Hybrid Nematic Cell Research Articles

Optical vector vortices generated with circularly planar and circularly hybrid nematic cells

Optical vector vortices generated with circularly planar and circularly hybrid nematic cells

Elastic constants and the formation of topological defects in hybrid nematic cells: A Monte Carlo study.

We present a Monte Carlo study of the effects of elastic anisotropy on the topological defects which can be formed in nematic films with hybrid boundary conditions. We simulate the polarized microscopy images and analyze their evolution in uniaxial systems for different values of the Frank elastic constants.

Total Internal Reflection in a Hybrid Nematic Cell Submitted to Weak Boundary Conditions

We calculate the trajectory of a monochromatic optical beam propagating in a planar-homeotropic hybrid nematic crystal cell submitted to weak anchoring conditions. We apply a uniform electric field perpendicular to the cell to control the trajectories for various values of the anchoring elastic energy. We have found that the anchoring energy has a strong influence on the ray penetration length and trajectory. Our calculations are consistent with a previously found Frederick’s type transition only present for weak anchoring in which electric fields above an anchoring-energy dependent critical field align completely the director field.

Landau–de Gennes theory for a −1 boojum in a degenerate hybrid-aligned nematic cell

ABSTRACTWithin Landau–de Gennes theory, the texture of a cylindrically confined degenerate hybrid nematic cell containing a −1 boojum has been investigated using a three-dimensional finite-difference iterative method. We impose strong anchoring conditions, with the nematic director perpendicular to the upper plate and planar degenerate at the lower plate. The director distribution of a degenerate hybrid-aligned nematic cell with a −1 defect is known to have no axial symmetry. The axially symmetric characteristic of the biaxial distribution is surprising. The geometric confinement and boundary condition induce an order reconstruction. The influences of cell thickness on the frustrated cell and eigenvalue exchange solution for a sufficiently small thickness with order reconstruction nucleus have been analysed. Defect rings appear when cell thickness is sufficiently small.

Dynamics of order reconstruction in a nanoconfined nematic liquid crystal with a topological defect.

At the wall in a hybrid nematic cell with strong anchoring, the nematic director is parallel to one wall and perpendicular to the other. Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 wedge disclinations in such a cell, using the two-dimensional finite-difference iterative method. Our results show that with the cell gap decreasing, the core of the defect explodes, and the biaxiality propagates inside the cell. At a critical value of dc* ≈ 9ξ (where ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable. Comparing to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 6ξ, the system undergoes a structural transition, and the defect core merges into a biaxial layer with large biaxiality. For weak anchoring boundary conditions, a similar structural transition takes place at a relative lower critical value. Because of the weakened frustration, the asymmetric boundary conditions repel the defect to the weak anchoring boundary and have a relatively lower critical value of da, where the shape of the defect deforms. Further, the response time between two very close cell gaps is about tens of microseconds, and the response becomes slower as the defect explodes.

Differential and integral extended Jones matrices for oblique light propagation through a deformed crystal

An integral Jones matrix for oblique light propagation through a deformed crystal is derived using the differential Jones matrix approach. We show that generally at oblique light propagation a crystal monotonically deformed along its normal should show both linear and circular birefringence, even without twist deformation. The capability of the approach is illustrated by the calculation of the integral extended Jones matrices for oblique light propagation through several deformed liquid crystal nematic cells. In particular, we predict that at oblique light propagation in certain directions the nontwisted hybrid nematic cell (in which the tilt angle of the optic axis with respect to the crystal plate normal smoothly varies through the thickness) shows optical activity, a phenomenon of spatial light dispersion akin to so-called weak optical activity in nonchiral polar solid crystals.

Diffusion of order reconstruction induced by 1/2 wedge disclination in a thin hybrid nematic liquid-crystal cell

Based on the Landau-de Gennes theory, the diffusion of order reconstruction induced by 1/2 wedge disclination in a thin hybrid cell is investigated by the relaxation iterative method. The core structure, the biaxial structure, and the diffusion of order reconstruction as the cell thickness decreases, are explored. The defect structure and the range of order reconstruction do not change when the cell thickness is larger than 15. As the thickness decreases from 15, the defect range broadens along the substrate direction, and the biaxial region as well as the range of order reconstruction also enlarges. When the thickness further decreases to below the critical value of 10, the biaxial region and the order reconstruction range merge into an entire cell, where the planar orientation is abruptly converted into the perpendicular one across the biaxial wall. The results obtained in this paper are important for further studying the regulating effect of topological defect on submicron colloidal particles in nematics.

Electrically controlled dispersion in a nematic cell

In this work, we show theoretically how the trajectories of a propagating optical beam traveling in a planar-homeotropic hybrid nematic crystal cell depend on the wavelength of the optical beam. We apply a uniform electric field perpendicular to the cell to modify these trajectories. The influence of both, the electric field intensity and the refraction index dependence on the wavelength, give rise to an electrically tuned dispersion that may be useful for practical applications.

Electrically controlled total internal reflection in nematic hybrid cells

In this work, we show theoretically how the trajectories of an optical beam propagating in a planar-homeotropic hybrid nematic crystal cell can be modified by applying a low frequency electric field perpendicular to the cell. We use a previously developed formalism for describing the propagation of a polarized beam through the cell. We include a low frequency electric energy for the calculation of the equilibrium orientational configurations of the director's field. The presence of the electric field gives rise to trajectories showing a nontrivial dependence of the beam's range and penetration length with the intensity of the applied electric field.

INTERACTION BETWEEN SPHERICAL PARTICLES IN A DEFORMED LIQUID CRYSTAL

We study how the initial deformation of a nematic liquid crystal affects the interaction between particles mediated by the elastic deformation of a nematic liquid crystal. We calculate the interaction energy between particles in a hybrid nematic cell, homeotropic on the surface of one confining plate and tangential (director can rotate freely) on the other. We find an analytic form of the interaction energy in the case of weak anchoring on the surface of the particle. This interaction energy can be non-monotonic and have a minimum in its landscape, which makes a clear contrast to that in a uniform nematic liquid crystal.

Ray tracing and reflectivity measurements in nematic hybrid cells

We propose an experimental procedure to show that an optical beam may propagate in a planar-homeotropic hybrid nematic crystal cell for incidence angles i larger than the critical one. Its effect on the reflectance coefficient R as a function of i is accounted for by using total internal reflection techniques. We model the propagation of a linearly polarized Gaussian beam of finite diameter through the cell and the reflectance curves R vs. i are calculated theoretically in the total internal reflection regime by using an analytical description. The experimental procedure, cell assembling and methodology are designed so that there is reasonable consistency with the model assumptions. In the optical limit approximation this model allows us to calculate analytically the phase shift, the trajectory and the reflectivity curves of a beam traveling with an extraordinary polarization (P-polarization) inside the cell. Since only some parts of the reflected beams will interfere, Berreman’s method is not suitable for the physical situation considered. The possibility of leakage through the tunneling effect across the cell is also considered and estimated quantitatively. The comparison between the theoretical results and the experimental data shows good agreement between our experimental results and the theoretical curves R vs. i. We compare our results with other measurements reported in the literature and find that the behavior of the reflectance curve is highly affected by the cell thickness-beam waist ratio. The values of this parameter can produce different behaviors of the reflectivity measurements. The analysis of the model predictions and experimental results suggest a possible interpretation of the physical origin of the reflectivity curves and give a qualitative insight into the phenomena behind the total internal reflection regime. Finally, we emphasize the limitations and advantages of the model and of the experimental results presented.

2A06 Hybrid nematic cell中の粒子間相互作用(物理・物性)

2A06 Hybrid nematic cell中の粒子間相互作用(物理・物性)

SIMULATION AND COMPARISON STUDY OF LIQUID CRYSTAL DIRECTOR DISTRIBUTIONS

A new finite-difference iterative method is used to calculate the distribution of the liquid crystal directors under an applied voltage.Numerical results for twisted nematic,super twisted nematic,and hybrid nematic cells are given.In comparison with Newton's method and the relaxation method, it in shown that the new method is simpler,more stable to give accurate results for various types of liquid crystal cells.

Fluctuation-induced interaction in a hybrid nematic liquid-crystal cell

We discuss the thermal Casimir effect in a thin hybrid nematic cell, and we analyse the enhancement of the fluctuation-mediated force due to the frustrating effect of competing surface interactions. The force exhibits a non-algebraic and non-monotonic dependence on the distance, and it dominates the total structural interaction in the system. Similar behaviour is found in the Fréedericksz cell, characterized by a bulk destabilizing external force.

Magnetic field controlled optical phase retardation in a hybrid nematic cell

We have studied the phase retardation of linearly polarized light in a hybrid nematic liquid crystal cell. For a certain range of directions of the applied magnetic field the phase retardation is found to change non-monotonically with the magnetic induction. The observed behaviour is described rather well by the standard Frank elastic theory. The corrections resulting from subsurface deformations, which are characteristic both for second order elasticity approach and for surface field theory, are also considered. The analysis of the experimental data suggests that the presence of distortions in the zero-field director configuration is the necessary condition for the non-monotonic phase retardation, which implies that such an experiment could be used for the detection of misalignment of the effective pretilts in a nematic cell.

Stable configurations in hybrid nematic cells in relation to thickness and surface order

The phase diagram of equilibrium configurations in thin hybrid nematic cells has been investigated in the framework of Landau-de Gennes theory extended to include weak surface anchoring. Surface interactions linear in the tensor order parameter were assumed and biaxiality was induced by the surfaces. It was found that in addition to the usual configuration where the director bends continuously from one plate to the other, there is also a possible configuration where bend does not occur, but where there is an eigenvalue exchange; i.e. the eigenvector of the tensor order parameter associated with the eigenvalue of the largest magnitude is different in different regions of the cell. Furthermore, for very small cell thickness a third possibility occurs: the eigenvector corresponding to the eigenvalue with the largest magnitude is uniform throughout the cell; it is either parallel or perpendicular to both plates depending upon the dominant surface interaction. Continuous transitions occur between the different configurations.

A New Optical Method to Measure Angular Tilts for Planar Anchored Nematic Liquid Crystals

Abstract We present a new optical method to measure the small tilt angles of quasi-planar anchored nematics. We use an hybrid nematic cell which contains twin domains of different birefringence for non zero tilt. We measure the small rotation of the cell which equalize the twin domains birefringence, at a common border. This rotation is proportional to the tilt. The calibration constant can be calculated from the knowledge of the elastic constants anisotropy and the refractive indices, or independently measured from a known tilted anchoring plate. The method is linear in angle, i.e. sensitive and independent of the sample thickness. Comparative measurements of tilt on PVA and SiO are presented and discussed, using this method and other optical measurements of birefringence.

Flexo-electric polarization and anchoring energy for a nematic liquid crystal in a hybrid cell

The flexo-electric polarization of hybrid nematic cells filled with 5 CB was measured as a function of cell thickness. To this aim the pyroelectric response of a nematic layer with small amount of light absorbing dye dissolved was detected in a nematic temperature range. From the experimental data the angle of the director deviation at the homeotropic boundary was calculated as a function of cell thickness. This dependence allows the qualitative determination of the shape of the potential well for the director deviation which is inconsistent with Rapini’s sinesquared form. The «apparent» Rapini’s anchoring energy varies with a surface director angle (at the homeotropic boundary) from 5·10−3 to 3.5·10−2 erg·cm−2.

Determination of the Director Orientation Inside a Hybrid Nematic Cell by Total Internal Reflection

Abstract In this paper we report computer calculations of the reflectivity of a hybrid nematic cell near the condition of total incernal reflection. The Berreman matrices method allows us to drop the approximations used in previous papers. In this way we get details about the interference pattern arising when the total internal reflection occurs inside the sample. We show how the reflectivity curve is affected by the director orientation which in a hybrid cell is essentially due to the boundary conditions. Our results suggest that measurements of this reflection pattern can give detailed information about the local director orientation.

Field Induced Molecular Reorientation in the Hybrid Nematic Cell

Abstract In this paper we review a general theoretical treatment of the interaction between a static or optical electric field and the molecular director of a hybrid aligned nematic liquid crystal. Preliminary experimental data are reported in the two cases for the first time.