Liquid Crystal Molecular Orientation Research Articles

Highly sensitive and label-free detection of biotin using a liquid crystal-based optofluidic biosensor.

A liquid crystal (LC)-based optofluidic whispering gallery mode (WGM) resonator has been applied as a biosensor to detect biotin. Immobilized streptavidin (SA) act as protein molecules and specifically bind to biotin through strong non-covalent interaction, which can interfere with the orientation of LCs by decreasing the vertical anchoring force of the alignment layer in which the WGM spectral wavelength shift is monitored as a sensing parameter. Due to the double magnification of the LC molecular orientation transition and the resonance of the WGM, the detection limit for SA can reach 1.25 fM (4.7 × 10-13 g/ml). The measurable concentration of biotin and the wavelength shift of the WGM spectrum have an excellent linearity in the range of 0 to 0.1 pg/ml, which can achieve ultra-low detection limit (0.4 fM), i.e., seven orders of magnitude improvement over conventional polarized optical microscope (POM) method. The proposed optofluidic biosensor is highly reproducible and can be used as an ultrasensitive real-time monitoring biosensor, which will open the door for applications to other receptor and ligand models.

Deflecting and routing nematicons via orientation programmable liquid crystal array.

By designing a liquid crystal cell with comb electrode structure, the alignment modulation of nematic liquid crystal in the cell can be realized after the electric field is applied. In different orientation regions, the incident laser beam can deflect at different angles. At the same time, by changing the incident angle of the laser beam, the reflection modulation of the laser beam on the interface of the liquid crystal molecular orientation change can be realized. Based on the above discussion, we then demonstrate the modulation of liquid crystal molecular orientation arrays on nematicon pairs. In different orientation regions of liquid crystal molecules, nematicon pairs can exhibit various combinations of deflections, and these deflection angles are modulable under external fields. Deflection and modulation of nematicon pairs have potential applications in optical routing and optical communication.

Customized Design of Aperiodic Liquid Crystal Grating for Generation of Multiple Optical Patterns

Liquid crystal (LC) grating has drawn extensive attention due to its merits, including tunability, easy preparation process, cost effectiveness, and high diffraction efficiency. However, limited efforts have been made to explore LC grating for the generation of customized diffraction patterns, despite the fact that the patterning size of LC molecular director orientation has been tremendously reduced even to subwavelength scale via high-precision photoalignment techniques. Here, we propose a versatile method based on the binary particle swarm optimization algorithm to efficiently design aperiodic LC gratings for customized optical patterns generation, including needle beam, multi-foci, and hollow beam. The optical performance of the LC gratings was simulated using both the vectorial Rayleigh–Sommerfeld method (with discrete zero or π phase, in this article) and the actual director orientations. These results matched each other well, which proves the effectiveness of the optimization method based on discrete phase. This study paves the way for designing LC gratings with engineered functionalities, which may find applications in optical tweezers, laser delivery systems, and laser scanning imaging.

Hierarchically self-constructed alignment layer of comb-shaped amphiphilic copolymers for spontaneous and stable vertical orientation of liquid crystals

Hierarchically self-constructed ordered nanoarchitectures for precise control of molecular alignment has received much attention in a wide range of applications from optoelectronic devices to flexible biosensors. Herein, we present a facile and effective fabrication strategy to create a hierarchically constructed self-alignment layer of comb-shaped amphiphilic copolymers for spontaneous vertical molecular orientation of liquid crystals (LCs). Amphiphilic poly(vinyl 4-heptylbenzoate)-g-poly(vinyl alcohol) (PVHB-g-PVA) as a comb-shaped copolymer was doped to an LC medium, and its interfacial self-assembly on indium tin oxide electrode induced the hierarchical self-constructed alignment layer through hydrophilic interactions of hydroxy groups. In addition, hydrophobic and cooperative interactions of alkyl phenyl moieties in amphiphilic copolymers ensured the stable vertical self-alignment of LC molecules. Compared to conventional polyimide layer, the proposed self-constructed alignment layer afforded high vertical alignment ability for LCs and outstanding device performance in terms of the threshold voltage and response speed. Furthermore, the long-term alignment reliability of the proposed copolymeric self-assemblies was comparable to that of commercial alignment layer.

Effects of the interlayer thickness on the optical characteristics of an ultrasound multilayered liquid crystal lens

Ultrasound multilayered variable-focus liquid crystal (LC) lenses have better optical performance compared to conventional ultrasound LC lenses. This paper investigates the effect of the distance between two LC layers on the optical characteristics of the lens. The LC molecular orientation in the lens was altered by the acoustic radiation force generated by the lens’ resonance flexural vibration mode, causing a change in the focal length. The focal length change was increased dramatically, and the electrical consumption was reduced in lenses with thinner interlayers.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals.

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.

Study of PDMS Microchannels for Liquid Crystalline Optofluidic Devices in Waveguiding Photonic Systems

Microchannels in LC:PDMS structures must be of good quality and suitable geometry to achieve the desired orientation of the liquid crystalline molecules inside. When applying a casting technique, with the molds obtained even by the most accurate method, i.e., photolithography, it is still crucial to inspect the cross-section of the structure and the surface roughness of the PDMS material. This paper presents a study of PDMS microchannels using a Scanning Electron Microscope (SEM) to make such a characterization as accurate as possible. By comparing images of the samples taken using standard polarized light microscopy and SEM, it is likely to understand the mechanism of the liquid crystal molecular orientation occurring in the samples. The results obtained in this work may be used for numerical simulations and further development of LC:PDMS structures.

Driving liquid crystal lens without LC molecular orientation defects induced by an electric field

Using the finite element method, we numerically investigated the transient behavior of molecular orientations of a liquid crystal (LC) lens with a circular electrode beside a hole-patterned electrode and a common flat electrode that has a resistive film. The transient properties of a three-dimensional electric field and the molecular orientations of the LC were simultaneously calculated when voltages were applied across a circular electrode/circularly hole-patterned electrode and a common electrode. The axially symmetric and bell-like distribution of the refractive index could also be obtained. When relatively high voltages were applied to the LC lens, LC molecular orientation defects, such as the disclination line, occurred in the inner region of the circularly hole-patterned electrode. The behavior of the LC directors at the defects were estimated, and transient properties of their phase profiles were predicted via numerical calculation. The spherical distribution of phase retardation without defects could be exhibited by applying a relatively high voltage with short switching on and switching off.

A Review of Tunable Electromagnetic Metamaterials With Anisotropic Liquid Crystals

The performance of metamaterial is limited to a designed narrow band due to its resonant nature, it is highly desirable to incorporate active inclusions in metamaterials to extend the operation bandwidth. This review summarizes the development in realizing the tunability of electromagnetic response in metamaterials incorporated with nematic liquid crystal (LC). From rigorous comparison, it is found that the anisotropic property of nematic LC is essential in predicting the influence of LC molecular director orientation on the resonant frequency of metamaterials. By carefully designing the metamaterials and properly infiltrating LC, the operation frequency of single/double negative parameters of metamaterials can be dynamically modulated with remarkable red/blue-shift, depending on the LC molecular orientation angle. Moreover, the recent liquid crystal-based developments and novel applications are investigated and highlighted.

Fully Electronically Phase Modulation of Millimeter-Wave via Comb Electrodes and Liquid Crystal

This letter presented a nematic liquid crystal (LC) based metal-dielectric-metal structure to fully electronically modulate the reflection phase of electromagnetic waves. The demonstrated structure is composed of a thin LC layer sandwiched between copper comb electrodes and a single slotted reflection array. Based on a proper application of the external electric field, the whole process of nematic LC molecular orientation is controlled. The innovative feeding method can actively prealign and deflect the orientation of LC molecules. Experimental results show that the maximum reflection phase shift is 189.2° at 114.9 GHz. This letter provides a practical way of regulating LC-based terahertz devices. The proposed structure is a potential candidate for the reconfigurable reflectarray antennas.

Molecular Orientation in a Variable-Focus Liquid Crystal Lens Induced by Ultrasound Vibration

A method to estimate orientation direction of liquid crystal molecules three-dimensionally under ultrasound excitation was proposed and the relationship between the ultrasound vibration and the molecular orientation was discussed. Our group have reported a technique to control orientation direction of liquid crystal molecules using ultrasound vibration which could be applied to an optical variable-focus liquid crystal lens. The lens consisted of a liquid crystal layer sandwiched by two glass circular discs and a piezoelectric ring. Ultrasound vibration induces change in the refractive index of the lens, enabling the variable-focus function. The three-dimensional orientation direction of the liquid crystal molecules in the lens was predicted from the transmitted light distributions under the crossed Nicol conditions. The liquid crystal molecules were inclined from vertical alignment by the ultrasound vibration, and larger ultrasound vibration gave larger inclination of the molecules. There was a strong correlation between the distributions of ultrasound vibration and the liquid crystal molecular orientation; the molecular orientation was changed remarkably between the antinodal and nodal parts of the ultrasound flexural vibration on the glass plate and the molecules aligned towards the antinode.

Simple Stokes polarimeter using a liquid crystal grating with ternary orientation domains.

We propose a liquid crystal (LC) grating with a ternary orientation domain for application to optical polarization measurements. The ternary orientation domain, which generates two-dimensional diffraction properties, is the key to simultaneous acquisition of multiple of polarization parameters. The LC molecular orientation state and the polarization dependence of the diffraction efficiency were investigated experimentally, focusing on the applicability to a practical Stokes polarimeter. An experiment was conducted using the proposed LC grating as a Stokes polarimeter, and the four Stokes parameters (S0, S1, S2, and S3) were determined for linearly and circularly polarized incident light. As a result, the feasibility of the proposed LC grating Stokes polarimeter has been demonstrated experimentally. Finally, the operational performance of the proposed LC grating Stokes polarimeter is discussed using a figure of merit that is numerically derived from the measured polarization dependence of the diffracted light intensity.

Self-organized wrinkling of liquid crystalline polymer with plasma treatment

Abstract

High pump efficiency of a second-order distributed feedback laser based on holographic polymer dispersed liquid crystals with preferred liquid crystal molecular orientation

We report on the fabrication and characterization of a surface-emitting distributed feedback (DFB) organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating.

Periodic pattern of liquid crystal molecular orientation induced by ultrasound vibrations

Methods for controlling the periodic molecular orientation of liquid crystals using ultrasound vibrations and estimating the orientational direction are proposed. An ultrasonic liquid crystal cell was used, consisting of a liquid crystal layer sandwiched by two glass plates fitted with piezoelectric ultrasound transducers. The transmitted light intensity distribution through the cell was measured by changing the polarization direction to investigate the orientation direction of the liquid crystal molecules. The transmitted light distributions changed periodically owing to the flexural vibration of the cell at resonance frequencies of 43.9 and 70.7 kHz. The orientational direction of the liquid crystal molecules correlated with the vibrational distribution of the glass substrates, and the molecular orientation was changed periodically by the acoustic radiation force. The interval and intensity of the transmitted light could be controlled by the driving frequency and voltage amplitude, respectively.

Focus control in the radial direction using an ultrasound liquid crystal lens

Nematic liquid crystal is widely used in optical devices such as liquid crystal displays and controlled by electric fields through the liquid crystal layer. Our group has proposed a control technique of liquid crystal molecular orientation using ultrasound vibration without indium tin oxide electrodes [1]. In this study, using ultrasound and nematic liquid crystal, we realized a variable focus liquid crystal lens. The liquid crystal lens has no transparent electrodes and consists of a simple structure; a nematic liquid crystal layer with a thickness of 50 μm is formed between two circular grass plates. An annular PZT ultrasonic transducer was bonded on the glass plate. By exciting the transducer at the resonance frequencies (28 kHz), the flexural vibration modes were generated. The acoustic radiation force from the vibration changed the molecular orientation of the liquid crystals, which induced the distribution of the execution refractive indices. The liquid crystal layer then worked as a lens. The focal points could be controlled by the radiation force and moved in the radial direction. [1] S. Taniguchi, D. Koyama, Y. Shimizu, A. Emoto, K. Nakamura, and M. Matsukawa, Appl. Phys. Lett. 108(2016), 101103.

Effect of liquid crystal molecular orientation controlled by an electric field on friction

Molecular orientation of the lubricating film will affect the lubrication state, especially in boundary lubrication. In this work, electrical field was used to affect the orientation of liquid crystal molecules during friction. We demonstrated that the use of an electric field applied between the steel ball and the chromium film on the surface of the glass plate provided an effective method for controlling the orientation of the liquid crystal molecules in a sliding point contact. The coupling effects of shear and electric field on friction were investigated with an in situ Raman spectrometer. This work shed new light on the influence of the adsorbed film on sliding friction coefficient, especially in thin-film lubrication regime.

PA38 ねじれ角が分布したマイクロパターン配向セルにおける液晶分子配向特性(液晶ディスプレイ,ポスター発表,2015年日本液晶学会討論会)

PA38 ねじれ角が分布したマイクロパターン配向セルにおける液晶分子配向特性(液晶ディスプレイ,ポスター発表,2015年日本液晶学会討論会)

Bistable liquid crystal device fabricated via microscale liquid crystal alignment

Bistable liquid crystal (LC) molecular orientation properties in micropatterned LC cells were investigated experimentally and theoretically. When an LC cell was heated to the phase-transition temperature and then cooled, an LC orientation with ±π/2-twist domains (±π/2-twist mode) was obtained. Furthermore, a different LC orientation with ±π-twist domains (±π-twist mode) was observed when a 10-V potential was applied across a sample LC cell. Both orientation states were stably retained over a long period. Herein, cross-sectional LC orientation models in the ±π/2- and ±π-twist modes are proposed to explain the generation and behavior of two different disclination lines. The total energies within one period in the ±π/2- and ±π-twist modes (F±π/2 and F±π, respectively) were estimated theoretically. These energies were found to depend on the LC layer thickness and to cross over at a certain thickness; this indicates that F±π is equal to F±π/2 at this equilibrium thickness. The best temporal stability is likely attained at this equilibrium thickness. We demonstrated a bistable color-switching device by combining a full-wave plate and crossed polarizers. When these optical components were configured properly, stable bistable switching between two colors was achieved.

Focus control of a nematic liquid crystal cell using ultrasound vibration

Nematic liquid crystals used in optical devices such as liquid crystal displays are controlled by the electric fields. Although indium tin oxide (ITO) is generally employed as the transparent electrode material to apply the electric field, it includes the rare metal indium and needs complicated processes such as sputtering deposition method. Our group has been investigating control techniques of molecular orientation of liquid crystals using ultrasound. In this study, we have applied these techniques to optical variable-focus lenses and the optical characteristics were examined. The ultrasonic liquid crystal cell has a simple structure with no transparent electrodes; it consists of has an annular ultrasound PZT transducer (inner diameter: 30 mm; thickness: 2 mm) and a liquid crystal cell in which a nematic liquid crystal layer (thickness: 25 μm) was formed between two glass substrates (thickness: 0.7 mm) and orientational films. The concentric flexural vibration modes could be generated on the cell at the several resonance frequencies, and the liquid crystal molecular orientation was changed by the acoustic radiation force. The transmitted light through the liquid crystal cell was focused, and the focal length could be controlled by the input signal.