Twisted Nematic Liquid Crystal Research Articles

Optical structure to improve the γ-curve distortion of a four-domain (4-D) twisted nematic liquid crystal cell

The γ-curve represents the viewing angle characteristics of a liquid crystal (LC) cell in all gray levels, including the dark state. In this paper, we propose a optical configuration of a four-domain (4-D) twisted nematic (TN) LC cell for improving the γ-curve distortion in the oblique direction, without any deterioration in the optical property of a conventional 4-D TN cell. A positive A-plate and a negative A-plate are applied at the top and the bottom of the 4-D TN LC layer, respectively. The two A-films applied can be optimized at all viewing angles by calculating the polarization difference (Δ p ) as functions of the retardation and the optical axis of the films, so that the transmittance distortion in the middle gray scale can be compensated for in all viewing directions. As a result of a calculation of the gamma distortion index (GDI), the γ-curve can be improved by more than 70% in the off-axis at 0° and 180°, and by more than 50% in the off-axis at 90° and 270°.

Acceleration of split-field finite difference time-domain method for anisotropic media by means of graphics processing unit computing

The implementation of split-field finite difference time domain (SF-FDTD) applied to light-wave propagation through periodic media with arbitrary anisotropy method in graphics processing units (GPUs) is described. The SF-FDTD technique and the periodic boundary condition allow the consideration of a single period of the structure reducing the simulation grid. Nevertheless, the analysis of the anisotropic media implies considering all the electromagnetic field components and the use of complex notation. These aspects reduce the computational efficiency of the numerical method compared with the isotropic and nonperiodic implementation. Specifically, the implementation of the SF-FDTD in the Kepler family of GPUs of NVIDIA is presented. An analysis of the performance of this implementation is done, and several applications have been considered in order to estimate the possibilities provided by both the formalism and the implementation into GPU: binary phase gratings and twisted-nematic liquid crystal cells. Regarding the analysis of binary phase gratings, the validity of the scalar diffraction theory is evaluated by the comparison of the diffraction efficiencies predicted by SF-FDTD. The analysis for the second order of diffraction is extended, which is considered as a reference for the transmittance obtained by the SF-FDTD scheme for periodic media.

Fabrication of an organic light-emitting diode inside a liquid crystal display

The fabrication of a hybrid device architecture fully integrating a transparent organic light-emitting diode (OLED) and a liquid crystal display (LCD) within two glass substrates is reported in this study. The transparent OLED was fabricated on the inner surface of the glass substrate. Twisted nematic liquid crystal (LC) materials were used to fill the space between the two glass substrates. The OLED was driven by an indium-tin oxide (ITO) anode on the glass substrate and a thin bi-metal (Al/Ag) cathode, which also served as the electrode of the LCD. The other electrode for the LCD-mode operation was the ITO on the other glass substrate. A commercially available ultraviolet (UV)-curable resin was spun onto the thin Al/Ag as the passivation layer to protect the OLED from attacks by the following polyimide layer (serving as the alignment layer of the LCD), rubbing process and LC materials. In this device structure, the electrical characteristic of the OLED-mode operation was almost the same as that of the control device. Current efficiency (in terms of cd/A) of the hybrid device from top-emission (towards the LCD) decreased by 26.5% due to optical interference effect, whereas efficiency from bottom-emission remained the same. The driving voltage of the LCD-mode operation increased by 1.6V due to the insertion of the passivation layer between the two electrodes. The contrast ratio decreased from 150 to 25 due to the reflection of the thin Al/Ag layer. Compared with that of the control device, the storage lifetime of the OLED increased as a result of filling the encapsulated cavity with LC materials, which helped repel ambient water and oxygen.

Adjustable Bistability and Hysteresis Loop in a Nd:GdVO4 Laser with an Intracavity Liquid Crystal Cell

Adjustable optical bistability and spatial hysteresis appearing in a Nd:GdVO4 laser with an intracavity liquid crystal (LC) cell were achieved. On the basis of optically induced LC reorientations and the varying magnitude of the order parameter, two types of LC were used to easily control the characteristics of hysteresis with a single control variable. Using an electrically driven twisted-nematic LC, an adjustable hysteresis area as well as a switching-on pump power was demonstrated and periodic spatiotemporal patterns were observed. By contrast, amplified hysteresis loops were obtained by increasing the rotation angle of homogeneous LC cells with a nearly invariable switching-on pump power.

Reflection resonance switching in metamaterial twisted nematics cell

Electric switching of reflection resonances at near-IR spectral range is experimentally demonstrated in a reflective metamaterial twisted nematic liquid crystal cell. Reflective metamaterial composed of nano-sized double-split ring resonator aperture is fabricated by a focused ion beam milling. Two-fold rotational symmetry of double-split ring resonators allows for two orthogonal polarization-dependent reflection resonances in the reflective metamaterial. With an external voltage of 10V across 12μm cell gap, a full switching is achieved between two reflection resonances. Dynamic measurements show the time constants of switch-on and switch-off are in the order of 100ms and 10ms, respectively.

Fully-Leaky Guided Mode Studies of Inverse Twisted Nematic Liquid Crystal

The fully-leaky waveguide technique has been used to explore the director of inverse twisted nematic liquid crystal cell. The reflection data, including all the polarization-conserving signals Rpp and Rss as well as the polarization-conversion signals Rsp and Rps, are recorded under various voltages applied to the cell of 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0V (rms), at a frequency of 1kHz. We found that this technique was sensitive to the director changes, especially for the polarization-conversion signals. Then by fitting angle-dependent reflectivity data to modeling theory, the arrangement of the director and some importance parameters of liquid crystal cell are obtained. It is proved that the reflectivity of fully-leaky waveguide mode can give the imperceptibility change in the cell.

Analysis of the chromaticity of near-field binary beam shapers

The chromatic properties of binary beam shapers inducing a spatially dependent transmission on the near field of an optical source after far-field filtering and imaging are derived. Beam shapers using pixels with two transmission states are highly versatile and have been experimentally realized with distributions of amplitude pixels, phase pixels, or polarization pixels that can be designed using a half-toning algorithm. The amplitude of the shaped beam in an image plane has a precisely controlled continuous profile given by the local density of the two pixel types in the object plane. The wavelength dependence of the field transmission corresponding to the full range of design transmission between 0 and 1 is studied analytically to assess the general performance of the corresponding beam shapers for broadband sources, with an example of a specific 2D transmission profile relevant to laser engineering. Amplitude beam shapers have no significant chromaticity, but have low damage threshold. The high-damage-threshold twisted nematic liquid crystal and phase shapers induce a wavelength-dependent transmission and phase on the shaped beam.

Switchable focus using a polymeric lenticular microlens array and a polarization rotator

We demonstrate a flat polymeric lenticular microlens array using a mixture of rod-like diacrylate monomer and positive dielectric anisotropy nematic liquid crystal (LC). To create gradient refractive index profile in one microlens, we generate fringing fields from a planar top electrode and two striped bottom electrodes. After UV stabilization, the film is optically anisotropic and can stand alone. We then laminate this film on a 90° twisted-nematic LC cell, which works as a dynamic polarization rotator. The static polymeric lenticular lens exhibits focusing effect only to the extraordinary ray, but no optical effect to the ordinary ray. Such an integrated lens system offers several advantages, such as low voltage, fast response time, and temperature insensitivity, and can be used for switchable 2D/3D displays.

Transmitted Ellipsometry Method for Extracting Physical Parameters of TN/VA/Inverse-TN Liquid Crystal Cells

A transmitted ellipsometry method is proposed for extracting the ellipsometric parameters (Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> and Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> ) of twisted nematic (TN), vertical alignment (VA), and inverse-TN (ITN) liquid crystal (LC) cells from the output Stokes parameters corresponding to five input polarized lights. The validity of the proposed approach is confirmed by comparing the experimental values of Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , and Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> with the simulated values obtained via a Genetic Algorithm curve-fitting approach. In experiments, the cell gap, pretilt angle, twist angle, and rubbing direction of TNLC and the cell gap, pretilt angle, and rubbing direction of VALC are extracted successfully. The sensitivity analysis results reveal that the ellipsometric parameters of the TNLC cell are sensitive to the cell gap, pretilt angle, twist angle and rubbing direction. However, those of the VALC and ITNLC cells are sensitive only to the cell gap, pretilt angle and rubbing direction. Overall, the results presented in this paper show that the proposed transmitted ellipsometry method provides a straightforward and accurate means of determining the ellipsometric and physical parameters of TN, VA, and inverse-TN LC cells.

Defect Core Structures in Twisted Nematic and Twisted Chiral Liquid Crystals

The defect structures of s = ±1/2 twist disclinations in twisted nematic and twisted chiral liquid crystals have been investigated within the Landau-de Gennes theory numerically. Our results show that there exists eigenvalue exchange across the defect core of both the two models. The defect core is essentially biaxial and never isotropic. The defect centre is uniaxial and is surrounded by a strong biaxial region.

Polarization modulation study of transmissive liquid crystal spatial light modulator using digital holographic polariscope

The study of phase modulation from the transmissive twisted nematic liquid crystal spatial light modulator (TN-LCSLM) with different incident states of polarization on its active area using Digital Holography method is presented. A pair of polarizer and quarter waveplate is used to illuminate the TN-LCSLM with elliptically polarized optical wavefront and transmitted optical wavefront is analyzed with a rotating analyzer. A single digital hologram is recorded from a complex optical wavefront passing through active display area of the TN-LCSLM at particular setting of different optical elements and quantitative phase is evaluated. We demonstrated that elliptically polarized light with certain ellipticity produces maximum phase modulation through the TN-LCSLM and the results are in good agreement with the eigenvectors suggested by Mueller matrix polarimetric characterization. This experimental setup can also be used as Grey-field Polariscope (GFP) in order to characterize important physical parameters such as the orientation of LC director molecules at input face and phase retardation with respect to addressed gray scale value on the TN-LCSLM active area. The TN-LCSLM is illuminated with circularly polarized light and change in this incident polarization after passing through the TN-LCSLM is recorded by four phase shifted digital holograms at four different analyzer orientations to measure these physical parameters.

MANIPULATION OF MAGNETIC NANOWIRE SUSPENDING IN HALF SPACE NEMATIC LIQUID CRYSTAL BY EXTERNAL MAGNETIC FIELD

Based on the elasticity energy, a magnetic nanowire suspended in a half-space twisted nematic liquid crystal (TNLC) was studied. Using equivalent capacitance, the equilibrium position of the nanowire, the deflexion of its axis from the anchoring direction and the eigenfrequency of the nanowire around its equilibrium position were calculated. The results show that the height of the nanowire increases slowly with increasing H in small magnetic field regime less than 0.01 Gs, and picks up quickly if H is larger than about 0.02 Gs. A calculation of the eigenfrequency of the nanowire shows that it decreases quickly with the increase of H at small magnetic field regime and then levels off towards a lower limit, which is about 170 Hz based on the present parameters under larger field. Moreover, the eigenfrequency also depends on mass with a maximum value which is found to be field-independent when the external field is large enough.

A wide-view twisted nematic liquid crystal cell with improved image quality obtained by decreasing γ-curve distortion

Wide-view (WV) film, which was introduced by Fuji Co., has been commercialized to block light leakage of twisted nematic (TN)-liquid crystal (LC) cells in a dark state when the viewing angles are in the off-axis direction. In this investigation, we designed an optical compensation structure of a TN-LC cell with an improved γ-curve, as well as a viewing angle property in the dark state. Basically, we used a pair of optical films consisting of a positive and a negative A-plate in order to avoid affecting the viewing angle property in the dark state. Then, we optimized the positive and negative A-plate to improve the viewing angle property in the gray levels by calculating polarization variations as a function of the optical axis and the retardation of the pair of the A-plates. From this calculation, we were able to show that the proposed normally white (NW) WV-TN cell exhibits wide viewing angle performance, including good γ-curve stability.

Twisted nematic liquid crystal polarization grating with the handedness conservation of a circularly polarized state

We propose a liquid crystal (LC) polarization grating that conserves the polarization state of incident light, wherein the variation range of the twist angle is 2π. The design scheme for theoretically 100% diffraction efficiency of the first-diffraction order is derived, and a prototype LC grating is evaluated. Under zero voltage, the fabricated LC grating exhibits high efficiency of the first-order diffraction, validating the proposed design scheme. The high efficiency of the second-order diffraction can also be achieved under a high voltage so that the LC director in the midplane is vertical to the substrate plane. The circular polarization sense of the second-order diffraction is identical to that of the incident light as in the case of the first-order diffraction. This grating functions as a beam deflector, steering the input beam in three different directions (zeroth-, first-, and second-order diffractions) by adjusting the applied voltage.

Experimental Study of Laser-Induced Orientational Effects in Twist and Planar Aligned Azobenzene Nematic Liquid Crystals

Laser-induced orientational effects are experimentally studied in BAAB-1005 type azobenzene NLC. Excitation of cis-form of azobenzene was induced by green laser with 0.532 μm wavelength. Nonlinearity was studied by He-Ne laser. 100 μm thick twist-aligned and planar-aligned LC-cells were used in the experiment. During the experiment occurrence of ring pattern was registered on the screen, which was formed during long time (about 10 seconds). Depending on direction of He-Ne laser polarization two types of rings were registered in case of twist-cell. This means that we have two nonlinear mechanisms. In case of planar-aligned LC cell only one type of rings was registered.

Electro-Optical Characteristics of ZrO2 Nanoparticle Doped Liquid Crystal on Ion-Beam Irradiated Polyimide Layer

It is well known that doping liquid crystals (LCs) with nanoparticles can readily change the physical and electro-optical properties of LC mixture. In this paper, we report on how the electro-optical properties and thermal stability of an LC system were enhanced by dispersing zirconia (ZrO2) nanoparticles in nematic LCs on ion-beam irradiated polyimide layers. Homogeneous LC alignment was achieved and ZrO2/LC mixture was applied in twisted-nematic (TN) mode. The addition of ZrO2 nanoparticles contributed to improvement of electro-optical properties in the TN LC cell by lowering voltage operation and decreasing response time. The TN LC cells with a ZrO2 nanoparticle concentration of 2.0 wt% showed the lowest threshold voltage of 2.0 V and the fastest response time of 15.3 ms. This enhanced electro-optical performance was likely due to van-der waals interactions and the screening effect of the ZrO2 nanoparticles in the LC medium. The thermal stability of the ZrO2/LC mixture was also improved compared to a pristine LC system.

Ultrathin Reflective Flexible Liquid Crystal Display Film

In this investigation, we present a thin reflective liquid crystal film with a mixed-mode twisted nematic mode. A low-temperature alignment process is used to obtain good orientation of the liquid crystal molecules. A microcell process is utilized to maintain a uniform cell gap of the flexible display under various bending conditions. To simplify the structure of a flexible mixed-mode twisted nematic liquid crystal display film, a layer of Zeonor optical plastic material is used in place of conventional plastic substrates; this material exhibits quarter wave retardation. The ultrathin device exhibits good cell gap uniformity, a high reflectivity of 35%, and a short response time of 6.67 ms.

Temperature-Frequency Converter Using a Liquid Crystal Cell as a Sensing Element

A new temperature-frequency converter based on the variation of the dielectric permittivity of the Liquid Crystal (LC) material with temperature has been demonstrated. Unlike other temperature sensors based on liquid crystal processing optical signals for determining the temperature, this work presents a system that is able to sense temperature by using only electrical signals. The variation of the dielectric permittivity with temperature is used to modify the capacitance of a plain capacitor using a LC material as non-ideal dielectric. An electric oscillator with an output frequency depending on variable capacitance made of a twisted-nematic (TN) liquid crystal (LC) cell has been built. The output frequency is related to the temperature of LC cell through the equations associated to the oscillator circuit. The experimental results show excellent temperature sensitivity, with a variation of 0.40% of the initial frequency per degree Celsius in the temperature range from −6 °C to 110 °C.

Voltage-induced defect mode coupling in a one-dimensional photonic crystal with a twisted-nematic defect layer

Defect modes are investigated in a band gap of an electrically tunable one-dimensional photonic crystal infiltrated with a twisted-nematic liquid crystal. Their frequency shift and interference under applied voltage are studied both experimentally and theoretically. We deal with the case where the defect layer thickness is much larger than the wavelength (i.e., the Mauguin condition). It is shown theoretically that the defect modes could have a complex structure with elliptic polarization. Two series of polarized modes are coupled with each other and exhibit an avoided crossing phenomenon in the case of opposite parity.

Note: Phase-locked loop with a voltage controlled oscillator based on a liquid crystal cell as variable capacitance

A phase-locked loop is demonstrated using a twisted-nematic liquid crystal cell as a capacitance that can be varied as a function of applied voltage. The system is formed by a phase detector, a low-pass filter, as well as a voltage controlled oscillator including such variable capacitance. A theoretical study is proposed and experimentally validated. Capture and locked ranges of hundreds of kHz have been obtained for the configuration used in this circuit. An application as frequency demodulator using a practical implementation of this circuit has been demonstrated.