Crystal Reorientation Research Articles

GND accumulation in bi-crystal deformation: Crystal plasticity analysis and comparison with experiments

A three-dimensional, large deformation crystal plasticity model is presented in which the lengthscale effect is incorporated through the explicit coupling of the density of geometrically necessary dislocations (GNDs) with the crystallographic slip rule. The channel-die bi-crystal compression tests carried out by Sun et al. [1998. Mesoscale investigation of the deformation field of an aluminium bicrystal. Scripta Material 39(4/5), 501–508; Sun, S., Adams, B.L., King, W.E., et al., 2000. Observations of lattice curvature near the interface of a deformed aluminium bicrystal. Philosophical Magazine 80 (1), 9–25] have been assessed and the crystal plasticity model has been employed to predict the distributions of GNDs local to the grain boundary. Quantitative comparisons of the predicted distributions with those determined experimentally show reasonable agreement. The qualitative comparisons of crystal reorientation predicted by the model with those observed by Zaefferer et al. [2003. On the influence of the grain boundary misorientation on the plastic deformation of aluminium bicrystals. Acta Materialia 51, 4719–4735] show good agreement. Further analyses of the bi-crystal have been carried out both with and without the development of GNDs. The results show that the crystal re-orientations local to the grain boundary are different if GND accumulation is included, suggesting that conventional, non-lengthscale crystal plasticity models are unlikely to be capable of capturing correctly the deformations arising at grain boundaries.

Experimental investigation of liquid crystal director reorientation by mechanical vibrations

Reorientation of a nematic liquid crystal undergoing mechanical vibrations of one of cell walls was studied experimentally. Orientational structures have been revealed, which have interesting properties, their characteristics and formation processes were investigated. It was shown that these structures have parameters that do not depend on the intensity and frequency of mechanical vibrations.

Voltage-driven in-plane steering of nematicons

Using an external voltage and interdigitated electrodes in a liquid crystalline cell, we demonstrate tunable steering of light beams and spatial solitons without the detrimental walk-off out of the plane of propagation. The results agree well with a simple model describing birefringence and reorientation in uniaxial nematic liquid crystals.

Anisotropic diffraction pattern formation from a nematic liquid crystals film induced by low-power linearly polarized beam

Anisotropic diffraction patterns were observed on far-field screen induced by a weak (0.16 W/cm2) linearly polarized collimated beam passing through a homeotropic C60-doped nematic liquid crystal (5CB) cell. Based on orientational photorefractive effect, two-wave coupling experiment was carried out to induce liquid crystal molecule re-orientation. The form of space-charge field with the Gaussian distribution induced by the Gaussian beam incident on the sample is investigated. The outline of far-field diffraction pattern is concentric ring, and shows symmetrical gaps in the direction perpendicular to the direction of polarization of light. In addition, diffraction pattern changes when the polarization of the incident light is changed, effective nonlinear index coefficient is n20.3 cm2/W. The results showed that our theoretical analyzsis was consistent with the observed experimental phenomena.

Modeling the Dynamics of Photo-induced Reorientation of Nematic Liquid Crystals doped with Azo-dye

We have used a continuum model of azo-dye doped liquid crystals to describe the dynamics of photo-induced enhancement of director re-orientation. By describing the dye distribution as a superposition of spherical harmonic functions we numerically integrate the dynamic equations and identify various time constants associated with cis to trans isomerization, excited state lifetime, and the various diffusion times of the different species. Our results give an experimental response time which is intensity dependent and characterized by a saturation intensity. This saturation intensity is dependent on the pump beams angle of incidence. Numerical results will be presented.

Light-induced orientational effects in periodic photonic structures with pure and dye-doped nematic liquid crystal defects

We theoretically study and compare the light-induced Freedericksz transition and above-threshold reorientation in pure and dye-doped nematic liquid crystals embedded into periodic dielectric structures. The presence of optical defect modes is found to significantly alter the effective influence of dyes as compared to the standard case of liquid crystal films without periodic structure, which is due to the overall resonant absorption of the dye-doped liquid crystal layer. Both statics and dynamics are discussed. In particular, it is found that optical response time can be up to several orders of magnitude faster with pure liquid crystals than with dye-doped ones. Thermal effects caused by absorption of dyes are also investigated to gauge whether it may induce nematic-to-isotropic phase transition.

The role of defects and boundary and volume inhomogeneities in the field-induced director reorientation in nematic liquid crystals confined between parallel plates

A deep understanding of the director dynamics in nematic liquid crystals confined between parallel plates is important for display technology as well as for basic science. Some aspects of this dynamics are still not clear, in spite of intense research and significant progress in the last thirty years. When a strong magnetic (or electric, with some precautions) field is applied to a well aligned nematic sample, depending on the angle between the director and the field one usually observes either a homogeneous or a spatially periodic (inhomogeneous) director reorientation to the new equilibrium position, parallel to the field, as shown by NMR observations well supported by the Leslie–Ericksen theory [A.F. Martins, P. Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986)]. These are collective modes (observed simultaneously over the whole sample) and the spatially periodic one originates in the long wavelength thermal fluctuations of the director orientation in equilibrium. Recent experiments [G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32, 1389 (2005)] have shown that in some practical cases the director reorientation in thin nematic samples confined between two parallel plates does not follows any of those two pictures. We focus on such experiments and present 2D computer simulations of the director dynamics in the presence of topological defects or boundary or volume inhomogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results show good agreement with the experimental data. We propose that in addition to the above mentioned collective modes, a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation in thin samples of low molecular mass liquid crystals.

Light-Induced Director Reorientation in Nematic Liquid Crystals Doped with Azobenzene-Containing Macromolecules of Different Architecture

The results of the studies of the orientational optical nonlinearity of a nematic host doped with high-molar-mass compounds of different molecular architecture such as a comb-like polymer and a dendrimer containing azobenzene chromophores are presented. The nonlinearity induced by these compounds was found to be higher than that induced by the low-molar-mass dyes similar in structure to the light-absorbing constituents of the polymer and dendrimer. Possible reasons for the nonlinearity enhancement for macromolecular dopants are discussed.

Second and Fourth Harmonic Frequencies in Electric Field-Induced Liquid Crystal Reorientations

The second and fourth harmonics of low frequency electric field-induced liquid crystal reorientations are analyzed theoretically and confirming results observed experimentally. These transient dynamic optical responses manifest when the phase of a homogeneous cell is nearby Nπ or , where N is an integer. These ripples would influence the grayscales of liquid crystal devices if the driving electric field frequency is comparable to the liquid crystal response rate.

Transient Properties of a Liquid Crystal Optical Device with an Elliptical Intensity Distribution

We developed a three-dimensional (3D) and real-time numerical calculation method for analyzing the dynamical behaviors of the reorientation of liquid crystal (LC) directors and of the phase retardation in an LC optical device. The LC optical device design employs an eight-divided circularly hole-patterned electrode structure in a flat nematic cell. The switching property of the elliptical phase retardation in the hole-patterned aperture can be estimated and the transient properties of its phase profile can be predicted fairly well by the calculation.

Enhanced alignment of plate ice crystals in a non-uniform electric field

An atmospheric electric field exerts a torque on both column and plate ice crystals. Columns tend to align along the direction of the field. One diameter of plate crystals also aligns parallel to a field, while a non-uniform diverging field leads to further orientation and proscribes a secondary orientation along the direction of least divergence. A laboratory investigation treats the electrical orientation of thin plate ice crystals in a non-uniform field as a two-step process. In the model, the average field provides the primary torque and aligns one diameter of the plate crystal while simultaneously a spatially varying component of the field aligns the perpendicular diameter. The first element of the process, the primary torque, has been investigated previously. The complete reorientation, caused by a secondary torque, is investigated herein. Experimentally a stronger field with sufficient non-uniformity is required to demonstrate the secondary torque. The realignment caused by this torque depends also on the strength of the aerodynamic torques on the falling ice crystals. Experiments in a laboratory cold chamber with small thin plate crystals in an electric field provided by a charged rod demonstrate crystal reorientation by changing the reflections of a beam of white light. A model incorporating electrical torques is applied to the laboratory observation and used to investigate the possibility of the effects occurring in the atmosphere. Fields near lightning strokes, which are strong but of short duration, may reorient small crystals. Larger crystal aggregates like planar snowflakes are governed by the same electrical torques. It is suggested that the secondary torque realignment might be observable for aggregates large enough to be detected by radar in cases pertaining to weaker but longer duration fields associated with cloud electrification. A simplification of the model is used to investigate electrically caused crystal motion and linear aggregation, with applications to the atmosphere and to lightning initiation.

New Insights into Lamellar Structure Development and SAXS/WAXD Sequence Appearance during Uniaxial Stretching of Amorphous Poly(ethylene terephthalate) above Glass Transition Temperature

An in situ study of structure formation in amorphous poly(ethylene terephthalate) (PET) during uniaxial stretching at a temperature 30 C above glass transition temperature was carried out using synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Three major deformation-induced structure transitions were confirmed. (1) At small strains, the applied load increased initially but leveled off afterward. Sporadic isotropic crystallization without preferred orientation was observed by WAXD, where no hierarchical structure was seen by SAXS. (2) At intermediate strains, strain hardening took place. Although WAXD showed persistent progression of isotropic crystallization, SAXS indicated formation of a layered structure as well as a fibrillar domain in large scale. This behavior is not consistent with the mechanisms for shish-kebab or spinodal-assisted structure formation. Instead, it can be explained by flow-induced demixing of crystal and amorphous phases through layerlike flocking motion perpendicular to the stretching direction. (3) At high strains, the ratio between the applied load and strain was about constant. In this stage, crystal reorientation and lateral crystal growth took place. The corresponding structure changes could be categorized into three subregions. In the first region, the (010) crystalline plane began to orient. In the second region, the (100) crystalline planemore » began to orient. In the last region, the structure change became stable and the sample eventually broke apart.« less

Optical properties of BaTiO3 thin films: Influence of oxygen pressure utilized during pulsed laser deposition

The crystallographic properties of BaTiO3 thin films, grown by pulsed laser deposition on MgO substrates, were found to be strongly influenced by the oxygen pressure used during growth. Low pressure grown films were c-oriented while increasing oxygen pressure produced films with preferred a-orientation. The crystal reorientation resulted in the shift of optical birefringence from +0.04 to −0.025 with low levels of birefringence in films possessing low tetragonal distortion. Mach-Zehnder electro-optic waveguide modulators were fabricated to characterize the electro-optic properties of the deposited films and to evaluate the suitability of these films for planar optical applications. An effective electro-optic coefficient of 23 pm/V was obtained for a c-axis oriented film near the crystal c→a reorientation point.

Biphotonic effect of azo-dye-doped liquid crystals using the sequential Z-scan technique

This study investigates the biphotonic effect of azo-dye-doped liquid crystals (ADDLCs) using the sequential Z-scan technique. A spot on the sample is illuminated by a green light for 6s, and then the same spot is illuminated simultaneously with a red light and a green light for 6s. Measurements are made by scanning the sample near the beam waist of the green laser. The results show that the biphotonic effect is important to the nonlinear coefficient of the sample. The variations of the optical Kerr constant with intensity of red light are measured. Measurement results demonstrate that the molecular reorientation of liquid crystals induced by the photoisomerization of the azo dyes dominates at low red-light intensity, but the thermal effect compensates for the molecular reorientational nonlinearity of the sample at high red-light intensity.

Branching and re-orientation of lamellar crystals in non-banded poly(butene-1) spherulites

The formation mechanism of non-banded polymer spherulites has been examined experimentally for isotactic poly(butene-1) grown from the melt by optical, atomic force, and transmission electron microscopies associated with quenching and chemical etching. At the growth front of the spherulites, the maximum width of lamellar crystals, λm, showed a square-root dependence on the growth rate. The dependence suggests an instability-driven branching. In terms of the correlation of lamellar orientation in the spherulites, an auto-correlation function has been determined from the image taken by polarizing optical microscopy. The correlation showed an exponential decay along the radial direction, and the correlation length was in proportion to λm. Those experimental evidences suggest that the structure is formed by the coupling of the branching instability and the random re-orientation of lamellar crystals on the occasion of branching in the non-banded spherulites of poly(butene-1).

Viscosity Response Measurement of Nematic Liquid Crystal under Application of Electric Field Using Shear Horizontal Wave

The Propagation characteristics of a shear horizontal (SH) wave in a trilayer structure having a viscous fluid layer is analyzed from a numerical analysis. Acoustic phase changes under application of electric fields in a nematic liquid crystal cell are measured using the SH wave. The acoustic phase advances with increasing the applied electric field. This result corresponds with the viscosity change based on a molecular reorientation of the liquid crystal. In dynamic response measurement, the response times in the phase and the optical measurements are different, because the measured area of the acoustic phase is only the vicinity of the glass surface.

Dynamic Characteristics of Vertically Aligned Liquid Crystal Device Using a Polymer Wall Associated with the Boundary Condition of Alignment Layer

Voltage-dependent liquid crystal (LC) reorientation was analyzed in a vertically aligned nematic liquid crystal display with a polymer wall (called a locked-super homeotropic (LSH) device) through the LC textures. The LC textures generated have a singularity of S = + 1, however, they were strongly dependent on the order of fabrication between the polymer wall and the vertical alignment layer, generating either pure radial or twisted radial orientation of the mid-director. In this article, we investigate how the boundary condition of the alignment layer on a wall surface affects the dynamics of LCs in the LSH mode.

Counterpropagating Dipole Beams in Nematic Liquid Crystals

We investigate the behavior of counterpropagating optical beam structures in nematic liquid crystals. We restrict our attention to the dipole{ dipole beam arrangements. A time-dependent model for the beam propagation and the director reorientation in nematic liquid crystals is numerically treated in three spatial dimensions and time. Stable dipole beams are observed in a very narrow threshold region of control parameters. Below this region the beams difiract, above the region spatiotemporal instabilities are observed, as the input intensity is increased and also as the distance between the dipole partners is decreased. A transverse beam displacement of counterpropagating dipole beams is also found. The difierence between the in-phase and out-of-phase components of the dipole is signiflcant, but only for a smaller distance between the dipole partners.

Self-controlled signal branch by the use of a nonlinear liquid crystal cell

A nonlinear input-output characteristic was observed when neodymium doped yttrium aluminum garnet laser passed through a silicon cell containing a twisted-nematic liquid crystal. The laser beam excited free carriers in the silicon plate, which triggered electrical reorientation of the liquid crystal. Consequently, the polarization direction of the laser beam changed, and the output beam passing through a polarizer became weaker as the input power increased. This function was utilized for self-branching of an optical pulse train.

Voltage-tuned resonant reflectance optical filter for visible wavelengths fabricated by nanoreplica molding

A guided-mode resonant filter incorporating an electro-optically tunable liquid crystal refractive index is demonstrated at a wavelength of 655nm and a tuning range of 4nm. Rigorous coupled wave analysis and finite difference time domain analysis are used to simulate the characteristics of the filter during liquid crystal reorientation. Tuning performance is demonstrated that is consistent with the device simulations. Tunable filters in the visible wavelength range that are inexpensively fabricated over large surface areas are expected to find applications in optical limiting and video display.