Reorientation Of Liquid Crystal Research Articles

The Orientational Fluctuations in the Re-Orientation Process of a Nematic Liquid Crystal

The field-induced molecular reorientation of liquid crystals is usually described by the Frank-Oseen equation with several simplifying assumptions. Among them the “classical” theory ignores thermal fluctuations in the liquid crystal structure. As a consequence, the static Frank-Oseen equation may give inconsistent results in some details of the reorientation process. This work is aimed to include formally the orientational fluctuations in the field-induced reorientation process in nematics. The simplified approach presented here is a “zero-order” approximation of the problem, nevertheless even in this form it allows for correct physical interpretation of the process for fields up to the threshold.

Optical Kerr Constant of Azo-Dye-Doped Nematic and Polymer-Dispersed Liquid Crystals Determined by Biphotonic Z-Scan Technique

We have recently investigated the optical Kerr constant (n2) of azo-dye-doped nematic liquid crystal (ADDLC) and polymer-dispersed liquid crystal (ADDPDLC) films using the Z-scan technique. The measurements of ADDLC samples are firstly performed with the simultaneous application of two pump beams, which are referred as the biphotonic Z-scan technique. The results show that n2 for ADDLC films is contributed primarily from the reorientation of liquid crystals induced by the photoisomerization of azo dyes and thermal effect, while that of ADDPDLC films with nano-sized LC droplets results from the thermal effect, and changes markedly with temperature.

Optical Reorientation and Trapping of Nematic Liquid Crystals Leading to the Formation of Micrometer-Sized Domain

We report on observation of optical reorientation of homogeneous nematic liquid crystals (LCs) thin slab of 4′-penthyl-4-cyanobiphenyl induced by a tightly focused near-infrared laser beam. We foun...

Alignment and Reorientation in Nematic Liquid Crystals Doped with Red Dye and Carbon Nanotube

Four different dyes (Methyl red, Disperse red 1, 13 and 60) were separately doped to each of four different nematic liquid crystals (E7, E8, E63 and ZLI-1132). Their solubilities, textures, phase transition temperatures and order parameters were determined. At the second stage, single-walled nanotubes in a small amount were separately added to each of these solutions, and the experiments were repeated as similar to previous ones. The solubility of dyes was sufficient for using in experiments. When adding dyes and nanotubes, an appreciable change in textures of liquid crystals was not observed. The phase transition temperatures stayed within the limitations required. The changes observed in order parameters depending on the voltage were very small. The highest value of order parameter (0.77) was attained when co-using red 60 dye and nanotube as dopants in the liquid crystal E63.

Tunable light diffractor associated with liquid crystal molecular reorientation from homogeneous alignment to periodic twist deformation in micrometre scale

We have demonstrated the formation of a transmissive tunable light diffractor with a fine grating period using a fringe-field switching (FFS) device. The presented device operates as a superposed amplitude and phase grating. The phase grating is formed by a thin slab of a nematic liquid crystal (LC) film and the amplitude grating is constructed by the electrode pattern of the FFS device. The electrode-position-dependent periodic deformation of the LC director as a function of the applied electric field is analysed and the respective occurrence of phase difference in monochromatic, coherent, linearly polarized He–Ne (632.8 nm) laser light is also calculated using the Jones matrix method at operating voltage. The same calculation results show that the transmitted beam of the present FFS diffractor is right-handed elliptically polarized. The operation characteristics of the thin diffracting device are investigated using a He–Ne laser source and a photodetector which shows a significantly large angular deviation of the diffracted beam owing to the small periodicity of LC deformation.

透過型Four Detectors Polarimeterの試作と液晶配向解析への応用

透過型Four Detectors Polarimeterの試作と液晶配向解析への応用

Silicon-based liquid-crystal cell for self-branching of optical packets

Self-controlled photonic switching was achieved by combining the photoconductivity of a semiconductor and the electrical tunability of a liquid crystal (LC). Pulse packets of 1.06 μm wavelength created free carriers in a silicon electrode of an LC cell, which triggered voltage application for LC reorientation. Consequently, polarization direction of the succeeding packets became perpendicular to that of the preceding packets, and they progressed in different directions after passing through a polarization beam splitter. The cascade LC cells divided a series of packets one by one in a self-controlled manner.

Electro-optic Kerr effect in polymer-stabilized isotropic liquid crystals

The electro-optic Kerr effect with a high Kerr constant around 10−9 m/V2 was observed in the polymer-stabilized isotropic liquid crystals over a wide temperature range. Optical isotropy at the field-off state was attained with randomly oriented domains of (chiral) nematic liquid crystals formed by a polymer matrix cured in the isotropic phase. Optical birefringence in the field-on state was induced by the reorientation of liquid crystals in the domains. Rapid rising and falling response times of the cells around 0.26 ms (0.52 ms total response time) were achieved. A theoretical model was proposed to explain the experimental results and agreed well with the results.

Photo-Reversible Liquid Crystal Alignment using Azobenzene-Based Self-Assembled Monolayers: Comparison of the Bare Monolayer and Liquid Crystal Reorientation Dynamics

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.

Liquid Crystal Addressing by Graphene Electrodes Made from Graphene Oxide

The electrooptic characteristics of the field-induced reorientation of a nematic liquid crystal are studied using graphene layers as transparent conductive electrodes. The covering of a large area with highly conductive graphene was achieved by the thermal reduction of a graphene oxide film. The conductivity of the graphene electrode provides electrooptic properties that are comparable to those of liquid crystal cells with two conventional indium tin oxide electrodes. This result confirms earlier studies and suggestions concerning graphene-based liquid crystal devices. It demonstrates that the fabrication of graphene layers via the deposition and subsequent reduction of graphene oxide is suitable for liquid crystal applications.

All-optical intensity modulation of near infrared light in a liquid crystal channel waveguide

We demonstrate a nonlinear optical channel waveguide made of E7 nematic liquid crystal infiltrated in a silica on silicon groove. Near infrared light at the wavelength of 1560 nm fiber coupled to the core of the liquid crystal waveguide was optically modulated by an optical beam with power below 25 mW by exploiting the optical Freedericks transition. By modeling the optical molecular reorientation in the nematic liquid crystal confined in a waveguiding geometry we are able to reproduce the experimental results.

Theory and numerical simulation of field-induced director dynamics in confined nematics investigated by nuclear magnetic resonance

We investigate the director reorientation in a nematic liquid crystal confined between two parallel plates and subjected to both a magnetic and an electric field. The permanent magnetic field is used first to align the director and, subsequently, to allow nuclear magnetic resonance (NMR) observation of the director response following the application of the electric field. For sufficiently strong electric fields, the director reorients and aligns parallel to the electric field. In this work we focus on the geometry where the electric and magnetic fields are orthogonal to each other. In this configuration the state of the system, immediately after applying the orthogonal electric field, is steady and unstable, at least in theory, since the director is assumed to be everywhere parallel to the magnetic field. In practice the real state of the system always deviates slightly from the perfect unstable steady state, which induces the start of the director reorientation toward the electric field. The nature and the characteristics of the initial deviation partially determine the reorientation process. In the classical approach, the small deviations from the ideal state are assumed to be due to thermal fluctuations, but this approach fails to account for some recent experimental results. For this reason we were led to investigate slightly non-uniform initial director configurations that are stable under the sole effect of the magnetic field but are sufficient to break the ideal unstable steady state created with the application of the orthogonal electric field. Such non-uniformities must be local or distributed over very small sample volumes, since their effects on the equilibrium NMR spectrum (before the application of the electric field) are not usually observed. In other words, we consider the presence of inversion walls in the bulk of the sample and local misalignments of the director on the boundary plates and investigate the effects of such non-uniformities on the response of the nematic to the application of the electric field, as observed by NMR. The model we propose, including such effects in parallel with thermal fluctuations, is able to account for the recently observed features of the field-induced director dynamics.

Widely tunable electro-optic distributed Bragg reflector in liquid crystal waveguide

We propose and numerically investigate a versatile and easy-to-realize configuration for a guided-wave voltage-tunable distributed feedback grating based on reorientation in nematic liquid crystal and coplanar comb electrodes. The device has a wide tuning range exceeding 100 nm and covers C and L bands for wavelength division multiplexing.

GPU-accelerated molecular dynamics simulation for study of liquid crystalline flows

We have developed a GPU-based molecular dynamics simulation for the study of flows of fluids with anisotropic molecules such as liquid crystals. An application of the simulation to the study of macroscopic flow (backflow) generation by molecular reorientation in a nematic liquid crystal under the application of an electric field is presented. The computations of intermolecular force and torque are parallelized on the GPU using the cell-list method, and an efficient algorithm to update the cell lists was proposed. Some important issues in the implementation of computations that involve a large number of arithmetic operations and data on the GPU that has limited high-speed memory resources are addressed extensively. Despite the relatively low GPU occupancy in the calculation of intermolecular force and torque, the computation on a recent GPU is about 50 times faster than that on a single core of a recent CPU, thus simulations involving a large number of molecules using a personal computer are possible. The GPU-based simulation should allow an extensive investigation of the molecular-level mechanisms underlying various macroscopic flow phenomena in fluids with anisotropic molecules.

Kinetics of director gliding on a polymer–liquid-crystal interface

Magnetic-field-induced surface reorientation of nematic liquid crystals on polyethyl-methacrylate films is studied. The experiments indicate that the time scale of director gliding expands from a fraction of a second to several hours. A power-law distribution function of relaxation times provides very good agreement between measurements and calculated gliding curves. From the model an upper limit of the extrapolation length can be extracted, which points to a surprisingly strong equilibrium azimuthal anchoring of the liquid crystal on the polymer layer.

Photoinduced Reorientation and Polarization Holography of Photoreactive Polymer Liquid Crystals with Bistolane Side Groups

New photo-cross-linkable liquid crystalline polymethacrylate with 4-methoxybistolane side groups was synthesized and its thermally enhanced photoinduced molecular reorientation behavior was investigated using a linearly polarized (LP) UV light. Since the bistolane mesogenic side groups were axis-selectively photoreacted under irradiating with LPUV light, annealing procedure generated the thermally enhanced molecular reorientation parallel to the polarization (E) of LP-365 nm light. The axis-selectively photoreacted bistolane moieties acted as the photo-cross-linked anchor to reorient non-photoreacted side groups along them, which direction was parallel to E. The maximum generated in-plane order parameter and birefringence were 0.43 and 0.18, respectively. The influences of degree of the photoreaction and the annealing temperature on the molecular reorientation behavior were investigated in detail. Furthermore, polarization holography using 325 nm He-Cd laser was carried out to fabricate pure polarization gratings with various polarization of the light source.

Optical trapping induced by nonlinear reorientation of liquid crystals

Optical trapping induced by nonlinear reorientation of liquid crystals

Dynamics of domain-wall motion in ferroelectric liquid crystals in an electric field

The director reorientation in smectic liquid crystals with ferroelectric properties has been considered in the case where the interaction of liquid-crystal molecules with the surface leads to a partial unwinding of the helical structure of the liquid crystal and the reorientation occurs as a result of the domain-wall motion. The dependences of the velocity of domain-wall motion on the electric field strength, electric field variation frequency, boundary conditions, spontaneous polarization, and viscosity of the liquid crystal have been determined. It has been demonstrated that an increase in the electric field variation frequency or the polar part of the anchoring energy and the spontaneous polarization of the liquid crystal at a constant field frequency results in an increase of the velocity of domain-wall motion. As a consequence, the time of the electro-optic response of the liquid crystal in weak electric fields (from 0.4 to 2.0 V/µm) decreases by a factor of more than three.

Transient Properties of a Liquid Crystal Optical Device with an Anamorphic Lens Property

We numerically calculate dynamical behaviors of the molecular reorientation of liquid crystal (LC) directors and of the phase retardation in an LC optical device with eight-divided circularly hole-patterned electrode and external transparent electrode. The three-dimensional (3D) distributions of the elliptical phase retardation in the hole-patterned aperture can be obtained by adjusting the applied voltage to each electrode. The transient properties of the phase profile can be estimated and be predicted fairly well by the numerical calculation.

Fabrication of a Liquid Crystal Microlens Array Film with Oligothiophene-Doped Polymerizable Liquid Crystals

Fabrication of a microlens array film was explored by means of photoinduced reorientation of dye-doped polymerizable liquid crystals (LCs). LC monomer/photoinitiator mixtures doped with a small amount of an oligothiophene derivative were irradiated with a 488-nm beam from an Ar+ laser. Then the sample cell was periodically moved. After these two processes were repeated for a few tens times, remaining photoinitiators were quenched by irradiation with UV light. Observation of the cells with a polarizing optical microscope revealed that the molecular alignment of LCs induced by the Ar+ laser beam remained unchanged after the beam was turned off and a microlens array with a square-lattice type was obtained. This indicates that formation of the microlens and photopolymerization of the LC monomers are brought about simultaneously in a small area. The fabricated microlens has polarization selectivity that reflects molecular alignment. The focal length of the microlens arrays could be changed by controlling the irradiation intensity. Furthermore, a freestanding film with microlens arrays was obtained by peeling from the sample cell.