Nematic Liquid Crystal Cell Research Articles

Optical measurements of the twist constant and angle in nematic liquid crystal cells

We present a reliable optical method for measuring the twist elastic constant K2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {K}_2$$\\end{document} and for assessing the total twist angle in a standard nematic twist cell. The method relies on the use of a non-standard configuration of crossed polarisers and a twist cell, which allows us to measure accurately the twist-cell parameters by reducing the degeneracy between them. Grid patching and an efficient beam propagation method are utilised in the numerical models used for fitting the experimental data. The modelling shows that the polarisation dynamics in a twist cell is non-trivial and much more complex than in a planar cell. The twist elastic constant of three commonly used liquid crystals (5CB, 6CHBT and E7) was successfully extracted from cross-polarised intensity measurements.

Genetic algorithm for the response of arbitrarily twisted nematic liquid crystals to an applied field.

When an external field is applied across a liquid-crystal cell, the twist and tilt distributions cannot be calculated analytically and must be extracted numerically. In the standard approach, the Euler-Lagrange equationsare derived from the minimization of the free energy of the system and then solved via finite-difference methods, often implemented in commercial software. These tools iterate from initial solutions that are compatible with the boundary conditions, providing limited to no flexibility for customization. Here we present a genetic algorithm that outputs fast and accurate solutions to the integral form of the equations. In our approach, the evolutionary routine is sequentially applied at each position within the bulk of the cell, thus overcoming the necessity of assuming trial solutions. The full range of twist angles from -90^{∘} to 90^{∘} is considered. In this way, the predictions of our routine strongly support the experimentally observed polarization transformations of light incident on different spatially varying twisted nematic liquid-crystal cells, patterned with different topologies on the two alignment layers.

Cascade Structured Plasmonic Liquid Crystal Biosensor for the Rapid Detection of Harmful Bacteria Dispersed in Potable Water

AbstractPathogenic microorganisms contaminating potable water are a serious water quality concern because they have severe consequences for human and environmental health. Managing water contamination requires the availability of fast and highly sensitive point‐of‐use detection systems responsive to a wide concentration range. In the present work, this goal is achieved by realizing a cascade‐structured biosensor that exploits innovative stimuli‐responsive materials such as gold nanorods (AuNRs) and photosensitive nematic liquid crystals (NLCs). The cascade structure is fabricated by interfacing a glass substrate in a back‐to‐front arrangement, hosting an array of bioactivated AuNRs and an NLC cell. The AuNRs array integrates microfluidic channels, allowing direct water sampling and the analysis of reduced water volumes with high sensitivity. The biosensor combines in the same device two independent optical transducers: a bioactive AuNRs array (plasmonic biosensor), sensitive to refractive index alterations, and an NLC cell that detects the presence of pathogens by responding to light intensity variations. The plasmonic biosensor performs exceptionally well for very low concentrations of bacteria. In contrast, the NLC biosensor works for high‐concentration bacteria, thus providing a cascade‐like detection system able to detect bacteria in a wide concentration range from 10 to 109 CFU mL−1.

INFLUENCE OF SURFACTANT CONCENTRATION ON CHANGES IN THE PLANE OF POLARIZATION OF LIGHT WHEN PASSING THROUGH THE LCD MEDIUM

The effect of surfactants in the form of cetyltrimethyl ammonium bromide (CTAB) in nematic liquid crystal cells of type 5CB have been studied by the polarization-optical method. The relationship between the concentrations of the introduced surfactant and the angle of rotation of the polarization plane is established. The clear boundaries of the linear and elliptical polarization of the light transmitted through the cell are indicated, depending on the concentration of surface-active substances (surfactants). Possible applications of such materials in semiconductor and solid-state laser technology and display technologies are proposed.

Moiré effect enables versatile design of topological defects in nematic liquid crystals

Recent advances in surface-patterning techniques of liquid crystals have enabled the precise creation of topological defects, which promise a variety of emergent applications. However, the manipulation and application of these defects remain limited. Here, we harness the moiré effect to engineer topological defects in patterned nematic liquid crystal cells. Specifically, we combine simulation and experiment to examine a nematic cell confined between two substrates of periodic surface anchoring patterns; by rotating one surface against the other, we observe a rich variety of highly tunable, novel topological defects. These defects are shown to guide the three-dimensional self-assembly of colloids, which can conversely impact defects by preventing the self-annihilation of loop-defects through jamming. Finally, we demonstrate that certain nematic moiré cells can engender arbitrary shapes represented by defect regions. As such, the proposed simple twist method enables the design and tuning of mesoscopic structures in liquid crystals, facilitating applications including defect-directed self-assembly, material transport, micro-reactors, photonic devices, and anti-counterfeiting materials.

Compact dual-focal augmented reality head-up display using a single picture generation unit with polarization multiplexing.

The augmented reality head-up display (AR-HUD) attracts increasing attention. It features multiple focal planes to display basic and AR information, as well as a wider field of view (FOV). Using two picture generation units (PGUs) to create dual-focal AR-HUDs leads to expanded size, increased cost, and reduced reliability. Thus, we previously proposed an improved solution by dividing one PGU into two partitions that were separately imaged into two virtual images with an optical relay system. However, the resolution of the PGU was halved for either virtual image. Regarding the drawbacks, this paper proposes a dual-focal AR-HUD using one PGU and one freeform mirror. Either virtual image utilizes the full resolution of the PGU through polarization-multiplexing. By performing optical design optimization, high image quality, except for the distortion, is achieved in an eyebox of 130 by 60 mm for far (10 m, 13° by 4°) and near (2.5 m, 10° by 1°) images. Next, we propose a distortion correction method by directly inputting the distorted but clear images acquired in the design stage into the real HUD with an inversed optical path. The proposed optical architecture enables a compact system volume of 9.5 L, close to traditional single-focal HUDs. Finally, we build an AR-HUD prototype, where a polarizing reflective film and a twisted nematic liquid crystal cell achieve polarization-multiplexing. The expected image quality of the two virtual images is experimentally verified.

Flexible control of terahertz chirality in twisted nematic liquid crystal cell with submillimeter thickness

Flexible control of terahertz chirality in twisted nematic liquid crystal cell with submillimeter thickness

Electrical Impedance Response of Liquid Crystals and Anomalous Diffusion: A Fractional Approach

We analyze the electrical impedance response established in terms of the time-fractional approach formulation of the Poisson-Nernst-Planck model by considering a general boundary condition. The total current across the sample is solenoidal, as the Maxwell equations require, and the boundary conditions can be related to different scenarios. We also study the behavior of the electrical conductivity and its connection with the ionic spreading in the sample to establish the diffusion regime present in the system in the low-frequency limit. Furthermore, we investigate impedance spectroscopy measurements of a nematic liquid crystal cell and observe a good agreement between the experimental data and the model.

Electrically induced orientational instability of the director in a homeotropic nematic liquid crystal cell and its effect on surface plasmon oscillations

ABSTRACT This study examines the electric field-induced orientational instability of the director in a homeotropically oriented nematic liquid crystal (NLC) cell. Transitions from homogeneous homeotropic state to non-homogeneous state and from non-homogeneous to homogeneous planar state were investigated. These transitions have a threshold and may exhibit hysteresis. Threshold voltages for these transitions were calculated and regions of existence of hysteresis were determined depending on the cell parameters. Increasing anchoring energy narrows hysteresis range for homeotropic to non-homogeneous transition and expands it for planar to non-homogeneous transition. The impact of the external electric field on the conditions and parameters of surface plasmon polariton (SPP) propagation in the Au–polymer film–NLC system was theoretically investigated. The effective refractive index of the SPP was found to increase with increasing anchoring energy of the NLC with the polymer film and with decreasing wavelength and applied voltage.

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.

Effect of rubbing symmetry on polarization distribution in ferroelectric nematic liquid crystal cells

It is a significant issue in controlling the polarization distribution in ferroelectric nematic liquid crystal cells. We investigated the polarization configuration of ferroelectric nematic liquid crystals on the surface and in the bulk of parallel/anti-parallel cells with rubbed substrates and proposed the polarization models in each cell. It is found that on the surface of rubbed polyimide films, the polarization direction is fixed and opposite to the rubbing direction. Splay and twist distribution of the polarization are formed in parallel and anti-parallel cells, respectively.

Dense packing of topological defects in nematic liquid crystal cells with pads, crossed-strips, and porous electrodes

Topological defect arrays in liquid crystal is an emerging optical material for smart windows, displays, gratings, and optical vortex generators. Formation of defect arrays is investigated using vertically aligned nematic liquid crystal cells with pad, crossed-strips, and porous electrodes. The location and types of the defects are identified using a polarized optical microscope. The pads and crossed-strips generate alternative radial and hyperbolic defects. Unexpectedly, the holes create dipoles of radial and hyperbolic defects, and the dipoles align in parallel order. The best dense packing of defects is achieved with the 15μm × 15μm unit cell.

Influence of dichroism on the optical properties of a twisted nematic liquid crystal cell in the terahertz region.

Polarization control is essential in terahertz (THz) imaging. Liquid crystals (LCs) have the potential to functionalize tunable polarization-control devices. Here, a twisted nematic (TN) cell using a hydrogen-bonded LC is fabricated, and the influence of dichroism in the THz region is discussed. Our results indicate that the polarization state in the Gooch-Tarry minimum condition is affected by the LC dichroism; a nondichroic LC is required for complete linearly polarized output. The output intensity of the dichroic LC-TN cell changed when electrically switched or when the incident THz wave polarization direction was rotated 90°. These intensity variations disappeared when using the nondichroic hydrogen-bonded LC.

Impact of red emissive ZnCdTeS quantum dots on the electro-optic switching, dielectric and electrochemical features of nematic liquid crystal: Towards tunable optoelectronic systems

In the present study, the concentration-dependent dielectric, electro-optical, and electrochemical properties of ZnCdTeS quantum dots (QDs) doped E7 nematic liquid crystal (NLC) mixtures were investigated. The dielectric permittivity components (ε‖ and ϵꓕ) and dielectric anisotropy (Δϵ = ε‖ ˗ ϵꓕ) of NLC samples containing varied concentrations of ZnCdTeS QDs (i. e. 0.10, 0.25, 0.50, 0.75, and 1 wt%) were measured at various temperatures. In the nematic phase, the results demonstrated that ε‖ increases much more than ϵꓕ upon an increase in the concentration of ZnCdTeS QDs. Δϵ enhanced as the concentration of QDs increased, reaching a maximum at 0.50 wt%, then decreased with further addition. Dielectric measurements revealed the formation of self-aligned QD arrays along the nematic director, which act similarly to multiple parallel capacitors in the NLC system. Moreover, electro-optical studies illustrated the significant effect of QDs doping on lowering the threshold voltage and response time. Interestingly, the optical switching-off time of NLC containing 0.50 wt% of the QDs decreased by ∼50% compared to that of the pure E7 sample. The reduced screening effect resulting from the QDs ion-capturing mechanism, enhanced effective intermolecular interactions, and increased dielectric anisotropy in the NLC system are the major factors responsible for the improved electro-optical characteristics. The impedance behavior of NLC cells was studied in the frequency range of 0.1 Hz–100 kHz. It indicated that the addition of ZnCdTeS QDs results in a remarkable increase of 96% in the electrical conductivity of the NLC system. Furthermore, the QDs doping significantly improved the NLC device's charge capacitance. Such studies would undoubtedly be beneficial for designing next-generation tunable optoelectronic systems since QDs can be utilized for tuning the dielectric anisotropy, electro-optical characteristics, charge capacitance, and conductivity of NLCs.

Optimisation of stratified anisotropic media for security films

ABSTRACT Optical security films have gained a lot of attention due to its potential application in authentication services. Electrode-free optical devices are in demand along with electrode devices due to its secure access to the decryption key and ability to hide information from public access. Our concern in this study is the examination of electrode-free optical security film. In such devices, hidden information can be viewed in black/white contrast or controlled birefringent colours, which are produced by patterned phase plate and polariser. In particular, we develop the model of stratified anisotropic media for theoretical description of reflective characteristics of optical security films. Our theoretical results are compared with experimental observations. Blue and yellow birefringent colours of supertwisted nematic liquid crystal cell are achieved by using theory of optical characteristics in anisotropic media and partial coherence. This theoretical analysis suggests new openings in the optimisation of liquid crystal security films.

Emergence of disordered branching patterns in confined chiral nematic liquid crystals

Spatial branching processes are ubiquitous in nature, yet the mechanisms that drive their growth may vary significantly from one system to another. In soft matter physics, chiral nematic liquid crystals provide a controlled setting to study the emergence and growth dynamic of disordered branching patterns. Via an appropriate forcing, a cholesteric phase may nucleate in a chiral nematic liquid crystal, which self-organizes into an extended branching pattern. It is known that branching events take place when the rounded tips of cholesteric fingers swell, become unstable, and split into two new cholesteric tips. The origin of this interfacial instability and the mechanisms that drive the large-scale spatial organization of these cholesteric patterns remain unclear. In this work, we investigate experimentally the spatial and temporal organization of thermally driven branching patterns in chiral nematic liquid crystal cells. We describe the observations through a mean-field model and find that chirality is responsible for the creation of fingers, regulates their interactions, and controls the tip-splitting process. Furthermore, we show that the complex dynamics of the cholesteric pattern behaves as a probabilistic process of branching and inhibition of chiral tips that drives the large-scale topological organization. Our theoretical findings are in good agreement with the experimental observations.

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.

Fast tunable metamaterial liquid crystal achromatic waveplate

Abstract Photonic metamaterials combined with liquid crystals (LCs) for tunability is a great niche for building miniature devices with high performance such as fast flat tunable lenses, tunable filters, and waveplates. Sub-wavelength or nano-grating surfaces are homogenized to uniaxial waveplates with negative birefringence of unique dispersion when the period is less than the wavelength by at least a few times. This uniaxial metasurface, combined with the LC layer, is shown to act as a tunable retardation achromatic waveplate with 8 μm thick LC layer operating over wide spectral and angular ranges, as compared to using two nematic liquid crystal (NLC) retarders of thicknesses on the order of 30–60 μm, when no metasurface is used. Hence the device becomes miniature and 50× faster due to the thinner liquid crystal layer. The silicon nano-grating of 351 nm pitch and 0.282 fill factor is designed and fabricated to operate in the short-wave infrared range (SWIR). Switching between three achromatic retardation levels: full-, half-, and quarter-waveplates is accomplished by changing the applied voltages on the NLC cell with a switching time of a few milliseconds. This device has applications in fast broadband shutters, low coherence phase shift interferometry, ellipso-polarimetry, dynamic control of light intensity, and smart windows.

Milky translucent haze of a large-scale topological defect array in nematic liquid crystal

ABSTRACTA large two-dimensional (2D) topological defect array in nematic liquid crystal (NLC) shows a translucent watery hazy texture. The haze is a composition of three optical phenomena: scattering of light, lens imaging and diffraction. The scattering of light causes the milky haze. Vertically aligned NLC cells with pixel electrodes were fabricated to produce the crowded irregular defects. The key parameters are the electrode shape (pad or annulus), the array structure (square or hexagonal) and the pixel size (from 15μm×15μm to 200μm×200μm). Location and topological charge of the defects are identified under a polarised optical microscope. 2D Fourier transform of the defect distribution gives the spatial frequency spectrum, which is the diffraction pattern of light through the defects. Large pad electrodes produce ordered defects and significant diffraction. Crowded irregular defects render the milky translucent haze and a continuous spatial spectrum. The traditional haze measurement is not able to distinguish the scattered and diffracted light. The spatial spectrum can accurately describe the appearance perceived by the viewers. Small annulus electrodes in a hexagonal array produce the messiest defects, the haziest texture and the most comfortable visual experience among the others.

Control over polarization of vertically emitting random lasers based on dye doped nematic liquid crystals by applying a transverse electric field locally

Flexible control over random lasers (RLs) is highly desired. In this manuscript, a control over the polarization of random lasing emitted vertically from dye doped nematic liquid crystal (NLC) cell by the transverse component of a local ac electric field was demonstrated. The polarizing direction of the RL could be tuned continuously and monotonously with the increase of the field strength, from the direction along the NLC director without an applied electric field. The maximum rotation angle of the polarizing direction reached nearly 90° at the applied voltage of 200 V (root-mean-square value) across the 230 μm wide slit. The degree of polarization declined from 1 at zero voltage, and after reaching the minimum rose with the field strength. The lasing spectrum consists of sharp spikes corresponding to the resonant modes for all the applied voltages. The mechanisms responsible for electrically tuning the polarization of the RL were discussed. Optimizing the voltage characteristic of the polarization tunability was suggested by the preliminary experimental results.