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.     

Electrically controlled director slippage over a photosensitive aligning surface; in-plane sliding mode

We have studied the electro-optical characteristics of a homogeneously aligned nematic liquid crystal (LC) with weak planar anchoring of the director at the bounding substrates. By using the in-plane switching (IPS) of the LC which is achieved by an in-plane electric field, the driving voltage was confirmed to be far less than that of the conventional IPS mode in which both substrates possess strong anchoring characteristics. Moreover, because of the absence of strong subsurface director deformations, the cell could operate optically in the Mauguin regime. Using these features we propose a new type of LC switching mode - in-plane sliding (IPSL) mode. We have realized this mode in a LC cell comprising one reference substrate with strong director anchoring and one substrate covered with photoaligning material with weak anchoring. In order to clarify the switching process, we derived a simplified expression for the threshold voltage on the assumption of uniformity of the in-plane electric field. For the dynamical response of the LC to the in-plane electric field, the switching on and off relaxation times of the IPSL mode were found to be longer than for the traditional IPS mode. However, we have proposed an optimized cell geometry for the IPSL mode with a response time comparable to that of the IPS mode.     

Suppressing pretilt angle for enhancing black quality in diagonal viewing angle of homogenously aligned liquid crystal display

The diagonal viewing angle light leakage in a black state of in-plane switching (IPS) liquid crystal display (LCD) associated with pretilt angle has been investigated. The mechanical rubbing process with a cloth causes relatively high pretilt angle in the homogenously aligned liquid crystals (LCs) so that the tilted LC director results in increase of a light leakage in a black state at diagonal viewing angles. In this study, we theoretically estimated using classical optic theory how the light leakage in a black state at diagonal viewing angle is associated with the pretilt angle and also proposed an effective method to reduce the pretilt angle from 1.5° to 0° in rubbed IPS LCD by utilising polymer stabilisation. With this approach, we could successfully acquire a better black quality in all viewing angles as compared with normal IPS LCD.     

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.     

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.     

Tunable backflow in chiral nematic liquid crystals via twist-bend nematogens and surface-localised in-situ polymer protrusions

Dynamic electro-optic response of the liquid crystal (LC) director shows a backflow effect that is manifested as an optical bounce in chiral nematic LCs (N*LC) during field-induced homeotropic-twisted transition. The bend elastic constant (K₃₃) strongly influences the dynamics of backflow at the N*LC in homeotropic-twisted transition. The cyanobiphenyl LC dimers – CB7CB, CB9CB and CB11CB – possess a unique characteristic of inherent bend molecular configuration that lowers K₃₃. With the modulation of the effective K₃₃ in dimer-doped N*LCs, we report the tunability of the optical bounce that decreases with the increase in the length of flexible spacers in LC dimers. The doped LC dimers with short spacer lengths not only generate a strong backflow with an enhanced twist degeneracy of the LC director across the cell, but also prolong the time of disappearance of the optical bounce. Furthermore, we demonstrate the suppression of the optical bounce with surface localised polymer protrusions having 50–100 nm diameters, which allow faster dynamic relaxation process and reduced backflow. We envision a novel design of a tunable microfluidic device for precise flow control of organic or inorganic matter in LC medium that exploits the tunable backflow in LC dimer-doped N*LCs.     

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.     

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.     

Light-controlled reorientation of nematic liquid crystal driven by an electric field

The joint influence of optical and (quasi-)static electric fields on the orientation of liquid crystal gives rise to peculiar effects. In this article we report on the generation of transient domains in liquid crystals, which are an order of magnitude larger than the size of the optical field profile. The formation of such a domain is due to the fact that the initially present optical field reverses the pre-tilt, and the voltage that is then applied gives rise to an amplification of the tilt angle. The resulting reorientation of the director strongly depends on the starting conditions of the preliminary present optical field. We demonstrate different switching conditions, depending on the relation between the incident angle of the beam and the pre-tilt angle. The resulting refractive index profiles give rise to lensing effects.     

Modelling zenithal bistability at an isolated edge in nematic liquid crystal cells

In recent experiments we observed bistable switching in devices made with long pitch square gratings on one surface. It was also discovered that the switching in these devices was localized mainly at isolated edges of the square grating profile. In this paper we present an initial study of surface-induced director configurations at isolated edges of a square profile in the absence of an applied voltage. Our emphasis is on understanding the effect of edge features such as the edge depth and edge inclination in forming stable high and low pre-tilt states. Models based on a Landau-de Gennes approach were used and solutions were found through numerical simulation using finite element methods; optical response was determined, based on wide angle beam propagation methods. Results from real cells are also presented. Our results show that static defect-stabilized states exist for a range of isolated edge depths and inclination angles. In particular, a combination of deep edge depths and steep edge inclinations produce stable high and low pre-tilt director configurations.     

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     

Dependence of viewing angle characteristics on pretilt angle in the in-plane switching mode

Viewing angle characteristics were systematically obtained when using in-plane switching (IPS) of liquid crystals. Although the IPS mode originally shows stable electro-optical performance regardless of viewing directions, the viewing angle characteristics are found to be strongly dependent on pretilt angle (slant angle of the liquid crystals from the substrate). Experimentally, the smaller the pretilt angle of the liquid crystal, the much wider the viewing angle characteristics, while larger pretilt angles of liquid crystals cause the characteristics to deteriorate. This deterioration occurs in a particular viewing direction, i.e. at right angles to the initial orientation direction of the liquid crystal when there is no in-plane electric field. The experimentally observed behaviour of the viewing angle dependence on the pretilt angle was also confirmed by computer simulations. Calculated iso-contrast contour lines, as a function of the pretilt angles, nearly coincide with the experimentally obtained viewing angle characteristics of the contrast ratio.     

Transflective blue-phase liquid crystal display with polar opposite electrodes

A submillisecond response, wide view and single-cell-gap transflective (TR) display employing a blue-phase liquid crystal is proposed. The device employs polar opposite in-plane switching (IPS) electrodes. To balance the optical phase retardation between transmissive (T) and reflective (R) regions, the IPS electrodes are formed with unequal gaps in the two regions. This display exhibits reasonably high optical efficiency and well-matched voltage-dependent transmittance and reflectance curves.     

Molecular director and layer response of chevron surface stabilized ferroelectric liquid crystals to low electric field

Recent papers on chevron surface stabilized ferroelectric liquid crystal cells claim that the chevron layer structure can be reversibly uprighted by application of the low to moderate electric fields typically employed to produce director reorientation. In this paper we show, using optical microscopy and X-ray scattering, that there is no significant change in the smectic layer thickness or chevron layer structure of our chevron surface stabilized ferroelectric liquid crystal cells under typical director switching conditions. Furthermore, we present arguments, based on the known elastic properties of smectics, that there is not likely to be a significant elastic layer response to these levels of applied electric field in any surface stabilized ferroelectric liquid crystal cell with anchored layers. Both the switching and observed continuous optical response to applied field can be understood on the basis of electric field induced reorientation of a non-uniform molecular director distribution. We further show that the typically observed broad distribution of layer orientations about the mean chevron structure arises from localized layering defects.     

The influence of flow on symmetric and asymmetric splay state relaxations

We present a detailed discussion of the relaxation of splayed states in untwisted devices — the asymmetric H state (Ha), and the recently observed symmetric H state (Hs). Experimental evidence suggests that the Hs does not experience the optical bounce due to induced backflow usually associated with splay state relaxation. A dynamic model using Leslie–Eriksen–Parodi theory has been developed, and is used to model the flow within the device during switching. We show that there is no backflow during Hs relaxation, and that the flow profile is similar to that present during relaxation of the V state (the state used for pi‐cell operation). This flow enhances the switching of the Hs, leading to a faster relaxation than might be expected. The influence of the different viscosity parameters is examined in detail, and a comparison between the experimental and simulated results is given.     

The influence of flow on symmetric and asymmetric splay state relaxations

We present a detailed discussion of the relaxation of splayed states in untwisted devices — the asymmetric H state (Ha), and the recently observed symmetric H state (Hs). Experimental evidence suggests that the Hs does not experience the optical bounce due to induced backflow usually associated with splay state relaxation. A dynamic model using Leslie-Eriksen-Parodi theory has been developed, and is used to model the flow within the device during switching. We show that there is no backflow during Hs relaxation, and that the flow profile is similar to that present during relaxation of the V state (the state used for pi-cell operation). This flow enhances the switching of the Hs, leading to a faster relaxation than might be expected. The influence of the different viscosity parameters is examined in detail, and a comparison between the experimental and simulated results is given.     

Thresholdless switching induced by polar anchoring in antiferroelectric liquid crystals

We present a novel thresholdless switching mode in an antiferroelectric liquid crystal cell which is stabilized by the presence of polar anchoring at the cell surfaces and the antiferroelectric nature of the material. We also suggest other possible configurations which are induced by strong polar anchoring and possess quite different director structures and optical characteristics.     

Thresholdless switching induced by polar anchoring in antiferroelectric liquid crystals

We present a novel thresholdless switching mode in an antiferroelectric liquid crystal cell which is stabilized by the presence of polar anchoring at the cell surfaces and the antiferroelectric nature of the material. We also suggest other possible configurations which are induced by strong polar anchoring and possess quite different director structures and optical characteristics.     

Response speed of in-plane switching mode liquid crystal displays

The response speed of in-plane switching mode liquid crystal displays with three initial director configurations—homogeneous, twisted nematic and 180° super twisted nematic—was investigated via simulations. These simulations studied the relationship between the change in the optical axis and the optical transmission in the three configurations, allowing us to calculate the optical response times. The time-dependent change in the director was calculated using the Erickson-Leslie equations and these two results combined. These results predict that the response time during both the rise and decay periods of a super twist cell is about four times faster than the other two configurations.     

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.