide-view characteristics for both the IPS and VA LC cells. In addition, we show the advanced wide-view technology for a reflective LC mode.     

Electro-optic characteristics of the fringe in-plane switching liquid crystal device for a liquid crystal with negative dielectric anisotropy

Fringe-field switching (FFS) liquid crystal (LC) mode is mainly used for high-end LC displays. At present, an LC with positive dielectric anisotropy is utilised, although light efficiency of the device in a white state is not maximised due to generation of tilt angle near the edge of electrodes along the field direction. In order to overcome the demerit, an LC with negative dielectric anisotropy has been challenged. In this article, FFS mode, which shows a high light efficiency and a low operating voltage, is investigated with the utilisation of fringe in-plane electric field. The optimised device shows improved electro-optic characteristics in comparison with not only conventional LC modes, but also previously proposed FFS device using a positive type of LC.     

Tunable negative index metamaterial employing in-plane switching mode at terahertz frequencies

We analysed the response of a tunable liquid crystal metamaterial transducer in the terahertz frequency range. Tunability of scattering parameters is achieving by an in-plane switching (IPS) effect. The metamaterial structure is based on Ω-shape resonators. A full-wave analysis technique based on the finite-difference time-domain (FDTD) method was performed using the QuickWave 3D electromagnetic solver. Terahertz transmission properties of the metamaterial structure can be controlled by the director of the liquid crystal layer. The effective refractive index for operation frequency varies from negative to positive values. A novel approach to switching of metamaterial transducer by IPS mode is presented.