Twisted Nematic Liquid Crystal Cell Research Articles

Technological process optimization and measurement of image quality of the electrically bifocal metalens.

This Letter describes the design procedure and process optimization of the electrically bifocal metalens. In our design, horizontal and vertical polarization is manipulated by applying a suitable voltage to a twisted nematic liquid crystal (TN-LC) cell. Each nanostructure is designed to be a rectangular prism, making different polarizations of light experience various phase delays, thus causing bi-focus. We selected lithographical methods to fabricate our metalens because of the minimum physical size, which can be as small as 50 nm, and the maximum aspect ratio, which is as high as 15. Furthermore, to increase the tolerance and make the sidewall vertical and smooth, we coated different characteristics of photoresist sensitivity to the upper and lower layers. After the development, the mushroom-type photoresist makes Ni easier to strip while in the lift-off process, thus increasing the quality of the whole metalens. Our experiment shows that the focal lengths and focusing efficiencies corresponding to the two polarizations are similar to the simulation results. The proposed electrically modulated bifocal metalens can be utilized in different applications and combined with other optical components.

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

The Effect of Chemically Modified Multi-Walled Carbon Nanotubes on the Electro-Optical Properties of a Twisted Nematic Liquid Crystal Display Mode

In this study, we have chemically modified multi-walled carbon nanotubes (MWNTs) with different side chains for better dispersion in liquid crystal solutions, and fabricated twisted nematic liquid crystal cells doped with such MWNT derivatives. The introduction of MWNT derivatives affects the alignment of LC molecules with or without external electric fields. Electro-optical property tests showed that the contrast ratio changed slightly with the sharp decrease in drive voltage, improving the drive ability of the twisted nematic liquid crystal display (TN-LCD) mode.

Simple Differential Interference Contrast Imaging System by Using Twisted Nematic Liquid Crystal Cells

Simple Differential Interference Contrast Imaging System by Using Twisted Nematic Liquid Crystal Cells

Opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) film-coated tandem-90°-twisted nematic liquid crystal cell

The opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) (PVK) film-coated tandem-90°-twisted nematic liquid crystal (TNLC) cell is investigated. The tandem-90°-TNLC cell assembled by two common 45°-TNLC cells can be adopted for incident linearly polarized lights at any linear polarization direction. The variations of diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell are demonstrated opto-electrically and electro-optically. The differences in the degree of linear polarization between the tandem-90°-TNLC and single-90°-TNLC cells for demonstrating diaphragm apertures are investigated. The dynamic transmittance, related to orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric fields generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is studied, and the possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is also proposed. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the developments of other PVK film-coated LC cells. Moreover, the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter if the above ion-induced issues are overcome.

Light intensity and FOV-controlled adaptive fluidic iris.

In this paper, we propose a light intensity and field of view (FOV) controlled adaptive fluidic iris. A 90° twisted-nematic liquid crystal (TNLC) cell and two orthogonal polarizers are attached on the substrate. When a voltage is applied to TNLC, it can change the phase of the incident light, leading to change in the light intensity. A black mask is then placed in the middle of the chamber; when changing the hydraulic pressure from the inlet and outlet, the black mask can move within the chamber. It is equivalent to changing the iris position along the optical axis. The iris, therefore, can control the FOV in an optical system. The experiments show that the device can control the light intensity from 100% to 0% by applying a voltage of 9V on the TNLC cell. The proposed device can be applied in imaging systems and optical attenuators.

Visual evaluation of surface anchoring strength by electrohydrodynamic convection of a nematic liquid crystal.

Visual evaluation of the surface anchoring energies in a nematic liquid crystal (LC) cell is characterized by the direction of the convection roll pattern that appears in the low-frequency conduction regime. The convection roll pattern in a twisted nematic LC (TNLC) cell is oriented perpendicular to the midplane LC director dominating the direction of convection flow, and its direction is determined by the relative surface anchoring energy between two surface boundaries. Thus the direction of the roll pattern generated at the TNLC cell with asymmetric LC alignment layers can provide information on the surface anchoring energies at the two boundaries. We demonstrate a method for determining the two anchoring energies through a measured midplane LC director applied to the Ericksen-Leslie equation.

Twisted nematic liquid crystal cells with rubbed poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) films for active polarization control of terahertz waves

We fabricated a terahertz (THz) polarization converter using a twisted nematic (TN) liquid crystal (LC) cell. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films coated on quartz glass substrates were used as electrode layers in the TN LC cell. The PEDOT/PSS films were rubbed unidirectionally using a rayon cloth to align the nematic LC, thereby also serving as an alignment layer. The azimuthal surface anchoring strength of the PEDOT/PSS films was measured to be 5 × 10−4 J/m2 using the Néel wall method, which is similar to that of typical polymeric alignment layers. The optical constants of the PEDOT/PSS film in the THz range were also characterized using the Drude-Smith model, and the results indicated that the PEDOT/PSS films could be used both as transparent electrodes in the THz range and as alignment layers for the LC. The electro-optical properties of the fabricated TN LC cell were also investigated using a polarized visible laser and THz time-domain spectroscopic system. In particular, the transmission spectra and polarization conversion property of the TN LC cell in the THz range were theoretically analyzed based on a stratified model that considers optical anisotropy, absorption, and multiple interference. This work substantiates the advantages of TN LC cells with rubbed PEDOT/PSS films useful for THz polarization converters with electrical tunability.

Liquid crystal alignment on ion-beam irradiated homogeneous hafnium strontium oxide films deposited by sol–gel process

We investigated homogeneous liquid crystal alignment on hafnium strontium oxide films deposited by a solution process, where the liquid crystal alignment was induced by ion-beam irradiation. Uniform liquid crystal alignment was achieved on the HfSrO films grown at 300 °C. We confirmed the effect of surface morphology and chemical composition of HfSrO films using field-emission scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, we observed the electro-optical characteristics of the twisted-nematic liquid crystal cells based on hafnium strontium oxide films to verify the possibility of their application in liquid crystal displays.

Order reconstruction in inverse twisted nematic cell with an applied electric field

Order reconstruction in an inverse twisted nematic (ITN) liquid crystal cell with an applied electric field is investigated based on Landau–de Gennes theory and the two-dimensional finite-difference iterative method. Twice eigenvalue exchange in three-axis layer configuration, thrice eigenvalue exchange in four-axis layer configuration, and negative order parameter uniaxial twisted state exist in this cell, which can be described by the order parameter tensor Q in equilibrium state. The twice eigenvalue exchange also has two degenerate configurations with reduced electric field E from 0.8 to 2.8 in 10ξ cell (ξ is the biaxial correlation length). Moreover, two critical cell gaps dc⁎⁎=7ξ and dc⁎=12ξ are included in the study of the ITN cell. When d≤dc⁎⁎, only the eigenvalue change state exists. When d≥dc⁎, only a positive order parameter uniaxial twisted state exists near the threshold electric field. When dc⁎⁎<d<dc⁎, a negative order parameter uniaxial twisted state exists near the threshold electric field. Comparison of the eigenvalue exchange solutions of different cell gaps of 5ξ, 10ξ, and 20ξ shows that smaller cell gap can inhibit the complexity of the eigenvalue exchange solution and reduce the number of axis layers. This research provides a theoretical basis for the change of multi-axis layer defect and promotes the concept of eigenvalue exchange.

用全漏光导波技术测量反射式混合型扭曲向列相液晶盒的指向矢分布

用全漏光导波技术测量反射式混合型扭曲向列相液晶盒的指向矢分布

Fast response in TN liquid-crystal cells: effect of functionalised carbon nanotubes

ABSTRACTThe optical response time of a nematic liquid crystal (NLC) decreases due to incorporation of carbon nanotubes (CNTs) in the liquid-crystal host. Such reduction is believed to be due to an increase in the elastic constant of the nanotube-doped LC system. In this paper, we present the effect on optical response due to doping an NLC with octadecylamine functionalised single-walled carbon nanotubes (ODA-SWCNT) in a twisted alignment mode. The electro-optic switching amplitude of ODA-SWCNT nanocomposites of NLC decreases compared to pure NLC. A fast response time is observed with an increase in the concentration of ODA-SWCNT in NLC host. Additionally, optical response of pure NLC in a twisted nematic (TN) cell fabricated using mixtures of polyimide (PI) and ODA-SWCNT as an alignment layer is investigated. The optical response time decreased by ~75% in a TN cell fabricated with a mixture of PI and ODA-SWCNT compared to that of a TN cell prepared using a pure PI alignment layer. The presence of ODA-SWCNT in the alignment layer enhances the surface anchoring of the NLC molecules leading to an increase in the elastic constant and a decrease in the optical response time of NLC.

Characterization of voltage-driven twisted nematic liquid crystal cell by dynamic polarization scanning ellipsometry

A dynamic polarization scanning ellipsometry technique based on Stokes polarimetry is proposed for dynamically characterizing a voltage-driven twisted nematic liquid crystal (TNLC) cell. In the proposed method, the six effective ellipsometric parameters are extracted under modulation voltages ranging from 0 V ~ + 10 V using four linearly polarized input lights. The profiles of the tilt angle and twist angle are calculated as a function of the modulation voltage. The validity of the proposed method is confirmed by comparing the experimental results for the effective ellipsometric parameters of a TNLC cell with the analytical results. Furthermore, a genetic algorithm (GA) based on a curve-fitting technique is used to inversely extract the pretilt angle, twist angle and rubbing direction of the TNLC cell. These extracted values are then compared to the known valued of the TNLC cell. In general, the results presented in this paper show that the proposed method provides a reliable means of obtaining the dynamic optical properties of a TNLC cell.

Engineered liquid crystal anchoring energies with nanopatterned surfaces.

The anchoring energy of liquid crystals was shown to be tunable by surface nanopatterning of periodic lines and spaces. Both the pitch and height were varied using hydrogen silsesquioxane negative tone electron beam resist, providing for flexibility in magnitude and spatial distribution of the anchoring energy. Using twisted nematic liquid crystal cells, it was shown that this energy is tunable over an order of magnitude. These results agree with a literature model which predicts the anchoring energy of sinusoidal grooves.

Extracting the Physical Optical Parameters of TNLC Cell Using Effective Parameters and Stokes Polarimetry Method

A method is proposed for determining the cell gap, twist angle and pretilt angle of twisted nematic liquid crystal (TNLC) cells. In the proposed approach, the effective optical parameters of the TNLC cells are measured using a Stokes polarimetry method and the physical optical parameters are then extracted from these effective parameters using a simple analytical technique. The experimental results for the TNLC cell show that the extracted values of the twist angle, pretilt angle and cell gap are $\Phi=89.864^{\circ}\pm 0.028^{\circ}$ , $\xi=1.806^{\circ}\pm 0.050^{\circ}$ and ${\rm d}=10.696 \mu{\hbox{m}}\pm 0.025 \mu{\hbox{m}}$ , respectively. The experimental results for the twist angle and cell gap deviate from the known values by just $\Delta\Phi=0.136^{\circ}$ and $\Delta{\rm d}=0.196 \mu{\hbox{m}}$ , respectively. Moreover, the pretilt angle is found to lie in the reliable range. In contrast to existing methods, the technique proposed in this study requires the use of only a single illuminating wavelength. As a result, it provides a cheaper and more straightforward approach for extracting the three physical parameters of TNLC cells than existing optical-parameter measurement schemes.

Optically anisotropic microlens array film directly formed on a single substrate

An optically anisotropic microlens array film directly formed on a single substrate is demonstrated. UV curable diacrylate monomers are coated as a film on the substrate. Under the action of fringing field, not only the film surface is flattened by the generated dielectric force but also the monomers are reoriented to form a gradient refractive index (GRIN) distribution in the film. Via UV exposure, the GRIN distribution is fixed and the polymeric film behaves as a microlens array. The fabrication process is simple and the film offers a switchable focus through controlling the polarization direction of the incident light. Integrating with a 90° twisted-nematic liquid crystal cell, our polymeric microlens array film shows great potential for switchable 2D/3D autostereoscopic displays.

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.

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.

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.