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.     

High-transmittance polymer-stabilised blue-phase liquid crystal display with double-sided protrusion electrodes

A polymer-stabilised blue-phase liquid crystal display (PSBP-LCD) with double-sided protrusion (DSP) electrodes structure is proposed. The oblique electric field between the protrusion electrodes inside both top and bottom glass substrates can induce more isotropic-to-anisotropic transition in the polymer-stabilised blue-phase liquid crystal (PS-BPLC) medium through Kerr effect than using the in-plane switching electrode. For the same electrode width, spacing and cell gap, the transmittance of PSBP-LCD with the DSP electrodes is ～29% higher than that using the IPS electrode.     

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).     

Blue-phase liquid crystal display with high dielectric material

A blue-phase liquid crystal display (BPLCD) with low operating voltage and high transmittance is demonstrated by using a high dielectric material, which is used as an insulation layer or protrusion fixed on the pixel and common electrodes in in-plane switching (IPS) mode. The operating voltage is reduced to about 14 V and the transmittance is improved for the BPLCD with high dielectric constant protrusion. Compared with the conventional protrusion electrode structure, the proposed protrusion can make manufacturing process simple and easy because the electrode has no complex shape. The results will be significant in designing optimal BPLCDs.     

Fast fringe-field switching of vertically aligned liquid crystals between high-haze translucent and haze-free transparent states

We report control of the haze value in a liquid crystal (LC) cell driven by a fringe electric field. When a fringe field is applied to a vertically aligned (VA) cell, a large spatial phase difference with a short grating period is induced in the LC layer. The average grating period of a VA cell driven by a fringe field is a quarter of the pitch of the interdigitated electrodes, which is half of the grating period of a VA cell driven by an in-plane field. Moreover, a sharper spatial phase profile is built around the edges of the interdigitated electrodes, which led to a high haze of 84.3% in the translucent state. The device was haze-free in the transparent state owing to the use of an LC layer without a polymer structure. To increase the haze value of the LC device while retaining a short response time, we developed an LC cell with crossed interdigitated electrodes where a large spatial phase difference is induced with little dependence on the azimuth angle. By applying a fringe electric field to a 20 μm thick LC cell using crossed interdigitated electrodes, we demonstrated a very high haze of 95.4% and a response time of less than 5 ms.     

Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.     

Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.     

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.     

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.     

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.     

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.     

Low voltage blue-phase liquid crystal display with insulating protrusion sandwiched between dual-layer electrodes

In order to lower the operating voltage of blue-phase liquid crystal display (BPLCD), a BPLCD with insulating protrusion, which is sandwiched between dual-layer electrodes, is proposed. There are four electrodes in this structure; thus, we investigate different driving methods to find a proper driving method. The effect of protrusion’s dielectric constant on operating voltage of the proposed BPLCD is explored under various electrodes’ parameters. As a result, the operating voltage of the proposed BPLCD with protrusion’s dielectric constant of 1,000 is 9.8 V, which is reduced by ~ 4.67× compared with that of conventional in-plane switching (IPS) BPLCD (45.8 V). Moreover, the zigzag electrode structure is adopted to reduce the large off-axis image distortion index. Besides, the azimuth distortion index is defined to describe the gamma shift between the minimum and maximum gamma shift curves at a certain polar angle. The results show that the off-axis image distortion index can be reduced to 0.0834, the azimuth distortion index is 0.0810 and the viewing cone of contrast ratio larger than 1,000:1 is over 50ºas the zigzag proposed BPLCD is used.     

Adaptive nematic liquid crystal lens array with resistive layer

ABSTRACT

We propose an adaptive nematic liquid crystal (LC) lens array using a dielectric layer with low dielectric constant as resistive layer. With the resistive layer and periodic-arranged iridium tin oxide (ITO) electrodes, the vertical electric field across the LC layer varies linearly over the lens aperture is obtained in the voltage-on state. As a result, a centrosymmetric gradient refractive index profile within the LC layer is generated, which causes the focusing behaviour. As a result of the optimisation, a thin cell gap which greatly reduces the switching time of the LC lens array can be achieved in our design. The main advantages of the proposed LC lens array are in the comparatively low operating voltage, the flat substrate surface, the simple electrodes, and the uniform LC cell gap. The simulation results show that the focal length of the LC lens array can be tuned continuously from infinity to 3.99 mm by changing the applied voltage.     

Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety

ABSTRACT

We designed and synthesised a reactive mesogen containing diphenylacetylene moiety in the mesogenic core and two polymerisable acrylate groups at both ends. By irradiating linearly polarised UV light on the conventional host LC mixture containing a small amount of the synthesised reactive mesogen in a sandwiched cell without an alignment layer, we demonstrated an in-situ photo-induced homogeneous alignment of liquid crystals without a pre-treated alignment layer, which was achieved by an irreversible polarisation-selective [2 + 2] photodimerization of diphenylacetylene moiety with linearly polarised UV irradiation at above the isotropic temperature of LC mixture. The resulting homogeneous alignment showed a superior initial dark state, negligible pretilt angle and excellent stabilities. Furthermore, the in-plane switching (IPS) LC cell prepared by this method exhibited a better dark state and electro-optic performance compared to that with conventional-rubbed polyimide alignment layer. The single photoirradiation process automatically resulted in a perfect alignment matching of optical axes between the top and bottom substrates in the LC cell, giving rise to an excellent dark state overcoming an intrinsic misalignment issue and complex fabrication process. The proposed in-situ alignment method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment technique in the manufacturing of high-resolution liquid crystal displays.     

Optical manipulation of charged microparticles in polar fluids

In this study, we report a systematic study of the response of a charged microparticle confined in an optical trap and driven by electric fields. The particle is embedded in a polar fluid, hence, the role of ions and counterions forming a double layer around the electrodes and the particle surface itself has been taken into account. We analyze two different cases: (i) electrodes energized by a step‐wise voltage (DC mode) and (ii) electrodes driven by a sinusoidal voltage (AC mode). The experimental outcomes are analyzed in terms of a model that combines the electric response of the electrolytic cell and the motion of the trapped particle. In particular, for the DC mode we analyze the transient particle motion and correlate it with the electric current flowing in the cell. For the AC mode, the stochastic and deterministic motion of the trapped particle is analyzed either in the frequency domain (power spectral density, PSD) or in the time domain (autocorrelation function). Moreover, we will show how these different approaches (DC and AC modes) allow us, assuming predictable the applied electric field (here generated by plane parallel electrodes), to provide accurate estimation (3%) of the net charge carried by the microparticle. Vice versa, we also demonstrate how, once predetermined the charge, the trapped particle acts as a sensitive probe to reveal locally electric fields generated by arbitrary electrode geometries (in this work, wire‐tip geometry).     

Graphene and liquid crystal mediated interactions

ABSTRACT

The two-dimensional graphene-honeycomb structure can interact with the liquid crystal’s (LC) benzene rings through π–π electron stacking. This LC–graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC’s nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphene-nematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC’s rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C* phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C* ordering resulting from the π–π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell.     

Uniform and fast switching of window-size smectic A liquid crystal panels utilising the field gradient generated at the fringes of patterned electrodes

A method to enable smectic A (SmA) liquid crystal (LC) devices to switch uniformly and hence fast from the clear state to a scattered state is presented. It will allow the reduction of the switching time for a SmA LC panel of 1 × 1 m² changing from a clear state to a fully scattered state by more than three orders to a few tens of milliseconds. Experimental results presented here reveal that SmA LC scattering initiates from the nucleated LC defects at the field gradient of the applied electric field usually along the edges of the panel electrode and grows laterally to spread over a panel, which takes a long time if the panel size is large. By patterning the electrodes in use, it is possible to create a large number of field gradient sites near the electrode discontinuities, resulting in a uniform and fast switching over the whole panel and the higher the pattern density the shorter the panel switching time. For the SmA LC panels used here, the ITO transparent electrodes are patterned by laser ablation and photolithography. It is shown that the defect nucleation time is much shorter than the growth time of the scattered region, hence it is possible to use the density of the field gradient sites to control the uniformity and switching time of a panel. Furthermore, the patterned SmA panels have a lower switching voltage than that of the non-patterned ones in general.     

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     

Fabrication of coloured liquid crystal device using photoluminescent biomolecular chlorophyll

We fabricated a coloured liquid crystal (LC) device using photoluminescent biomolecular chlorophyll. The chlorophyll molecule, which is inexpensive because of its abundance in nature, was doped in the LCs for manufacturing the coloured LC device. We confirmed the green colour property of the LC doped with chlorophyll dye using UV-vis spectroscopy. Although the LC cell filled with 0.5 wt% chlorophyll showed good vertical alignment and fast response time, the doping of LC medium with 1.5 wt% chlorophyll resulted in imperfect vertical alignment and slowed response time due to the aggregation of chlorophyll molecules at high concentration. Furthermore, the photoluminescence (PL) characteristics of the LC doped with chlorophyll were investigated using PL spectroscopy. The coloured LC cell doped with chlorophyll dye showed emission in the red wave number region under UV light. As the LC doped with chlorophyll exhibited good colour performance, conventional colour filter layers could be avoided with the employment of the proposed coloured LC device. By controlling the light source, it is possible to apply the advanced LC device for colour switching. Moreover, a full-colour-switching LC device can be realised using various biomolecular dyes that can emit other colours.