Liquid Crystal Director Orientation Research Articles

A novel radial orientation treatment of nematic liquid crystals using magnetic field lines with permanent magnet and applying the assisted electric field

AbstractRecently, there has been considerable research on optical devices, such as liquid crystal (LC) lenses and special optical plates, using LCs. In such devices, relatively small LC cells are frequently used, or unique LC orientations are required. As an LC orientation process, we focused on the LC director's orientation induced by the magnetic force line distribution of a small neodymium magnet. We propose a simple method for obtaining radial orientation, which is rather difficult to obtain using the ordinary rubbing method. The initial orientation in the LC cell is a vertical orientation cell with almost zero azimuth anchoring. With the proposed method, the reorientation process is performed with an assisting electric field and a small permanent magnet, unlike the conventional magnetic field orientation process that requires a large electromagnet. Furthermore, a polymer stabilization treatment is used to fix the obtained radial orientation pattern in the LC cell. After the treatment, the applying voltage can control the tilt angle of the director in weak polymer treatment, and a completely fixed orientation pattern can be obtained that in strong.

Optical Tamm states in a hybrid structure with a holographic polymer-liquid crystal grating

ABSTRACT The reflection spectrum of a hybrid structure formed by a metal film and a holographic grating consisting of a sequence of polymer and liquid crystal (LC) layers is theoretically studied. It is shown that in such a hybrid structure, a narrow dip in the reflection spectrum appears in the region of the grating band gap, which is associated with the formation of an optical Tamm state (OTS). The spectral position of the reflection dip depends on the parameters of the polymer and LC. The type of metal and its thickness affect the value of the OTS reflection dip, but they do not affect its spectral position. The dependence of the OTS wavelength on the LC director orientation makes it possible to control the spectral position of the reflection dip using an external electric or magnetic field.

Influence of the flexoelectric effect on director alignment of nematic liquid crystals confined to the planar anchoring cylinder

ABSTRACT The influence of the flexoelectric effect on the director distribution of nematic liquid crystals confined to a cylinder with planar anchoring under an applied electric field is investigated by the continuum theory and the difference iterative method. Numerical analyses of the influence of the dielectric effect, flexoelectric effect and the coupling of these two effects on director alignment of nematic liquid crystal in a cylinder are obtained on the basis of the equilibrium equations using continuum theory. The analyses revealed that the change of director of nematic liquid crystals was caused by flexoelectric effect and dielectric effect in different directions. It provides a good theoretical basis for controlling the orientation of liquid crystal director in a cylinder and measuring of flexoelectric coefficient of liquid crystal materials.

Oblique wrinkling patterns on liquid crystal polymer core–shell cylinders under thermal load

Smart soft materials, which can flexibly respond to external multi-physics stimuli, have attracted considerable attention over the past few years. Here, we present tunable wrinkling patterns in cylindrical core–shell systems under thermal load via the orientation of director in nematic liquid crystal polymer (LCP). To quantitatively analyze mechanical behavior and morphological evolution of LCP core–shell cylinders, we develop a core–shell model that accounts for director-induced anisotropic spontaneous strains. By tuning the alignment of liquid crystal director, we explore the effects of anisotropy of spontaneous strains on instability pattern formation and evolution. When the director is along the circumferential direction, wrinkling pattern can be axisymmetric or diamond-like, determined by a single dimensionless parameter Cs which characterizes the stiffness ratio and curvature of the system. When the director is parallel to the axial direction, leading to circumferential expansion upon heating, the core–shell cylinder usually buckles into churro-like mode. For general spatial director alignment, the ultimate wrinkling patterns depending on the anisotropy state of spontaneous strains, can be diamond-like, parallel bead-chain or oblique stripe mode. We draw director-affected phase diagrams to provide an overall view of pattern selection affected by liquid crystal director orientation, which could be used to quantitatively guide the effective design of wrinkling morphology-related smart surfaces.

Using liquid crystals to control surface plasmons

ABSTRACTWe propose using a liquid crystal (LC) layer placed on top of graphene ribbons or a graphene monolayer to control surface plasmons. The plasmon frequency depends on the LC director orientation which can be controlled by an applied voltage. The surface plasmons excitation is sensitive to the light polarisation and can be manipulated using the orientation properties of LCs. We further suggest using periodically anchored LCs instead of a relief grating to excite and control the plasmons in monolayer graphene. The suggested scheme can be used as the basis for a tunable optical filter or a modulator.

Electrically tunable VCSEL with intra-cavity liquid crystal: Design, optimization, and analysis of polarization- and mode-stability

Tunable vertical-cavity surface-emitting lasers (VCSEL) provide broad wavelength tuning range at very high modulation speed, but usually employ micro electromechanical systems, which are sensitive to vibrations. In this paper we propose a tunable 1550 nm VCSEL design with enclosed intra-cavity liquid-crystal (LC) cell. The laser is electrically pumped and the wavelength selectivity is obtained by varying LC’s refractive index through electro-optical tuning of the LC director.By performing one-dimensional transfer-matrix optical calculations, supplemented with a fully-vectorial three-dimensional self-consistent modeling of thermal, electrical and optical phenomena, we optimize the design and show the dependency of the emitted wavelength on LC cell thickness and the LC director orientation. The analysis is concluded with a possibility of achieving 68.5 nm tuning range in a continuous-wave operation regime and without polarization or transverse mode switching.

Numerical modelling of light propagation in surface plasmon resonance sensor with liquid crystal

The five-layer nanorod-mediated surface plasmon sensor with inhomogeneous liquid crystal layer was theoretically investigated. The reflectance as the function of the incident angle was calculated at different voltages applied to the liquid crystal (LC) for different analyte refractive indices. By changing the LC director orientation one can control the position of the reflective dips and choose the one that is the most sensitive to the analyte refractive index. At the chosen angle of incidence, the analyte refractive index can be found from the reflectance value. The director reorientation effect is stronger when the prism refractive index is between ordinary and extraordinary refractive indices of the LC. In this case, the voltage increase and the prism refractive index decrease have a similar effect on the reflectance features.

Modeling Static Electric Field Effect on Nematic Liquid Crystal Director Orientation in Side-Electrode Cell

A two-dimensional model of Fredericks effect was used for the investigation of the static electric field influence on nematic liquid crystal director orientation in the side-electrode cell. The solutions of the equations describing the model were obtained by finite-difference methods. Fredericks transition threshold for the central part of the cell, as well as dependencies of the distribution of the director orientation patterns on the electric field and location, were obtained. The numerical results are found to agree qualitatively with the experiment. Further investigations are needed to elucidate completely the Fredericks effect.

Electrically switchable photonic liquid crystal devices for routing of a polarized light wave

The new mode of LC alignment based on photoalignment AtA-2 azo dye where the refractive interface between orthogonal orientations of the LC director exists without voltage and disappeared or changed with critical voltage has been proposed. The technology to fabricate electrically controlled liquid crystal elements for spatial separation and switching of linearly polarized light beams on the basis of the total internal reflection effect has been significantly improved. Its distinctive feature is the application of a composite alignment material comprising two sublayers of Nylon-6 and AtA-2 photoalignment azo dye offering patterned liquid crystal director orientation with high alignment quality value q=0.998. The fabricated electrically controlled spatially structured liquid crystal devices enable implementation of propagation directions separation for orthogonally polarized light beams and their switching with minimal crosstalk.

Microwave modulation characteristics of twisted liquid crystals with chiral dopant

Adding a chiral dopant in twisted nematic (TN) liquid crystal cell can stabilize the orientation of liquid crystal molecules, particularly in high TN (HTN) or super TN (STN) liquid crystal cells. The difference in pitches in liquid crystal is induced by the chiral dopant, and these different pitches affect the orientation of liquid crystal director under an external applied voltage and influence the characteristics of microwave modulation. To illustrate this point, the microwave phase shift per unit length (MPSL) versus voltage is calculated on the basis of the elastic theory of liquid crystal and the finite-difference iterative method. Enhancing the pitch induced by the chiral dopant in liquid crystal increases the MPSLs, but the stability of the twisted structures is decreased. Thus, appropriate pitches of 100d, 4d, and 2d can be applied in TN, HTN, and STN cells with cell gap d to enhance the characteristics of microwave modulation and stabilize the structures in twisted cell. This method can improve the characteristics of liquid crystal microwave modulators such that the operating voltage and the size of such phase shifters can be decreased.

Effect of temperature on the morphology and electro-optical properties of liquid crystal physical gel

Liquid crystal physical gels were (thermally) prepared with cholesteryl stearate as a gelator in nematic liquid crystal, 4-cyano-4′-pentylbiphenyl. The electro-optical performance of liquid crystal physical gels is almost entirely dependent on the gels' inherent morphology. This study involved an empirical investigation of the relationships among all of the gelation temperature, morphology, and electro-optical properties. Besides continuous cooling at room temperature, isothermal cooling was also performed at both 18 and 0 °C, corresponding to near-solid and solid phases of 4-cyano-4′-pentylbiphenyl respectively. Nevertheless, the liquid crystal physical gel was also isothermally rapidly cooled using liquid nitrogen. Polarizing optical microscopy showed that the gel structure became thinner when isothermal cooling was carried out. These thinner gel aggregates then interconnected to form larger liquid crystal domains. Moreover, it was also revealed that the gel networks were randomized. Electron spin resonance results showed that the liquid crystal director orientation was severely randomized in the presence of gel networks. Conversely, isothermal cooling using liquid nitrogen generated a higher liquid crystal director orientation order. The 6.0 wt% cholesteryl stearate/4-cyano-4′-pentylbiphenyl physical gel that was isothermally cooled using liquid nitrogen showed the lowest response time in a twisted nematic mode optical cell.

Surface Dipole Control of Liquid Crystal Alignment

Detailed understanding and control of the intermolecular forces that govern molecular assembly are necessary to engineer structure and function at the nanoscale. Liquid crystal (LC) assembly is exceptionally sensitive to surface properties, capable of transducing nanoscale intermolecular interactions into a macroscopic optical readout. Self-assembled monolayers (SAMs) modify surface interactions and are known to influence LC alignment. Here, we exploit the different dipole magnitudes and orientations of carboranethiol and -dithiol positional isomers to deconvolve the influence of SAM-LC dipolar coupling from variations in molecular geometry, tilt, and order. Director orientations and anchoring energies are measured for LC cells employing various carboranethiol and -dithiol isomer alignment layers. The normal component of the molecular dipole in the SAM, toward or away from the underlying substrate, was found to determine the in-plane LC director orientation relative to the anisotropy axis of the surface. By using LC alignment as a probe of interaction strength, we elucidate the role of dipolar coupling of molecular monolayers to their environment in determining molecular orientations. We apply this understanding to advance the engineering of molecular interactions at the nanoscale.

Study of light-induced reorientation of the nematic liquid crystal director by birefringence dynamics

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.

Diffraction and fringing field effects in small pixel liquid crystal devices with homeotropic alignment

Reducing the pixel dimensions of liquid crystal microdisplays in search of high resolution has a fundamental impact on their electro-optic behavior. The liquid crystal director orientation becomes distorted due to fringing fields and diffraction effects influence the optical characteristics of the device once the structure features approach the wavelength of the incident light. Three-dimensional finite element simulation of the liquid crystal dynamics with a variable order approach is combined with a full-vector beam propagation analysis to investigate how elasticity and diffraction limit the resolution as a function of the pixel size for transmissive and reflective architectures with vertical liquid crystal alignment. The key liquid crystal properties are considered and the importance of materials with high birefringence is confirmed for small pixel devices as these improve the contrast for a fixed pixel size.

Liquid crystal display response time estimation for medical applications

Accurate characterization of diagnosis instruments is crucial in medical applications such as radiology and clinical neurosciences. While classical CRT medical displays have been replaced almost exclusively with liquid crystal devices (LCDs), the assessment of their temporal properties (response times) is still largely based on heuristic methods, which have not been evaluated thoroughly yet. The authors introduce a novel approach and show that it improves the accuracy and reliability compared to the common heuristic recommended by ISO 9241-305 substantially for a wide range of settings. The approach is based on disentangling the signal from the modulatory backlight through division (division approach). They evaluated this method in two different ways: First, they applied both methods to luminance transition measurements of different LCD monitors. Second, they simulated LCD luminance transitions by modeling the LCD optical responses according to a physical liquid crystal director orientation model. The simulated data were generated for four different response times, each with four different backlight modulation frequencies. Both the novel and the ISO convolution method were applied to the data. Application of the methods to the simulated data shows a bias of up to 46% for the ISO approach, while the novel division approach is biased at most 2%. In accordance with the simulations, estimates for real measurements show differences in the two approaches of more than 200% for some LCD panels. The division approach is robust against periodic backlight fluctuations and can reliably estimate even very short response times or small transitions. Unlike the established method, it meets the accuracy requirements of medical applications. In contrast, the popular convolution approach for estimating response times is prone to misestimations of time by several orders of magnitude and tend to further worsen as advances in LCD technology lead to shorter response times.

Full prediction of the broadband optical modulation performance of a twisted nematic liquid crystal cell

We present a procedure to obtain the physical parameters responsible of twisted nematic liquid crystal (LC) cells optical modulation. The novelty of our approach is based on the use of spectroscopic measurements of the light transmitted by the system polarizer–LC cell-analyzer, combined with a previously proposed simple physical model of the LC twist and tilt distribution along cell. The procedure involves two steps: the first one yields off-state parameters like the LC director orientation, the twist angle, and the optical path difference (cell gap); the second step yields the effective retardances of the central and edge LC layers. The use of a spectroscopic method provides a full characterization of the LC cell as a function of both the voltage and the wavelength. The complete procedure leads to a very accurate prediction of the transmitted light broadband spectrum, as well as the complex (amplitude and phase) modulation for any wavelength within the calibration range.

Anchoring transitions of liquid crystals on SiO x

Anchoring transitions were observed of nematic liquid crystal (LC) mixtures on obliquely evaporated SiO x by varying the relative abundance of two components in the mixture. Of these two components, one has a longitudinal cyano group and another has lateral cyano groups. It was also found that both temperature and SiO x thickness variations are able to shift the anchoring transitions. The anchoring on SiO x has two origins: long‐range van der Waals potential and short‐range surface interactions. Since the two origins have opposite preference in LC director orientation the observed transition is caused by the change of their relative strength. Thermal absorption–desorption of molecules on SiO x surface is important in determining the strength of short‐range interactions, whereas the layer thickness and optical properties of SiO x are dependent on the van der Waals potential. Based on previous work by de Gennes we propose a model to describe the observed phenomenon.

Simulated anchoring of a nematic liquid crystal at a polymer surface

Liquid-crystal anchoring at a polymer surface arises from interactions at several different length scales. At the molecular level, a liquid-crystal molecule may tend to align with the substrate polymer chain, while at the nanometer length scale grooves can exist that arise from the periodic repeat structure of a polymer chain or from nanometer-scale undulations due to surface stresses. On a still longer scale there is the secondary effect of grooves or surface inhomogeneities. We have performed a total of more than 900 ns of atomistic molecular dynamics simulations in order to study the relative importance of the molecular-level interaction and the topography of the polymer surface in liquid-crystal anchoring. Substrates were constructed in which grooves were induced along a direction perpendicular to the constituent molecular chains. In the results presented for the case of 32 5CB molecules on a poly(vinyl alcohol) substrate, the liquid-crystal director orientation appeared to be determined principally by the substrate chain orientation. Only for the deepest grooves did the director align along the grooves and perpendicular to the substrate molecular chain direction.

Single Molecule Spectroscopy of Conjugated Polymer Chains in an Electric Field-Aligned Liquid Crystal

Using single molecule polarization spectroscopy, we investigated the alignment of a polymer solute with respect to the liquid crystal (LC) director in an LC device while applying an external electric field. The polymer solute is poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (or MEH-PPV), and the LC solvent is 5CB. The electric field induces a change in the LC director orientation from a planar alignment (no electric field) to a perpendicular (homeotropic) alignment with an applied field of 5.5 x 103 V/cm. We find that the polymer chains align with the LC director in both planar and homeotropic alignment when measured in the bulk of the LC solution away from the device interface. Single molecule polarization distributions measured as a function of distance from the LC device interface reveal a continuous change of the MEH-PPV alignment from planar to homeotropic. The observed polarization distributions are modeled using a conventional elastic model that predicts the depth profile of the LC director orientation for the applied electric field. The excellent agreement between experiment and simulations shows that the alignment of MEH-PPV follows the LC director throughout the LC sample. Furthermore, our results suggest that conjugated polymers such as MEH-PPV can be used as sensitive local probes to explore complex (and unknown) structures in anisotropic media.

Evaluation of Nematic Liquid Crystal Director Orientation in Vicinity of Glass Substrate Using Shear Horizontal Wave Propagation

A nematic liquid crystal director orientation in a vicinity of a glass/liquid-crystal interface is investigated in relation to an acoustic phase change of a shear horizontal (SH) wave propagating in a cell structure. The relationships between phase change and electric field applied to the liquid crystal layer are given for various layer thicknesses. The numerical analysis of wave penetration in the liquid crystal cell is performed by considering distribution profiles of liquid crystal direction in relation to shear stress at the glass/liquid-crystal interface. The experimental acoustic phase changes are well consistent with the calculated shear stresses as flow resistance. The acoustic phase is strongly dependent on viscosity distribution and particle velocity in the liquid crystal layer.