Liquid Crystal Cell Research Articles

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.

Energy-Efficient Liquid Crystal Smart Window with a Clear View

In this study, we enhance the angular-selective light absorption capabilities of guest–host liquid crystal (GHLC) cells by introducing a novel design featuring a uniform lying helix (ULH) structure. Previously GHLC cells, predominantly vertically aligned cells absorbed obliquely incident light but compromised x-direction visibility. In stark contrast, our ULH-based design allows incident light to seamlessly traverse transmittance in both z- and x-directions while efficiently obstructing oblique incident light in the y-direction. Our innovative ULH-based GHLC cell achieves an impressive optical performance. Specifically, it attains a substantial transmittance rate of 56.7% in the z-direction. Furthermore, in oblique views encompassing both the x- and y-directions, it maintains competitive transmittance rates of 44.2% and 29.5%, respectively. This strategic design not only ensures clear and unobstructed views for building occupants in the z- and x-directions but also contributes significantly to energy conservation by preventing oblique incident light from penetrating, thus reducing cooling requirements. Our ULH-based GHLC cell represents a breakthrough in smart window technology, offering an elegant solution to the challenge of balancing energy efficiency and occupant comfort in architectural settings. This advancement holds promising implications for sustainable building designs by enhancing indoor environmental quality while mitigating energy consumption for cooling, ultimately redefining the potential of smart windows in contemporary architecture.

Effect of Azo Dye on Electro-Optic and Optical Band Gap Characteristics in ZnO Nanoparticles Induced Vertically Aligned Liquid Crystal Cells

Alignment of liquid crystal (LC) molecules in vertical direction is the most promising and interesting aspect for display devices in view of scientific and technological growth. Moreover, in accordance with the photovoltaic device applications, liquid crystals (LCs) show numerous applications. In the present work, the impact of dye as dopant with appropriate amount, over the electro-optic (E-O) and band gap properties as well as phase behaviour of vertically aligned liquid crystals (VALCs) has been studied and discussed. Initially, zinc oxide (ZnO) nanoparticles (NPs) were mixed to induce the vertical alignment (VA) in confined cell. Then, to prepare the dye doped LC sample, 0.125% of azo dye as dopant was uniformly mixed in the host sample. The results showed enhanced E-O characteristics with reduction in optical band gap as calculated using UV Visible study in 0.125% dye doped cell as compared to host sample cell.

Influence of Capillary Flow on Self-Organization of Water Droplets in Nematic Liquid Crystal

The results of studying the motion of isotropic liquid drops in a homeotropic layer of liquid crystal Е7 with long-lived instabilities induced by a damped shear flow that occurs when a flat capillary is filled due to the action of capillary forces (capillary flow). An intense flow of nematics with initial homeotropic orientation leads to the appearance of orientational instabilities. In the experiments, a liquid-crystal cell with a variable gap (from 60 to 100 μm) was used. The object of the study was a 10:1 emulsion "liquid crystal – isotropic liquid". It was found that drops of isotropic liquid form various types of spatial structures. In particular, chains of drops in the region of strong deformations of LC orientation (for example, at the boundaries of the existence of long-lived regions with a changed orientation) were observed. In addition, an increased concentration of isotropic droplets occurred in the region separating the sections of the liquid crystal under the action of an electric field and in the absence of its action. Based on the analysis of microscopic images, the velocity of mutual motion of two isolated drops was determined. It made possible to estimate the interaction force between the drops (Fa = 5.6 pN) that was caused by the distortions of orientational structure in the vicinity of the drops. The results obtained are discussed within the existing theory of liquid crystals.

Influence of mechanical vibration actions on the memory effect of PDLC films doped with SiO<sub>2</sub> nanoparticles

The effect of mechanical vibration in the frequency range from 200 Hz to 2500 Hz on the memory effect in 5CB polymer dispersed liquid crystals doped with SiO2 nanoparticles has been studied. As a result of capacitance-voltage measurements, the dependences of the dielectric properties of liquid crystal cells with 0.1 wt% SiO2 on the bias voltage. It has been established that the memory effect detected by dielectric hysteresis in polymer dispersed liquid crystals with silicon dioxide nanoparticles monotonically decreases with increasing oscillation frequency and completely disappears at a frequency of 2000 Hz due to the destruction of the network of silica nanoparticles and, respectively, the appearance of an isotropic orientation liquid crystals molecules.

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

A liquid crystal smart window for energy saving and harvesting

Energy conservation, particularly in buildings, has become a critical focus area. Among the various components of a building, windows play a significant role in the exchange of heat and light. Electrochromic systems, known for their ability to switch between transparent and opaque states with the aid of external energy, have served as the basis for conventional smart windows. This research introduces a novel approach to smart window technology by incorporating a luminescent solar concentrator (LSC) that incorporates thin photovoltaic cells at the window's edges, coupled with a thermally and optically adjustable liquid crystal cell. The device intelligently transitions between an energy-saving, opaque mode (day mode) and a transparent, idle mode (night mode), based on ambient temperature and incident light levels. Through this innovative design, the proposed smart window offers enhanced energy efficiency and harvesting capabilities, promising significant benefits for sustainable building practices.

Robust and Accessible True Random Number Generator Exploiting the Structural Frustration of Nematic Liquid Crystals

AbstractThe study proposes a highly credible true random number generator based on the structural frustration of nematic liquid crystals, which refers to the appearance of two energetically equivalent structures in a nematic system. Frustration is induced in a simple liquid crystal cell by applying an electric field and observing optical textures under crossed polarizers. A clockwise or counterclockwise spiral image appears on 630 separate circular electrodes under the electric field, and random binary sequences are obtained by assigning a digit of 0 or 1 to each image. These sequences pass the strict statistical randomness tests of the National Institute of Standards and Technology with a pass rate of 0.992 or higher for the ten applicable tests and exhibit excellent uniformity. Furthermore, a simple method is introduced to convert the binary results to random decimal numbers with only a minimal amount of data processing.

Compressive hyperspectral microscopy for cancer detection.

Hyperspectral microscopy grants the ability to characterize unique properties of tissues based on their spectral fingerprint. The ability to label and measure multiple molecular probes simultaneously provides pathologists and oncologists with a powerful tool to enhance accurate diagnostic and prognostic decisions. As the pathological workload grows, having an objective tool that provides companion diagnostics is of immense importance. Therefore, fast whole-slide spectral imaging systems are of immense importance for automated cancer prognostics that meet current and future needs. We aim to develop a fast and accurate hyperspectral microscopy system that can be easily integrated with existing microscopes and provide flexibility for optimizing measurement time versus spectral resolution. The method employs compressive sensing (CS) and a spectrally encoded illumination device integrated into the illumination path of a standard microscope. The spectral encoding is obtained using a compact liquid crystal cell that is operated in a fast mode. It provides time-efficient measurements of the spectral information, is modular and versatile, and can also be used for other applications that require rapid acquisition of hyperspectral images. We demonstrated the acquisition of breast cancer biopsies hyperspectral data of the whole camera area within . This means that a typical biopsy can be measured in . The hyperspectral images with 250 spectral bands are reconstructed from 47 spectrally encoded images in the spectral range of 450 to 700nm. CS hyperspectral microscopy was successfully demonstrated on a common lab microscope for measuring biopsies stained with the most common stains, such as hematoxylin and eosin. The high spectral resolution demonstrated here in a rather short time indicates the ability to use it further for coping with the highly demanding needs of pathological diagnostics, both for cancer diagnostics and prognostics.

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.

Bright-Field and Edge-Enhanced Imaging Using an Electrically Tunable Dual-Mode Metalens.

The imaging of microscopic biological samples faces numerous difficulties due to their small feature sizes and low-amplitude contrast. Metalenses have shown great promise in bioimaging as they have access to the complete complex information, which, alongside their extremely small and compact footprint and potential to integrate multiple functionalities into a single device, allow for miniaturized microscopy with exceptional features. Here, we design and experimentally realize a dual-mode metalens integrated with a liquid crystal cell that can be electrically switched between bright-field and edge-enhanced imaging on the millisecond scale. We combine the concepts of geometric and propagation phase to design the dual-mode metalens and physically encode the required phase profiles using hydrogenated amorphous silicon for operation at visible wavelengths. The two distinct metalens phase profiles include (1) a conventional hyperbolic metalens for bright-field imaging and (2) a spiral metalens with a topological charge of +1 for edge-enhanced imaging. We demonstrate the focusing and vortex generation ability of the metalens under different states of circular polarization and prove its use for biological imaging. This work proves a method for in vivo observation and monitoring of the cell response and drug screening within a compact form factor.

Photoaligned Tunable Liquid Crystal Lenses with Parabolic Phase Profile

We present a theoretical model of a cylindrical tunable liquid crystal lens based on the modulation of anchoring energy. The latter can be easily obtained using photoalignment techniques. The liquid crystal cell we propose exhibits strong anchoring at the top substrate and anchoring energy with a parabolic profile at the bottom substrate. The model describes the dependence of the focal length on the applied voltage and presents a theoretical study of the lens aberrations. The results obtained are of general relevance and can be used to optimize the performances of every type of liquid crystal lens with a parabolic profile.

Eliminating Ambiguities in Electrical Measurements of Advanced Liquid Crystal Materials

Existing and future display and non-display applications of thermotropic liquid crystals rely on the development of new mesogenic materials. Electrical measurements of such materials determine their suitability for a specific application. In the case of molecular liquid crystals, their direct current (DC) electrical conductivity is caused by inorganic and/or organic ions typically present in small quantities even in highly purified materials. Important information about ions in liquid crystals can be obtained by measuring their DC electrical conductivity. Available experimental reports indicate that evaluation of the DC electrical conductivity of liquid crystals is a very non-trivial task as there are many ambiguities. In this paper, we discuss how to eliminate ambiguities in electrical measurements of liquid crystals by considering interactions between ions and substrates of a liquid crystal cell. In addition, we analyze factors affecting a proper evaluation of DC electrical conductivity of advanced multifunctional materials composed of liquid crystals and nanoparticles.

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

AbstractPhotodimerization of coumarin derivatives in photoactive polymers under linearly polarized UV (LPUV) light irradiation is a photochemical reaction often used to prepare the surface alignment layer that can orient liquid crystals (LCs) due to the anisotropic interfacial interactions. Herein, the synthesis of a series of novel coumarin‐containing copolymers and their use for the photoalignment surface layer of LCs is reported. These copolymers have the particular feature of bearing two coumarin side groups in each monomer unit, which is designed to have a high content of the chromophores. The alignment of LCs induced by the photoalignment layers of the coumarin‐containing copolymers is investigated. It is found that the photoalignment layers can effectively induce orientation of LC molecules along the direction of the UV light polarization used to prepare the surface layers, with an order parameter of around 0.5 achieved by adjusting the preparation conditions. Moreover, the electro‐optical behaviors of LC cells made with the photoalignment layers are investigated, confirming the alignment of LCs without the use of rubbed surfaces. Optically inscribing the photoalignment layer for nonuniform and organized LC alignment is also demonstrated.

Controlled wiring of disclination lines between patterned photoalignment layers in nematic liquid crystal

On demand creation of topological defect configurations is a relevant topic for theoretical studies but also enables new developments in the field of photonics, stimuli-responsive actuators and colloidal assemblies. Liquid crystals (LCs) provide a versatile and technologically relevant material system to study topological defects, and photoalignment enables complex surface patterning that includes the formation of defects. This work is based on periodic defect grids of +1/2 and −1/2 defects, imposed at the top and bottom surface of a LC cell. The periods at the two substrates are slightly different and the azimuthal orientation of the defects is varied, so that different types of disclination interconnections are formed in the bulk of the LC. Some disclination lines connect defects at opposing substrates, while others connect defects on the same substrate. The experimentally generated configurations are simulated with the help of finite element Q-tensor simulations in order to get a detailed understanding of the defect formation. The optical appearance in polymerizing optical microscopy images is compared to optical simulations to verify the validity of the director configurations.

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.

Development of thin film quarter-waveplate for polarization-independent liquid crystal on silicon phase modulators

Liquid crystal on silicon (LCoS) phase modulators spatially modulate the phase of light across the panel. The orientation order and elongated molecules of nematic liquid crystals (LCs) means that traditional LCoS phase modulators are inherently polarization-dependent, resulting in either complicated polarization manipulation systems or high optical power loss for unpolarized incident light. We propose a polarization-independent LCoS (PI-LCoS) device that combines existing technologies, which allows for cost-effective fabrication. Our PI-LCoS phase modulator is suitable for a wide variety of applications in telecommunication, adaptive optics, and display technologies. We have demonstrated the feasibility of the proposed PI-LCoS device by fabricating polarization-independent LC cells using a thin-film quarter-wave plate composed of a photoalignment layer and a layer of LC polymer. We show experimentally that the proposed design can efficiently modulate the phase of light with arbitrary input polarization.

Optimizing liquid crystal cell thickness in electro-optical Fresnel lenses through theoretical calculations and experimental validation.

Tunable liquid crystal (LC) lenses have gained significant attention in recent decades due to their lightweight, low cost, and versatility in applications such as augmented reality, ophthalmic devices, and astronomy. Although various structures have been proposed to improve the performance of LC lenses, the thickness of the LC cell is a critical design parameter that is often reported without sufficient justification. While increasing the cell thickness can lead to a shorter focal length, it also results in higher material response times and light scattering. To address this issue, the Fresnel structure has been introduced as a solution to achieve a higher focal length dynamic range without increasing the cell thickness. In this study, we numerically investigate, for the first time (to our knowledge) the relationship between the number of phase resets and the minimum required cell thickness to achieve a Fresnel phase profile. Our findings reveal that the diffraction efficiency (DE) of a Fresnel lens also depends on the cell thickness. Specifically, to achieve a fast response Fresnel-structured-based LC lens with high optical transmission and over 90% DE using E7 as the LC material, the cell thickness should fall within the range of 13 to 23 µm.