Homeotropic Nematic Liquid Crystal 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.

Magnetic field-induced vortex triplet and vortex lattice in a liquid crystal cell

ANID-Millennium Science Initiative Program ICN17_012 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1180903

On the Problem of the Mechanism of Binary Acoustic Action on a Homeotropic Structure of Nematic Liquid Crystals

A universal model of the distortion mechanism of a homeotropic nematic liquid crystal layer under joint action of the periodic shear and compression has been verified and experimentally confirmed for the first time. The model takes into account all physical factors responsible for this phenomenon and adequately describes it in a wider frequency range including ultrasonic frequencies than that for which such a description was obtained previously. The relationship between the optical response of the mesophase layer and the amplitudes and frequencies of the binary action is found.

Orientational instability of nematic liquid crystal in a homeotropic cell with boundary conditions controlled by an electric field

ABSTRACTWe study the orientational instability of the director in a homeotropic nematic liquid crystal (NLC) cell in a DC electric field. The electric field is applied along or perpendicular to a cell surface depending on whether anisotropy of dielectric permittivity of NLC is positive or negative. The easy axis on one of the cell polymer substrates is allowed to deviate in perpendicular to the substrate plane due to the influence of the NLC and the electric field. It was established that the orientational instability of the director can have a threshold as well as be thresholdless which depends on the character of the coupling of the easy axis with the electric field. The temporal behaviour of a director and the easy axis during transition to the stationary state after turning on electric field and returning to the initial homogeneous state after turning off the field was investigated. In the case of the negative anisotropy of the static permittivity, the comparison of experimental and calculated time dependences of the easy axis reorientation angle let us to estimate values of the mobile easy axis viscosity coefficient and coupling parameter describing the coupling between the easy axis and the electric field.

Photo-controlled patterned wrinkling of liquid crystalline polymer films on compliant substrates

Photo-chromic liquid crystalline polymer (LCP) is a type of smart materials which are sensitive to light. Here we harness its photo-mechanical response to flexibly control surface patterning, through modeling a film involving homeotropic nematic liquid crystals with director perpendicular to the polymer film attached on a compliant substrate. Theoretical and numerical analyses were conducted to explore the surface instability of such film/substrate systems under both uniform and non-uniform illuminations by ultraviolet (UV) light, respectively. By minimizing energy, the film can buckle into checkerboard patterns at the critical photo load. Fourier spectral method is applied to study the post-buckling evolution of various 3D wrinkling patterns including wavy shaped, ring-like, checkerboard, stripe and herringbone patterns. Besides, non-uniform illumination is investigated through square and circular patterned lights, respectively, where the wrinkles can be ordered and controlled remotely and flexibly. Furthermore, the geometric and size effects of local illumination are discussed and characterized by some critical dimensionless parameters. Phase diagrams are provided and agree with experimental observations, which could be used to guide the photo design of wrinkling morphogenesis that is particularly attractive for remote control applications.

Photo-manipulated photonic bandgap devices based on optically tristable chiral-tilted homeotropic nematic liquid crystal.

We report on the spectral properties of an optically switchable tristable chiral-tilted homeotropic nematic liquid crystal (LC) incorporated as a tunable defect layer in one-dimensional photonic crystal. By varying the polarization angle of the incident light and modulating the light intensity ratio between UV and green light, various transmission characteristics of the composite were obtained. The hybrid structure realizes photo-tunability in transmission of defect-mode peaks within the photonic bandgap in addition to optical switchability among three distinct sets of defect modes via photoinduced tristable state transitions. Because the fabrication process is easier and less critical in terms of cell parameters or sample preparation conditions and the LC layer itself possesses an extra stable state compared with the previously reported bistable counterpart operating on the basis of biased-voltage dual-frequency switching, it has much superior potential for photonic applications such as a low-power-consumption multichannel filter and an optically controllable intensity modulator.

Optical vortex generation in homeotropic NLCs in the presence of DC electric field

ABSTRACTThe generation of optical vortices in nematic liquid crystals (NLCs) due to the photorefractive effect has been studied. The vortices can appear in the NLCs with both positive and negative dielectric anisotropy. The efficiency of the Gaussian beam (TEM00) to vortex conversion approached about 30%.

Light induced thermomechanical hydrodynamics in homeotropic nematic liquid crystals

A new method of inducing thermomechanical hydrodynamic flow in homeotropic nematic liquid crystals (NLC) is proposed based on using different anchoring conditions on the cell boundaries. Effect of different pairs of boundary conditions including hard, weak and conical on the director orientation, director gradient and NLC flow are studied, theoretically. Absorption of a linearly polarized laser light is considered to induce the need temperature gradient for the thermomechanical effect (TM) in the NLC cell. Coupling of fluidity to the director orientation distorts the director. Consequent optical nonlinearity is calculated and compared with the optical nonlinearity due to other kinds of the TM effect.

Optical vortex formation in the field of the Gaussian beam with high degree of wavefront curvature when passing through undeformed nematic liquid crystal

Diffraction of the focused circularly polarized Gaussian beam passed through an undeformed homeotropic nematic liquid crystal is considered. The parameters of two coaxial Gaussian beams and two optical vortices formed behind the liquid-crystal layer are determined. The difference in the positions of foci of Gaussian beams and vortices appears in the experiment as interference ring patterns. The efficiency of the beam transformation to optical vortices is calculated.

Optical nonlinearity due to thermomechanical effect in the planar and homeotropic nematic liquid crystals

Possibility of observing third thermomechanical (TM) effect in uniform nematic liquid crystals (NLC) with proper selection of boundary conditions on the cell walls is theoretically studied. Absorption of a light wave induces the needed temperature gradient for the TM effect. The molecular director reorientation due to third TM effect and the induced phase shift on the probe beam are calculated. The forth TM coefficient can be measured directly by the method proposed in this work.

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.

Rayleigh–Benard Convection in the homeotropic nematic liquid crystal by a Gaussian laser beam absorption

The Rayleigh–Benard Convection (RBC) were studied theoretically in the homeotropic nematic liquid crystal (NLC) cell due to the absorption of a normally incident Gaussian light beam from upper side of the cell. It was shown that a convection of hydrodynamic motion in the NLC can be induced and eventually the molecular director coupling with hydrodynamics reorients the director. An external magnetic field parallel to the director decreases the director reorientation and the radial flow velocity, while the velocity in the z direction is unaffected. The results were found to be in a range that can easily be checked experimentally.

Spectral modulation of a bistable liquid-crystal photonic structure by the polarization effect

The light polarization has an effect on spectral properties of a multilayered photonic crystal infiltrated with a bistable chiral-tilted homeotropic nematic liquid crystal (LC) as a defect layer. By varying the direction of polarization of incident, linearly polarized light interacting with the birefringent LC, the tunability of defect modes in wavelength and amplitude and the broadening of the low-transmittance range can be realized in the transmission spectrum. The LC features two optically stable states and two voltage-sustained states. The bistability makes the device of low energy consumption. Such a hybrid can be used as not only a wavelength selector, optical shutter or multichannel switch but also a stopband-tunable device.

Soluble polyimide films as alignment layers for bistable chiral-tilted homeotropic nematic liquid crystal display applications

Soluble polyimides (PI) with and without aliphatic side chains were synthesized by a one-pot synthesis method and acted as homogeneous or homeotropic liquid crystal alignment layers. Homeotropic PI was synthesized by grafting an alkyl side chain on the backbone of homogeneous PI. The vertical alignment properties can be tuned by adjusting the grafting density and the length of the side chains. In addition, various homogeneous and homeotropic PIs were blended by a different ratio to adjust the pretilt angle of the alignment film. The long-term alignment properties and thermal resistance of the alignment films were investigated. Combining the dual frequency liquid crystal with the alignment layers which have an intermediate pretilt angle of 75°, a bistable chiral-tilted homeotropic nematic liquid crystal display device is demonstrated.

Dynamical Behavior of Prewavy Pattern near Nematic–Isotropic Transition

We report new results about a type of electrohydrodynamic instability, the prewavy instability, in a homeotropic nematic liquid crystal in a high-frequency electric field. This instability is seen in the form of a series of bright and dark stripes in between two crossed polarizers. The stripe formation corresponds to the periodic modulation of the rotation angle of the director in the \(xy\)-plane with periodicity along the \(x\)-direction. The dynamical behavior of prewavy patterns near the nematic–isotropic transition is reported. We found that the isotropic phase, due to the Joule heating effect, starts to grow in the dark stripes, i.e., in the vicinity of in-plane rotation angle α = 0.

Vertically Aligned Carbon Film Synthesized from Magnetically Oriented Polyacetylene using Morphology Retaining Carbonization

Polyacetylene (PA) films with vertically aligned fibril morphology were synthesized in homeotropic nematic liquid crystal (N-LC) solvent by using a magnetic field of 5 Tesla as an external perturbation. Scanning electron microscope (SEM) photographs indicated that the lengths of fibrils from the substrate were 5−35 µm, depending on polymerization time. Carbonization was carried out using iodine-doped PA film and a morphology-retaining car - bonization method. From the SEM results, we confirmed that the vertical morphology of the PA remains unchanged even after carbonization at 800 o C. The weight loss of the films due to carbonization at 800 o C is about 20% of the weight of the film before iodine doping. It is ex - pected that vertically aligned carbon might be a precursor for preparing vertical graphite ma- terials, which materials could be useful for electrochemical energy storage and cell electrodes.

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.

Transient Mode Selections in Soft-Mode Turbulence by Controlling the Nambu–Goldstone Modes

We have studied the soft-mode turbulence (SMT), a type of spatiotemporal chaos observed in the electroconvection of the homeotropic nematic liquid crystals, induced by nonlinear interaction between the long-wave length Nambu–Goldstone modes and the short-wave length convective modes. By applying an external magnetic field H, the NG mode can be controlled and as a result an ordered pattern may be observed. Transiently it is achieved by the dynamical changes of the many domains consisting of regular rolls. Removing the magnetic field, then, the ordered state relaxes to its original SMT pattern. The transient dynamics of such a relaxation process is also investigated. An order parameter called as M p is used as a quantitative indicator for the transient process. We reveal two regimes of relaxation process: regime I and regime II correspond to the superposition of the modes and the free rotation of wavevector q ( r ) of the electroconvective pattern, respectively.