Orientation Of Liquid Crystal Molecules Research Articles

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.

A superfast responsive and ultrathin self-constructed bilayer nanoarchitecture for automatic vertical orientation of liquid crystals

Ultrathin bilayer nanoarchitectures are structurally and functionally tunable yet robust, which are hierarchical building blocks with a wide range of applications from flexible displays to wearable optoelectronic devices. Here, we report an unprecedented cost-effective fabrication strategy to construct an ultrathin, superfast responsive, and highly durable bilayer nanoarchitecture for automatic vertical arrangement of liquid crystals (LCs) by using the spontaneous and cooperative self-construction of small mesogenic amphiphiles. The self-constructed bilayer nanoarchitecture is achieved by the simple doping and subsequent in situ self-assembly of a small amount of mesogenic amphiphiles in the confined cell geometry. It is experimentally demonstrated that an ultrathin and rough bilayer nanoarchitecture with vertical nanopillar array formed by surficial hydrogen bonding and cooperative π–π interaction of mesogenic amphiphiles enables an automatic vertical orientation of LC molecules to be realized during self-construction process. In addition, this simple approach provides a superfast responsive and highly reliable nanoarchitecture with a greatly enhanced electro-optical switching performance compared to conventional polyimide alignment layer. The ultrafast response time of 1.5 ms, which is more than ten times faster than that obtained from a conventional alignment layer, and low voltage driving achieving about 33% energy savings are accomplished by our method.

Factors influencing flexoelectric polarization in liquid crystals

The work experimentally investigated the factors affecting the formation and physical parameters of flexoelectric polarization in liquid crystals (LC) with nematic as well as smectic A- and C-phases. Factors such as the heating temperature of the sample, orientation effects near the nematic – isotropic state (N-I) phase transition, and the initial orientation of LC molecules were studied. The thickness of the studied layers is from 10 to 100 μm, the frequency of exposure is 1000 Hz. It was found that the flexosignal takes maximum value in homeotropically oriented samples in the nematic phase. When approaching the N-I phase transition, the magnitude of the flex signal increases, which is associated with an increase in the deviation angle of the director of the LC. As the dipole moment grows, the quadrupole moment also increases. This is due to the tendency of LC molecules to form dipole pairs with antiparallel orientation of electric moments. The estimate of the flexural value U1ω yielded (for example, for MBBA) with a planar orientation 3 times less than with a homeotropic one. The presence of the second harmonic U2ω is a consequence of the orientational instability of surface polarization. The rapid growth of U2ω is associated with an increase in the square of the director deviation angle .

Liquid crystal-based sensing platform for detection of Pb2+ assisted by DNAzyme and rolling circle amplification

Lead ions (Pb2+) are one of the most widespread heavy metal contaminants that pose detrimental impact on environment and human health. We demonstrate a highly sensitive and specific liquid crystal (LC)-based sensing platform for detecting Pb2+ assisted by DNAzyme and rolling circle amplification (RCA). Magnetic beads (MBs) are functionalized with DNA duplexes of the catalytic strands (DNAzymes) and the substrate strands. In the presence of Pb2+, the substrate strands are disassembled due to activation of the DNAzyme, which allows initiation of DNA RCA on MBs. The amplified DNA strands can disrupt arrangement of octadecy trimethyl ammonium bromide monolayers (OTAB), thereby inducing planar orientation of LC molecules at the interface of aqueous and LCs. Thus, LCs exhibit bright appearance. In contrast, RCA cannot be triggered in the absence of Pb2+. Therefore, LC molecules adopt perpendicular orientation at the interface, which induces the dark morphology of LCs. The limit of detection reaches as low as 16.7 pM. It is an improvement of more than two orders of magnitude compared to that of previously reported LC-based sensing approaches. This approach also shows excellent performance in monitoring Pb2+ in tap water and lake water.

Application and Technique of Liquid Crystal-Based Biosensors.

Liquid crystal biosensors are based on changes in the orientation of liquid crystal molecules induced by specific bonding events of biomolecules. These biosensors are expected to serve as a promising system to detect biomolecules, biomolecular activity, and even small chemical molecules because they are inexpensive, sensitive, simple, effective, and portable. Herein, we introduce the principle and fabrication of liquid crystal biosensors and review the research progress in signal-amplified technology for liquid crystal sensing and its application in the detection of viruses, bacteria, proteins, nucleic acids, and small chemical molecules. In addition, the current theoretical and practical issues related to liquid crystal biosensors were investigated.

Effect of the Physicochemical Modification on Bismuth-doped Zinc Oxide in the Anisotropic Orientation of Liquid Crystal Molecules

Alignment of liquid crystals by ion-beam (IB) irradiated bismuth-doped zinc oxide (BZO) films was investigated. The BZO alignment layer was deposited using a simple solution process, and IB irradiation of the BZO was used to induce surface-alignment of LC molecules uniformly. Solution-processed thin films have numerous advantages, including low manufacturing cost, simplicity of fabrication, and high productivity. IB irradiation of the BZO film induces a drastic improvement in the uniformity of the BZO surface and creates physico-chemical modifications that preserve the anisotropic characteristics of the surface, thereby unidirectionally aligning the LC molecules.

Tunable Beam Steering at Terahertz Frequencies Using Reconfigurable Metasurfaces Coupled With Liquid Crystals

Metasurfaces with a spatially varying phase profile enable the design of planar and compact devices for manipulating the radiation pattern of electromagnetic fields. Aiming to achieve tunable beam steering at terahertz frequencies, we numerically investigate metasurfaces consisting of one dimensional arrays of metal-insulator-metal (MIM) cavities infiltrated with liquid crystals (LCs). The spatial phase profile is defined by a periodic voltage pattern applied on properly selected supercells of the MIM-cavity array. By means of the electro-optic effect, the voltage controls the orientation of LC molecules and, thus, the resulting effective LC refractive index. Using this approach, the spatial phase profiles can be dynamically switched among a flat, binary, and gradient profile, where the corresponding metasurfaces function as mirrors, beam splitters or blazed gratings, respectively. Tunable beam steering is achieved by changing the diffraction angle of the first diffraction order, through the reconfiguration of the metasurface period via the proper adjustment of the applied voltage pattern.

LIQUID CRYSTAL MATERIALS ORIENTATION USING NEW APPROACH

It is well known that the liquid crystal (LC) mesophase is actively used in display technique and biomedicine devices. Unfortunately, the switching time of the devices based on the LCs is not fast enough; thus, it is very important to find novel perspective ways to obtain the good switching time of the LC dipoles used in these devices. Initial orientation of the LC molecules influences the dynamic parameters, for example, the switch-on and switch-off characteristics and the diffraction efficiency of the final devices. Among the different methods and approaches to find the optimized orientation of the inertial LC molecules a nanotechnology approach has shown the best results. This approach allows increasing the transparency, to decrease the resistivity and the number of the functional layers in the sandwich LC structures. Thus, it results in a decrease in the applied bias voltage. The effect is based on the fact that the ITO coating can be considered as the conducting layer and as the orienting (alignment) layer simultaneously. In the current paper, we continue our steps in the direction to find the best way of the LC molecules orientation. It is proposed to consider the LC media sensitization process as the method to change the surface relief when this relief is prepared from the polymeric orienting materials doped with the carbon nano-objects. Based on the solid fullerene-doped polyimide thin films and other organics it can be shown that the content of the fullerenes influences the wetting angle significantly. The fullerene concentration is correlated with the different surface relief view applied in the aligning of the LC molecules. The switching of the LC can be improved; furthermore the novel relief depended on the fullerene content can be used for the optical limiting of the laser irradiation.

Two-Dimensional Hybrid Photonic Crystal With Graded Low-Index Using a Nonuniform Voltage

Abstract We proposed a new method for designing graded index lens using liquid crystal infiltration into annular photonic crystals. Applying an external nonuniform voltage in the transverse direction perpendicular to the direction of light propagation yields different orientation of liquid crystal molecules inside the photonic crystal unit cells. As a result, a gradient refractive index was modulated. We numerically investigate focusing properties of the designed graded index structure using plane-wave expansion and finite-difference time-domain methods. The gradient refractive index profile was adjusted by varying the nonuniform voltage excitations, which consequently altered the focal distance of the graded index structure. A wide tuning range of 1856 nm was achieved for focal distance by the proposed graded index structure. This feature can be implemented for planning a flat lens with tunable focal distance based on electro-optic effect. These achievements may have future applications in some optical devices such as near-field imaging and scanning.

Tunable optical filter based on one-dimensional periodic structure composed of SiO2 and anisotropic metamaterial (AMM) with a liquid crystal defect layer sandwiched by two SiO2

This paper reports the tunable transmission properties of asymmetric one-dimensional periodic structure (1DPS) composed of SiO2 and anisotropic metamaterial (AMM) layers with defect of liquid crystal (LC) sandwiched by two SiO2 layers, i.e., (SiO[Formula: see text]AMM)[Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3 using the transfer matrix method (TMM). We have studied the optical properties of the periodic structure (SiO[Formula: see text]AMM)3 [Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3 with a different incident angle of the electromagnetic wave for particular director angle of LC molecules. The tunability of the transmission property of the considered 1DPS shows that the transmittance depends upon the orientation of LC molecules. Such an asymmetric periodic structure (1DPS) composed of SiO2 and AMM with a defect of LC sandwiched by two SiO2 layers, (SiO2 [Formula: see text]AMM)[Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3, may be used as a tunable optical filter and bi-stable device.

Preparation of DNA-functionalized surfaces for simultaneous homeotropic orientation of liquid crystals and optical recognition of analytes: application to the determination of progesterone.

The work describes a simplified method for the preparation of liquid crystal (LC) bioassay using DNA-based capture molecules and having lower detection limits. The capture DNA probes of the stem-loop structure were immobilized on the surface of a glass slide. A homeotropic orientation of LC molecules can be obtained with the proper surface coverage of capture DNA probes. In the presence of analytes (specifically shown here for the progesterone as a model analyte), the molecular binding between capture DNA probes and progesterone opens the loop of the capture DNA probes. The opened sequence is then amenable to hybridization with a reporter DNA probe that is immobilized on gold nanoparticles. This changes the surface microstructure, disrupts the orientation of LC molecules, and results in an enhanced optical response, expressed as the average grey value of the images. This new kind of surface treatment for simultaneous recognition of target molecules and homeotropic anchoring of LCs reduces the number of preparation steps and makes the process of LC bioassay easier. This method has a detection limit as low as 0.1pmol·L-1 of progesterone. Graphical abstract Schematic presentation of the liquid crystal-based DNAassay. DMOAP: Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride; TEA: Triethoxsilylbutyraldehyde; 5CB: 4-cyano-4'-pentylbiphenyl; P4: progesterone.

Fiber Mach-Zehnder-interferometer-based liquid crystal biosensor for detecting enzymatic reactions of penicillinase.

A novel, to the best of our knowledge, liquid crystal (LC) biosensor, based on an optical fiber Mach-Zehnder interferometer (MZI), is proposed. The proposed optical fiber MZI consists of two single-mode fibers and a tapered photonic crystal fiber (PCF). The PCF is coated with 4'-pentyl-biphenyl-4-carboxylic acid (PBA)-doped 4-cyano-4'-pentylbiphenyl (5CB). Being a pH-sensitive material, PBA can manipulate LC molecules to different orientations according to their pH values. When the orientation of LC molecules changes with varying pH, the effective refractive index of the cladding modes also is accordingly affected. Enzymatic reactions of penicillinase can release H+, which causes the decrease of the pH. Therefore, the enzymatic reactions of penicillinase can be sensed by monitoring the peak shift in the interference spectrum. The effects of the tapered diameter on the sensitivity of the sensor were experimentally investigated as well.

Influence of BaTiO3 particles on second harmonic generation in liquid crystal 5CB

Second harmonic generation (SHG) in the colloid based on barium titanate particles and the liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) was detected. Furthermore, the SHG signal from the 5CB + BaTiO3 colloid was observed at lower power of exciting radiation than in the pure 5CB. It is shown that the SHG intensity has the maximum value at polarization of exciting radiation along the preferred direction of LC molecules. In this case, it depends on the value of the applied electric field. Experimental results were explained by the photo-stimulated polarization of ferroelectric particles and orientation of LC molecules, formation of local electric field around these particles and also momentary symmetry of the system at action of short laser impulses.

Control and evaluation of liquid crystal molecules using ultrasound vibration

Nematic liquid crystals are widely used in optical devices such as liquid crystal displays and controlled by electric fields through the liquid crystal layer. The authors have proposed a technique to control the orientation of liquid crystal molecules using ultrasound vibration without indium tin oxide electrodes. An ultrasound liquid crystal cell was fabricated; it consists of a liquid crystal layer sandwiched by two glass plates having two ultrasound PZT transducers. When exciting the transducers at the resonance frequencies, the flexural vibration modes were generated on the cell and the acoustic radiation force acted to the liquid crystal molecules so that the orientation direction can be changed. The relationship between the molecular orientation and the vibration distribution of the cell was investigated from the transmitted light distribution of the liquid crystal cell under the crossed Nicol condition. In addition, the orientation of liquid crystal molecules was evaluated as ultrasonic wave velocity in the GHz range by the Brillouin scattering method. The ultrasonic wave velocity in the long axis direction of the liquid crystal molecules was higher than that in the short axis direction, which indicates the uniaxial anisotropy of the liquid crystal molecules.

Characteristics of a liquid-crystal-filled composite lattice terahertz bandgap fiber

A new type of terahertz fiber is presented based on composite lattice photonic crystal bandgap. The cladding is filled selectively with the nematic liquid crystal 5CB which is sensitive to the electric field. The terahertz wave can be modulated by using the electric field to control the orientation of liquid crystal molecules. The plane wave expansion method and the finite element method are employed to theoretically analyze bandgap characteristics, polarization characteristics, energy fraction and material absorption loss. The results show that this fiber structure can be used as tunable terahertz polarization controller.

A Tunable Polarization-Dependent Terahertz Metamaterial Absorber Based on Liquid Crystal

In this paper, a tunable polarization-dependent terahertz (THz) metamaterial absorber based on liquid crystal (LC) is presented. The measurement results show that absorption peak is at 239.5 GHz for a TE-polarized wave and 306.6 GHz for a TM-polarized wave, without exerting the bias voltage on the LC layer. An increase in bias voltage affects the orientation of LC molecules and causes redshifted resonant frequencies. By adjusting the bias voltage from 0 to 10 V, frequency tunabilities of 4.7% and 4.1% for TE- and TM-polarized waves, respectively, were experimentally demonstrated. Surface current and power loss distribution was analyzed to explain the physical mechanism of the absorber, while the absorption dependence on geometrical parameters and incident angles was also studied in detail. According to the obtained results, the proposed absorber is shown here to be capable of achieving tunable polarization-dependent absorption, and to have potential application in terahertz polarization imaging, terahertz sensing, and polarization multiplexing.

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

ABSTRACTThe nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.

Electro-optical properties of liquid crystal displays based on the transparent zinc oxide films treated by using a rubbing method

Liquid crystal (LC) alignment on inorganic films has been found to be affected by surface modification via ion-beam irradiation. In this study, ZnO films treated by rubbing with a velvet cloth were shown to be capable of aligning LC molecules in the direction of the rubbing. Uniform and homogeneous LC alignment was achieved on the rubbed ZnO films. By analysing the optical axes before and after the rubbing treatment, we confirmed an increase in the anisotropy of the ZnO films; this optical anisotropy contributed to the uniform orientation of LC molecules. Further, the electro-optical characteristics of the twisted nematic cells based on ZnO films were superior to those based on conventional polyimide layers. Our results indicate that the rubbing approach could be applied in the fabrication of high-performance LC displays.

Thermally stable poly(styrene-maleic anhydride) layer modified by ion-beam for liquid crystal orientation

We report uniform and homogeneous liquid crystal (LC) alignment using ion beam (IB)-irradiated poly(styrene-maleic anhydride) (SMA) films as an alignment layer when the IB incidence angle is controlled. The molecular polarity on the SMA surface increased with IB irradiation, especially at an incidence angle of 30°, resulting in strong van der Waals forces between the LC molecules and the surface. Atomic force microscopy and X-ray photoelectron spectroscopy were used to determine the LC alignment mechanism. Chemical structure analysis indicated that the orientation of LC molecules was due to van der Waals forces and selective breaking of C–C bonds by IB irradiation. The potential application of SMA substrates as alignment layers for display devices was further supported by their higher thermal stability (220 °C) compared with that of conventional rubbed SMA (160 °C) and rubbed polyimide (160 °C). Therefore, LC cells with IB-irradiated SMA are potential candidates for alternative alignment layers in LC applications.

Liquid-crystal ordering mediated by self-assembly of surfactant solution confined in nanodroplet: a dissipative particle dynamics study

In recent years, photonic crystal technology has received much attention. In this study, we focus on liquid crystal droplets considered as one of photonic crystals. It was reported that the orientation of liquid crystal molecules can be controlled by covering the droplet surface with surfactant solutions (Moreno-Razo et al. Nature. 2012;485:86). We investigated a system in which surfactant solutions are added to liquid crystal droplets with based on their study. The effect of surfactant concentration on the orientation behaviour of the liquid crystal molecules was observed. Phase transition of liquid crystal was also observed with changes to the temperature of the liquid crystal droplets. Various self-assembled structures and changes in liquid crystal droplet properties were discussed. We believe that the results of our work will help in further pursuing the research on photonic crystal devices.