Twisted Nematic Cell Research Articles

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.

Physical properties of hydrogen bonded nematic liquid crystals and electro-optical properties in terahertz waves [Invited

4-m-alkyl cyclohexane carboxylic acids and 4-n-alkylbenzoic acids were mixed to form hydrogen bonded liquid crystals (HBLCs), which exhibited a nematic phase at room temperature. Dielectric constants and three elastic constants of HBLC mixtures were investigated. Benzoic acid derivatives with some polar groups were added to the HBLC and the dielectric anisotropy was successfully enhanced. Considering the absorption loss of HBLCs in the terahertz wave region, the electro-optical properties of the homogeneous and twisted nematic cells were numerically evaluated using measured physical properties, and the transmitted polarization state was compared with that of conventional nematic LCs.

Refractive-type varifocal liquid-crystal Fresnel lenses for smart contacts.

We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 VRMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.

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.

Fabrication of Polarization Grating on N-Benzylideneaniline Polymer Liquid Crystal and Control of Diffraction Beam

Photoresponsive photoalignable liquid crystalline polymers composed of phenyl benzoate terminated with N-benzylideneaniline were evaluated. These polymers are capable of axis-selective photoreaction, photoinduced orientation, and surface relief grating formation. Polarization holography using an He-Cd laser beam at a wavelength of 325 nm demonstrated the formation of a surface relief grating with a molecularly oriented structure based on periodic light-induced reorientation and molecular motion. Electrical switching of diffracted light using an electric field response of twisted-nematic cell containing a low-molecular-weight liquid crystal in combination was also demonstrated.

Shear direction switching phenomena in twisted nematic liquid crystal cells for novel differential interference contrast imaging systems

AbstractA lateral shear occurs between ordinary and extraordinary rays in general optical uniaxial crystals, and we can observe a double image through the crystal, indicating a typical birefringence phenomenon. Small lateral shears occur in typical liquid crystal (LC) cells under small voltage applications, where the molecular alignment direction is generally inclined. The small lateral shear is useful for differential interference contrast (DIC) imaging systems, although it must be negligible in a typical LC display panel. In particular, unique lateral shearing properties are exhibited by twisted nematic (TN) LC cells, which are popular and important LC display modes. The 45°‐oblique lateral shear occurs according to the direction of the surface molecular alignment, and the inclination direction (+45° or −45°) depends on the molecular twist direction. If it is possible to switch the twist direction in the TN cell, the lateral shear direction can be switched by 90°, and a full DIC imaging system can be attained because the normal DIC system can obtain one‐directional derivative information along the shear direction. In this work, twist direction switching phenomena in TN cells are investigated by applying a magnetic field in the direction parallel to the cell surface.

Analysis of Electro-Optical Behavior in Liquid Crystal Cells with Asymmetric Anchoring Strength

Liquid crystal director distributions have been numerically analyzed between asymmetric anchoring surfaces, that is, infinitely strong and very weak anchoring strength interfaces. In a hybrid aligned nematic (HAN) cell and a twisted nematic (TN) cell, HAN and TN orientations turn to a homogeneous orientation when the weak anchoring strength is lower than a critical one. Relationships between the anchoring strength and elastic constants of the liquid crystal were analyzed to be of a quasi-homogeneous orientation. The quasi-homogeneous orientation returned to the original HAN and TN orientations under voltage application. Low-driving electro-optical properties with no threshold voltage can be obtained in a quasi-homogeneous HAN cell. A unique voltage–transmission curve of 0–100–0% appeared in a quasi-homogeneous TN cell between the crossed polarizers.

Orientation-induced properties of anisotropic polyacrylamide thin layer via plasma treatment in liquid crystal system

Uniform liquid crystal (LC) alignment was achieved on a polyacrylamide (PAM) film treated by a plasma ion beam (IB), and the alignment status of the LCs was confirmed by polarized optical microscopy and pretilt angle analysis using antiparallel LC cells assembled using the IB-treated PAM (IB-PAM) layer. Physically modified characteristics on the PAM surface caused by the IB treatment were observed by atomic force microscopy. Etching with the IB caused roughness reduction and smoothing on the surface with increased irradiation time. Chemical modifications on the surface were investigated by X-ray photoelectron spectroscopy, whose results demonstrated stoichiometric changes causing formation of CO bonding and polarity on the surface. The IB incident at 40° drives the surface polarity anisotropy and van der Waals interactions with the LC molecules, thereby achieving uniform LC alignment. Thermal endurance tests of the IB-PAM layer showed maintenance of the uniform LC alignment status up to 120 °C. Twisted-nematic cells based on the IB-PAM layer showed suitable electro-optical performance with low operating characteristics. The IB-PAM films also showed high and stable optical transparencies. Thus, IB treatment on polymer layer is deemed suitable for the LC alignment process, and IB-PAM film is an excellent choice for next-generation LC systems.

Fast responsive 2D/3D switchable display using a liquid crystal microlens array.

A fast responsive two-dimensional/three-dimensional (2D/3D) switchable display is demonstrated based on an active liquid crystal (LC) microlens array and a twisted nematic (TN) cell. Compared with the traditional LC microlens array, the fabricated LC microlens array has more ideal phase profile and better focusing effect. The TN cell can switch the polarization direction of the incident light with a very short switching time (4.3 ms) and a small driving voltage (5Vrms). By introducing the tilted elemental image arrays and tilted LC lens array, the moiré patterns are eliminated. The fast switching of the 2D/3D display can be realized by applying or removing voltage to the TN cell. The fast responsive 2D/3D switchable display with the LC microlens array and the TN cell is thin and compact without moiré pattern compared with other 2D/3D switchable display devices.

Multi-Functional Film Based on Aligned CNT Sheet for Liquid Crystal Display Application

Carbon nanotube (CNT), an extremely long hollow cylinder with nanometer-sized diameter, is a promising carbon material with unique thermal, electrical, and physical properties. Due to their peculiar and fascinating properties, CNTs have been studied and used in various industries since their discovery. The CNTs are potentially viable alternatives for conventional materials in electric devices considering their mechanical resilience, high electrical conductivity, and optical transparency. The alignment of CNTs into ordered structures garnered a special interest as it enhances electrical conductivity and optical transparency, and generates a new intriguing anisotropic optical absorption property. The optical properties of highly oriented CNT sheets have been investigated to apply as optical polarizer and polarized light source. Further, the flexible electrodes derived from an aligned CNTs have unveiled applications in super-capacitors, solar cell, and lithium-ion battery. Therefore, to utilize CNTs in LCD and many other flexible electronic device applications it is essential to attain highly ordered CNTs which should be static even under external stresses such as bending, pressing, and strain.In this work, we fabricated the aligned CNT sheet films (a-CSFs) in which the grown CNTs are stretched unidirectionally on the substrate and its orientation is fixed via polymerization of a coated monomer and then measured its optical and mechanical properties, testing feasibility of its application for multi-functioning films such as polarizer, electrode, and liquid crystal (LC) alignment. The fabricated a-CSF exhibits polarization efficiency (PE) (87.23% of 10 layers of a-CSF), sheet resistance (361.7 Ω/sq), and an excellent aligning capability of LC molecules as shown in Fig. 1 (a~c). Besides, flexibility and stability test of a-CSF via repeating bending test of 1,000 cycles at bending radius of 2 mm exhibited very low variation in the change ratio of sheet resistance and transmittance less than 8% and 10%, respectively.Utlilizing the mutifunctionalty of a-CSF, a liquid crystal display (LCD) with a twisted nematic(TN) mode was fabricated and exhibited feasibility of dynamic swicthing between bright and dark states on application of voltage off/on even without using conventional components of LCD such as transparent electrode, film-type polarizer, and polyimide-type alignment layer. We measured the voltage-dependent transmittance of both devices based on the a-CSF with 5 layers and 8 layers. The threshold voltage (Vth) and operating voltage (Vop) are defined as the voltage required to achieve 90% and 10% of transmittance relative to maximum transmittance, respectively. The measured Vth of both devices with 5 and 8 layers are 1.7 Vrms and 1.3 Vrms, respectively and the measured Vop of both devices are 7.6 Vrms and 4.3 Vrms, respectively. In the case of 8 layers, it shows slightly lower Vth and Vop, which is mainly due to the lower sheet resistance of a-CSF with 8 layers than 5 layers as shown Fig 2.In conclusion, we fabricated the multi-functional a-CSF with properties of the electrode, polarizer, and alignment layer based on the a-CSF using the simple fabrication process. We confirmed the a-CSFs exhibit better conductivity and PE in the visible regime. Furthermore, it has the excellent grooved morphology along the drawn direction of a-CS for homogeneous alignment of LC molecules. We carried out the bending test to confirm the stability of the films from mechanical stress. It showed excellent stability after bending tests of 1,000 cycles. The performance of the fabricated TN cells using a-CSFs were considerably working good. Figure 1

Formation of the Shock Pressure Regime in the Form of a Traveling Wave in Nematic Twisted Cells

The paper presents a numerical study, within a nonlinear generalization of the classical Ericksen–Leslie theory, of a new nonlinear mechanism for the formation of the regime of shock pressure on the boun-ding walls of a microsized twisted nematic cell (TNC), realized in the form of the kinklike traveling wave $$\mathcal{P}$$(z – $${v}t$$) under the effect of the externally applied electric field and induced by a strong local distribance of the director field in the form of a Gaussian (normal) distribution. The mechanisms responsible for the formation of the traveling wave of shock pressure propagating in the TNC from one of its boundaries to another are studied, and it is shown how the magnitude of the electric field and the shape of the localized initial perturbation of the director field affect the similarity of the traveling wave to the kinklike wave. Studies of the dynamic relaxation of the director field in TNCs also showed that, at temperatures exceeding the temperature for the nematic–smectic A (NA) TNA phase transition by several dozen mK, fluctuations in the order parameter of the emerging smectic phase suppress the effect of the electric field and favor the singular behavior of the azimuthal anchoring energy density as T → TNA.

Формирование режима ударного давления в форме бегущей волны в нематических твистовых ячейках

Abstract The paper presents a numerical study, within a nonlinear generalization of the classical Ericksen–Leslie theory, of a new nonlinear mechanism for the formation of the regime of shock pressure on the boun-ding walls of a microsized twisted nematic cell (TNC), realized in the form of the kinklike traveling wave $$\mathcal{P}$$ ( z – $${v}t$$ ) under the effect of the externally applied electric field and induced by a strong local distribance of the director field in the form of a Gaussian (normal) distribution. The mechanisms responsible for the formation of the traveling wave of shock pressure propagating in the TNC from one of its boundaries to another are studied, and it is shown how the magnitude of the electric field and the shape of the localized initial perturbation of the director field affect the similarity of the traveling wave to the kinklike wave. Studies of the dynamic relaxation of the director field in TNCs also showed that, at temperatures exceeding the temperature for the nematic–smectic A (NA) T _NA phase transition by several dozen mK, fluctuations in the order parameter of the emerging smectic phase suppress the effect of the electric field and favor the singular behavior of the azimuthal anchoring energy density as T → T _NA.

Special Issue on Liquid Crystal on Silicon Devices: Modeling and Advanced Spatial Light Modulation Applications

Since the first liquid crystal displays (LCDs) at the beginning of the seventies—based on the twisted-nematic cell configuration [...]

Bistable switching of polarization-grating diffractions enabled by a front bistable twisted nematic film.

Bistable electrical switching of high-efficiency grating diffractions is realized by adding a front π-bistable twisted nematic (π-BTN) cell to a passive liquid crystal polarization grating (LCPG). The π-BTN cell can be switched between stable non- and 180°-twisted states, acting as a polarization converter that switches the polarization of a laser beam and thus changes the diffraction behavior of the LCPG. Both states of the π-BTN cell are stable and can be reversibly switched to each other by applying a voltage pulse of different frequencies. We experimentally demonstrate two bistable-switching operations: (i)the BTN-LCPG can either split or deflect the laser beam; and (ii)the BTN-LCPG can selectively diffract the laser beam to the +1st and -1st orders, while maintaining a high diffraction efficiency of ∼90%. With electrical switchability, a simple optical design, and low power consumption, the proposed BTN-LCPG concept is favorable in various applications.

Ion-beam-induced surface modification of solution-derived indium-doped zinc oxide film for a liquid crystal device with stable and fast switching properties

The characteristics of a solution-derived indium-doped zinc oxide (In:ZnO) film exposed to ion-beam (IB) irradiation as a liquid crystal (LC) alignment layer were investigated. Solution processing was conducted to deposit a thin layer on a substrate and irradiation at various IB irradiation energies was used as an LC alignment method, all of which achieved uniform and homogeneous LC alignment. Atomic force microscopy using three-dimensional imaging and numerical analyses showed that the IB irradiation reduced the surface roughness. Through the X-ray photoelectron spectroscopy, it was revealed that the modification of the chemical composition due to the IB irradiation affected the state of the LC alignment. Increased polarizability of the surface and oxygen vacancies induced strong van der Waals forces between the In:ZnO film surface and the LC molecules, thereby subsequently achieving uniform and homogeneous LC alignment. The electro-optical (EO) characteristics of a twisted-nematic cell made with IB-irradiated In:ZnO film at an IB irradiation energy of 2200 eV, with which outstanding EO performance was observed. Therefore, using the solution-derived In:ZnO film with the IB irradiation as an alignment shows remarkable potential for use in LC device.

Liquid crystal alignment on ion-beam irradiated bismuth-doped tin oxide films and their application to liquid crystal displays

ABSTRACTUniform and defect-free homogeneous alignment of liquid crystal (LC) molecules on solution-derived bismuth-doped tin oxide (TBO) films has been achieved using ion-beam (IB) irradiation. We performed measurements and physicochemical analysis to verify and establish the cause of the successful LC alignment. In addition, we measured the electro-optical characteristics of twisted-nematic cells with IB-irradiated TBO films to explore the suitability of this material for liquid crystal displays (LCDs). The results indicate that this approach will allow the fabrication of high-performance enhanced LCD devices.

Ionic effects on electro-optics and residual direct current voltages of twisted nematic liquid crystal cells

ABSTRACTBy using electro-optical and dielectric absorption [1–4] measurements, we report our investigations of ionic effects on electro-optics and residual direct current voltages (VrDCs) of two polyimide (PI)-aligned twisted nematic (TN) cells with same liquid crystal mixture but different PI-alignment materials. We have also carried out new experimental methods to find out that the observed VrDCs were caused by LC-PI-interfacial trapped ions generated and transported from the LC medium for one TN cell, and from the PI layers for the other TN cell. Our measured VrDCs indicated that the former had two different exponential-decay rates similar to the published results by M. Mizusaki et al. [2] but the latter had only a single exponential-decay rate.

Mid-wave infrared beam steering based on high-efficiency liquid crystal diffractive waveplates.

We demonstrated two liquid crystal diffractive waveplates: one optimized for near-infrared (1.06 µm), and another for mid-wave infrared (MWIR, 3~5 µm). By employing a low loss liquid crystal mixture UCF-M3, whose absorption loss is below 2% in the 4~5 µm spectral region, the grating achieves over 98% diffraction efficiency in a broad MWIR range. To switch the grating, both active and passive driving methods can be considered. In our experiment, we used a polymer-stabilized twisted nematic cell as the polarization rotator for passive driving. The obtained rise time is 0.2 ms and decay time is 10 ms.

An Electrically Tunable Liquid Crystal Lens with Coaxial Bi-Focus and Single Focus Switching Modes

A hole-patterned electrode liquid crystal (LC) lens with electrically switching coaxial bi-focus and single focus modes of tuning is demonstrated. The proposed LC lens mainly consists of a two LC layer (TLCL) structure with different thicknesses to achieve higher focusing power than the conventional hole-patterned electrode LC lens with the same aperture size. In the TLCL structure, one LC layer, doped with 3 wt % RM257, was photopolymerized to achieve a fixed focusing power of 18.5 Diopter. Due to polarization dependence in TLCL lenses, an additional 90° twisted nematic (TN) cell was used to change the incident polarization in order to switch lens functions on or off. As a result, a fixed focusing power of 18.5 Diopter was achieved when voltages of 10 Vrms were applied to the 90° TN cell. In addition, the switching capabilities of the bi-focus and single focus modes were achieved when operating individually with applied voltages from 20 Vrms to 90 Vrms, and higher voltages of over 90 Vrms, respectively. The maximum focusing power in the fabricated TLCL lens is 30.9 Diopter.

Investigation of laser speckle noise suppression by using polymer-stabilized liquid crystals within twisted nematic cell

We propose a robust method to suppress laser speckle using a polymer-stabilized liquid crystal (PSLC) device with high initial transmittance. With applied voltage, a large modulation depth has been produced through light scattering because of the refractive index mismatch between the rotated nematic liquid crystals and the polymer networks. By using PSLCs with 5 wt % monomer, a speckle noise reduction rate of approximately 54.7% can be achieved with an applied voltage of 6 V. The lowest speckle contrast of approximately 0.025 with relatively high discrimination from the projected AF image has also been demonstrated through a wedge PSLC cell.