Liquid Crystal Director Research Articles

24.3: Optical Rewritable Diffraction Grating based on Photo‐Alignment Materials

AbstractIn this article, an optical rewritable diffraction grating is disclosed. A periodically grating pattern is written on the photoactive material, the photo‐alignment material periodically defines the liquid‐crystal director distribution in a cell sandwiched between two glass plates. The pattern can be erased, and then after can be rewritten with different period, simply by exposing the photo active layer with different polarization of blue light (Λ=450nm). 1D and 2D gratings can be achieved, for the same cell, using different masks with diffraction efficiency, for the 1st order, up to 13.0% and 19.0% respectively.

Light-induced first-order orientational transitions in a nematic liquid crystal in the presence of an ordinary wave

The effects of light-induced reorientation of the director of a nematic liquid crystal doped with dendrimer are investigated experimentally. The influence of light polarisation on the firstorder orientational transition that occurs with changing the light beam power is studied. An irreversible first-order orientational transition when changing the polarisation of light is discovered and examined. A theoretical description of the observed effects is presented.

Optically generated reconfigurable photonic structures of elastic quasiparticles in frustrated cholesteric liquid crystals

We describe laser-induced two-dimensional periodic photonic structures formed by localized particle-like excitations in an untwisted confined cholesteric liquid crystal. The individual particle-like excitations (dubbed "Torons") contain three-dimensional twist of the liquid crystal director matched to the uniform background director field by topological point defects. Using both single-beam-steering and holographic pattern generation approaches, the periodic crystal lattices are tailored by tuning their periodicity, reorienting their crystallographic axes, and introducing defects. Moreover, these lattices can be dynamically reconfigurable: generated, modified, erased and then recreated, depending on the needs of a particular photonic application. This robust control is performed by tightly focused laser beams of power 10-100 mW and by low-frequency electric fields at voltages ~10 V applied to the transparent electrodes.

Director Model for the Electro-Optics of Blue-Phase Liquid Crystal

We propose a theoretical model to explain the electro-optical properties of a blue-phase liquid-crystal (BPLC) cell. This is the first time that a BPLC cell can be simulated using a simple model. Instead of employing the Kerr effect, we model the BPLC cell using a series of nematic liquid-crystal layers, and study the behavior of the liquid-crystal director. With the help of this model, the electro-optical properties and the response time can be calculated by a commercial simulator, and the calculated results are in good agreement with the experimental data.

Liquid crystal gratings based on alternate TN and PA photoalignment

A diffraction grating is proposed by periodically defining the liquid-crystal director distribution to form alternate parallel aligned and twist nematic regions in a cell placed between two crossed polarizers. Based on the combined phase and amplitude modulation, both 1D and 2D tunable gratings are demonstrated. Low voltage ON/OFF switching of 1st order diffracted light with extinction ratio over 80 is achieved within a small voltage interval of 0.15 Vrms. Unique four-state feature of the cell is obtained and their applications in optical logic devices are discussed.

Numerical analysis and design of patterned electrode liquid crystal microlens array with dielectric slab

The phase profile of hole patterned electrode liquid crystal (LC) microlens array (MLA) was studied by a numerical method. It indicated that the formation of inverse domain and tilt wall in the liquid crystal cell limits the optical performance of the microlens array. The physical mechanism that determines the formation of inverse domain and tilt wall was elucidated by analyzing the phase profile and the liquid crystal director distribution. According to our numerical results, a new configuration for hole patterned electrode liquid crystal microlens array was proposed by introducing a dielectric slab between the patterned electrode and the liquid crystal. The physical parameters that influence the optical performance of this new configuration LC MLA were studied, which provide fundamental information for the design. The results indicate that the phase profile and focusing power of the new configuration is greatly improved.

Analysis of the rotation of the polarisation direction in a cylindrical liquid crystal lens where the direction of the incident linear polarisation is not parallel to the axis of the cylindrical lens

A liquid crystal (LC) lens is used to change the propagating direction of the incident light at the lens-shaped boundary of two different media and shows strong dependence on the polarisation direction of the incident light. The performance of the cylindrical LC lens is investigated in the situation that the directions of the incident linear polarisation and the axis of the cylindrical lens are not parallel. Two LC cell configurations were considered where LC directors aligned twisted or homogeneously. In case of the twisted LC configuration, the polarisation direction of the propagating light follows the twist of the LC directors if the directions of the incident linear polarisation and the LC alignment on the side of the lower substrate are parallel. For these two configurations, the LC lens can control the propagating light in the LENS-ON state. In case of homogeneously aligned LC configuration, polar angle distributions of LC directors are not uniform due to the lens-shaped boundary. So in the LENS-OFF state where LC directors rotate under the driving voltage, a LC domain boundary may occur in some cases.

Chiral induction in thioester and oxoester liquid crystals by dispersed carbon nanotubes

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral signatures for the thioester liquid crystals were found to be an order of magnitude larger than those of the oxoesters. We speculate that the much larger strength of the thioester's chiral properties is a result of stronger non-covalent interactions between the liquid crystal molecule and carbon nanotube.

Mixtures of Magnetic Nanoparticles and the Ferroelectric Liquid Crystal: New Soft Magnetoelectrics

A mixture of maghemite ferromagnetic nanoparticles and the ferroelectric liquid crystal Felix 015/100 was experimentally analysed in the vicinity of the ferroelectric smectic C* (SmC*) phase. In order to verify the existence of magnetoelectricity in prepared soft material, a SQUID-based magnetometer was used. It was shown that the orientation of magnetic nanoparticles is directly coupled to the liquid crystal molecular director field. Such coupling allows possibility of indirect coupling between the nanoparticle magnetization and liquid crystal polarization. This makes such mixtures potential for new soft magnetoelectrics.

胆甾相液晶盒贝纳德效应实验

The conception of Bernard effect and cholesteric liquid crystal texture are introduced.The sample of cholesteric liquid crystal in planar texture is prepared,the texture change with temperature field by polarizing microscope is observed.The Bernard effect or pane grate effect under the cleaning temperature is appeared.The experimental phenomena show that the temperature field induced the liquid crystal molecule convection,the birefringence interference is occurred,and liquid crystal director distribution is periodical variation,so that the microcosmic Bernard effect pattern formed.The experimental phenomena wait for explanation rightly,and have some significance for liquid crystal physics basic research.

Three-dimensional complex-shaped photopolymerized microparticles at liquid crystal interfaces

Microparticles of arbitrary shapes immersed in the bulk of nematic fluids are known to produce dipolar or quadrupolar elastic distortions that can mediate long-range colloidal interactions. We use two-photon photopolymerization to obtain complex-shaped surface-bound microparticles that are then embedded into a nematic liquid crystal host with a uniform far-field director. By means of three-dimensional imaging with multi-photon excitation fluorescence polarizing microscopy, we demonstrate low-symmetry, long-range elastic distortions induced by the particles in the liquid crystal director field. These director distortions may provide a means for controlling elastic interactions in liquid crystals between custom-designed photopolymerized microparticles attached to confining solid substrates and nematic fluid-borne colloids, thus enabling elasticity-mediated templated self-assembly.

反扭曲向列相液晶的导波研究

Using the fully-leaky waveguide technique,the inverse twisted nematic(ITN) negative liquid crystal cell is investigated.The intensity of reflected light dependent on the internal angle through the fully-leaky liquid crystal waveguide geometry is obtained from experiment with various voltages.Compared with theoretical result,some parameters of liquid crystal material and liquid crystal cell are determined,and then the information of liquid crystal director within the liquid crystal cell is also obtained.

CONVECTIVE MOTIONS IN NEMATIC LIQUID CRYSTAL HOMEOTROP AND PLANAR CELLS INDUCED BY GAUSSIAN LASER BEAM

Laser beam heating the medium induces instability in the liquid crystal (LC) cell. This instability in conjunction with influence of gravitational force results in convective motions in the cell. In this paper theoretical modeling for studying convection induced by Gaussian laser beam in nematic LC homeotrop and planar cells is presented for the first time. Velocity field and LC director distribution are obtained for various light powers and LC cell sizes by solving Navier-Stokes, heat transfer and director equations simultaneously. The modeling allows us to solve the problems of convections induced by Gaussian laser beam due to Rayleigh-Benard and Marangoni mechanisms as well. There is a good qualitative agreement between theoretical calculations and prior experimental results. The possibility of control and stability of convective motions are studied. Instabilities of the Benard cells are of thermal origin because the Prandtl number for the medium under study is considerably larger than unity.

Analysis of optical bounce associated with two-step molecular reorientation in the fringe-field switching mode

The dynamic electro-optic response of liquid crystals (LCs) having positive dielectric anisotropy has been investigated in the fringe-field switching device. Simulation and experimental results show a huge optical bounce near the centre of the pixel electrodes of the device during decaying time of the LC director from an operating voltage. The causality of the said behaviour has been detected as two-step molecular relaxation, first tilt down and then rotating of the LC, which is associated with splay/bend and then twist relaxation, respectively. Reduction of dielectric anisotropy of the used LC material in the device has been suggested as a remedy of reducing optical bounce. The reduction of rotational viscosity also produces optical bounce of lesser duration; however, the intensity of the bounce remains invariant.

Reversible long range network formation in gold nanoparticle - nematic liquid crystal composites

Nanoparticles (NPs) are dispersed into liquid crystals (LCs) to create ordered NP assemblies and thereby modify the LC and NP properties. Although low NP concentrations are normally used to avoid aggregation, high concentrations can lead to new organization through coupling of the interparticle attractive forces with the LC elastic properties. Gold nanoparticles (AuNPs) with mesogenic coatings, tailored to be highly miscible in the liquid phase of n-alkyl-cyanobiphenyl LCs, form reversible micron-scale networks on cooling at the clearing point by enrichment of the NPs at the nematic-isotropic liquid interfaces. The network topology and LC director field orientation are controlled by the cooling rate, surface alignment, film thickness, AuNP concentration and ligand shell composition. Thin film networks consisted of branches and circular areas of LC enriched in AuNPs. Nucleating nematic droplets evolve into homeotropic alignment of the host nematic matrix, accompanied by birefringent disclination lines and loops. Thick film AuNP networks in LCs form complex structures with stable radial director configurations in small domains and Schlieren domains elsewhere. Controlled formation of networks via the use of LC phase transitions offers an additional approach to produce quasi-periodic NP assemblies that are both long range and reversible in nature.

A Review of Electrically Tunable Focusing Liquid Crystal Lenses

Electrically tunable focusing liquid crystal (LC) lenses are reviewed in this paper. The distribution of the orientations of LC directors which is controlled by electric fields results in a distribution of refractive indices of LC directors. The incident light can be modulated by the electrically tunable lens-like phase difference of the LC lens. We introduce the basic operating principles of LC lenses and discuss the structures of LC lenses. The major challenges of LC lenses are also discussed. We believe this paper provides a guideline for basic understanding of LC lenses.

Large-area optoelastic manipulation of colloidal particles in liquid crystals using photoresponsive molecular surface monolayers

Noncontact optical trapping and manipulation of micrometer- and nanometer-sized particles are typically achieved by use of forces and torques exerted by tightly focused high-intensity laser beams. Although they were instrumental for many scientific breakthroughs, these approaches find few technological applications mainly because of the small-area manipulation capabilities, the need for using high laser powers, limited application to anisotropic fluids and low-refractive-index particles, as well as complexity of implementation. To overcome these limitations, recent research efforts have been directed toward extending the scope of noncontact optical control through the use of optically-guided electrokinetic forces, vortex laser beams, plasmonics, and optofluidics. Here we demonstrate manipulation of colloidal particles and self-assembled structures in nematic liquid crystals by means of single-molecule-thick, light-controlled surface monolayers. Using polarized light of intensity from 1,000 to 100,000 times smaller than that in conventional optical tweezers, we rotate, translate, localize, and assemble spherical and complex-shaped particles of various sizes and compositions. By controlling boundary conditions through the monolayer, we manipulate the liquid crystal director field and the landscape of ensuing elastic forces exerted on colloids by the host medium. This permits the centimeter-scale, massively parallel manipulation of particles and complex colloidal structures that can be dynamically controlled by changing illumination or assembled into stationary stable configurations dictated by the "memorized" optoelastic potential landscape due to the last illumination pattern. We characterize the strength of optically guided elastic forces and discuss the potential uses of this noncontact manipulation in fabrication of novel optically- and electrically-tunable composites from liquid crystals and colloids.

The Study of Thermal Effect in Liquid Crystal Director

The model of thermal effect for liquid crystal director was built. The temperature field of liquid crystal was numerically studied. The effects of laser irradiation time and the crystal size on the thermal effect were analyzed and compared. With the increase of laser irradiation time, the temperature field became stable.

A polarization independent liquid crystal phase modulation adopting surface pinning effect of polymer dispersed liquid crystals

A polarization-independent liquid crystal (LC) phase modulation using the surface pinning effect of polymer dispersed liquid crystals (SP-PDLC) is demonstrated. In the bulk region of the SP-PDLC, the orientations of LC directors are randomly dispersed; thus, any polarization of incident light experiences the same averaged refractive index. In the regions near glass substrates, the LC droplets are pinned. The orientations of top and bottom droplets are orthogonal. Two eigen-polarizations of an incident light experience the same phase shift. As a result, the SP-PDLC is polarization independent. Polarizer-free microlens arrays of SP-PDLC are also demonstrated. The SP-PDLC has potential for application in spatial light modulators, laser beam steering, and electrically tunable microprisms.

Homeotropically aligning phase separated columnar structures for fabrication of flexible electrooptical devices

A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photo-curable pre-polymer is reported. Vertical polymer columns and micro-fibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LC-substrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electro-optical devices with high contrast coupled with fast response times. This simple single-step technique eliminates the need for the traditional polymer alignment layer pre-deposited on substrates.