Homeotropic Nematic Liquid Crystal Cell Research Articles

Electrically induced orientational instability of the director in a homeotropic nematic liquid crystal cell and its effect on surface plasmon oscillations

ABSTRACT This study examines the electric field-induced orientational instability of the director in a homeotropically oriented nematic liquid crystal (NLC) cell. Transitions from homogeneous homeotropic state to non-homogeneous state and from non-homogeneous to homogeneous planar state were investigated. These transitions have a threshold and may exhibit hysteresis. Threshold voltages for these transitions were calculated and regions of existence of hysteresis were determined depending on the cell parameters. Increasing anchoring energy narrows hysteresis range for homeotropic to non-homogeneous transition and expands it for planar to non-homogeneous transition. The impact of the external electric field on the conditions and parameters of surface plasmon polariton (SPP) propagation in the Au–polymer film–NLC system was theoretically investigated. The effective refractive index of the SPP was found to increase with increasing anchoring energy of the NLC with the polymer film and with decreasing wavelength and applied voltage.

Selection rules and a new model for stable topological defect arrays in nematic liquid crystal

ABSTRACT Orientation of nematic liquid crystal (NLC) around a topological defect is expressed by the equation, , where , , and are the director spatial phase, topological charge, azimuthal angle and a spatial phase shift, respectively. Conventionally, defect array is modelled by simply adding the fields of all the defects. The resultant array is a combination of the defects imposed by design () and the defects due to the lattice geometry (). However, defect arrays generated in homeotropic confinement show hyperbolic hedgehogs () between the s, which are out of the scope of the conventional model. In this research, two-dimensional defect ‘crystals’ with various lattice structures (square and hexagonal), various defect shapes (radial or circular) and various lattice constants (pixel size) are generated by pixelated patterned electrodes in homeotropic NLC cells. The results verify the two selection rules, 1) The total must be zero, 2) must be a constant throughout the array, and a new model composed of , and is established. Calculation proves that the selected modelled array has minimum free energy. The -- model is versatile and easy to apply, which benefits the design of new topological defect arrays.

Light-matter interaction induces a single positive vortex with swirling arms.

Homeotropic nematic liquid crystal cells with a photosensitive wall and negative dielectric anisotropy exhibit, under the influence of local illumination, stable vortexes with swirling arms that are trapped at the illuminated area. Close to the Fréedericksz transition an amplitude equation is derived, which allows us to understand the origin of the induced vortex and the competition between the illuminating profile and the elastic anisotropy generating the swirling of the arms.

Vortex Induction via Anisotropy Stabilized Light-Matter Interaction

By sending circularly polarized light beams onto a homeotropic nematic liquid crystal cell with a photosensitive wall, we are able to locally induce spontaneous matter vortices that remain, each, stable and trapped at the chosen location. We discuss the dual light-matter nature of the created vortices and demonstrate the ability of the system to create optical vortices with opposite topological charges that, consistent with angular momentum conservation, both derive from the same defect created in the liquid crystal texture. Theoretically, we identify a self-stabilizing mechanism for the matter vortex, which is provided by the concurrency of light-induced gradients and anisotropy of the elastic constants that characterize the deformation of the liquid crystal medium.

Excitation of Rayleigh-Benard Convection in Homeotropic Nematic Liquid Crystal by a Gaussian Laser Beam from Above

We theoretically studied the Rayleigh-Benard convection in homeotropic nematic liquid crystal cells due to the absorption of a Gaussian light beam, which penetrates the cell normally from above. It was shown that a convection of hydrodynamic motion in nematic liquid crystals can be induced as a result of absorption Gaussian beam and consequently, can lead to hydrodynamic motion and significant director reorientation in liquid crystal cells. The results can be easily measured experimentally.

Effects of Fluid Flow on Electric-Field-Induced Director Reorientation in Homogeneous and Homeotropic Nematic Liquid Crystal Cells, Probed by Transient Current Measurements

Flow effects on the dynamic behavior of the director in homogeneous nematic liquid crystal (NLC) cells with positive dielectric anisotropy (Δε> 0) and homeotropic NLC cells with negative dielectric anisotropy (Δε< 0) have been studied by analyzing the transient currents. The transient currents of homogeneous and homeotropic NLC cells were excited by step voltage application. The measured transient currents were well reproduced by computer simulation, based on the dynamic theory of NLCs in which the flow effects are taken into account. It is found that the flow effects with the free-slip boundary condition accelerate the electric-field-induced director reorientation in homeotropic NLC cells with Δε< 0, while the flow effects are negligible in homogeneous NLC cells with Δε> 0.

Effects of viscosities on the transient current in homeotropic nematic liquid crystal cells

Effects of the Leslie viscosity coefficients α i ( i = 1, 2, …, 5) of nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δ ε < 0) on the electric-field-induced director reorientation in homeotropic NLC cells have been studied from the analysis of the transient current induced by step voltage application. The transient current in a homeotropic NLC cell with Δ ε < 0 was well reproduced by computer simulation, based on the theory of NLCs in which the flow effects and the free-slip boundary condition are taken into account. It is found that the response time of vertical alignment NLC displays is dominantly governed by α 2 and α 4 + α 5 of NLCs with Δ ε < 0.

AC electric field assisted photo-induced high efficiency orientational diffractive grating in nematic liquid crystals

An orientational grating with reduced scattering noise is formed in fullerene-doped and undoped homeotropic nematic liquid crystal cells by simultaneous application of both spatially modulated light and ac electric field. The first-order diffraction efficiency obtained in the fullerene-doped cell reaches ~30 %, approaching the maximum value predicted by theory. Effective nonlinear index coefficient is ~0.9 cm2/W and ~1000 times larger than previous observations in C(60)-doped nematic liquid crystals using a dc field. Grating formation time can be as short as 1 s, which is ~100 times shorter than those found by us using a dc field and previous studies.

A computational approach to the optical Freedericksz transition

The optical Freedericksz transition in a homeotropic nematic liquid crystal cell is modeled using a solver which combines direct solution of Maxwell’s equations with a relaxation algorithm for the liquid crystal director. We find that even in the equal elastic constant case the continuous optical Freedericksz transition can be driven first order. For films in which the optical retardation of the extraordinary wave is sufficiently large, a whole set of discontinuous jumps in transmission coefficient can occur. These jumps correspond to the existence of optical resonances in the liquid crystal film. Our results agree in the short wavelength limit with paraxial approximation calculations, and provide a strong test of the FDTD method for anisotropic materials such as liquid crystals.

Laser induced self-phase modulation in nematic liquid crystals and effects of applied dc electric field

We investigated the ring-pattern characteristic of self-phase modulation in a homeotropic nematic liquid crystal cell induced by a focused Ar-ion laser beam. Laser induced self-phase retardation versus incident angle was experimentally studied, and good agreement with theory was found. Effects of applied dc electric field on the ring-pattern were studied, to our knowledge, for the first time. When the electric field was below a critical value, laser induced self-phase retardation could be varied by changing the electric field strength. Above this value, irregular shaped diffraction pattern accompanied by intense light scattering was observed, which is attributable to honeycomb-like domain formation.

Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incident light

Two simple and fast optical methods are presented for the cell thickness and the surface pretilt angle measurements of a planar-oriented liquid-crystal (LC) cell, in which the LC director always lies in a plane of a Cartesian coordinate system. The methods employ an obliquely incident light propagating in the same plane containing the liquid-crystal molecules. Simple analytic formulas for the angular-dependent optical transmission are obtained. The phase retardation between the ordinary and extraordinary waves is used to measure the cell thickness with the use of a compensator. Independent data can be obtained for the cell thickness measurement by varying the angle of incidence of the incident light. The phase retardation-extremum method is used to measure the pretilt angle. Approximate formulas relating the pretilt angle and the angle of incidence corresponding to the transmission minimum are also obtained for the pretilt angle measurement for vertically and parallel-oriented LC cells with a small pretilt angle. For the parallel cell, the approximate formula agrees with that obtained by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices, edited by G. J. Sprokel (Plenum, New York, 1980), p. 115]. The methods are applicable to a planar LC cell, including the planar parallel nematic LC, homeotropic nematic LC cell, obliquely oriented nematic LC cell, and the planar-oriented ferroelectric smectic LC cell.

Nematic Liquid Crystal 1024 Bits Page Composer

We report design considerations and experimental results concerning a nematic liquid crystal (NLC) cell used in an ac crossbar addressing mode to achieve a page composer for holographic memories. Our display is a 32 x 32 binary elements page composer utilizing the electrooptic (EO) effect of index modulation or field effect in an homeotropic NLC cell, and this composer acts as a spatial wave polarization modulator.