Nematic Liquid Crystal Layer Research Articles

Soliton-like defects in nematic liquid crystal thin layers

The nonsingular soliton-like defects in plane nematic liquid crystal (NLC) layers and spherical NLC drops are experimentally detected and studied when the interaction of NLC molecules with a bounding surface is varied. The dynamics and the annihilation of nonsingular defects of opposite signs on a plane surface are investigated. Periodic transformations of the soliton-like defects in NLC drops in an electric field are detected. The theory of elasticity is used to show that the surface energy taken into account in the total free energy of NLC in the case of weak anchoring leads to the possibility of nonsingular solutions of a director equilibrium equation. The calculated pictures of director distribution in a plane NLC layer and in a spherical NLC drop characterized by weak surface anchoring agree well with the results of polarized light optical observations.

Interaction of electromagnetic waves in nematic waveguide

ABSTRACTControlled by an electric field, the interaction of two coupled TE-modes at the dynamic grating in a planar waveguide with a thin layer of nematic liquid crystal has been studied theoretically. The dynamic grating is induced by an external electric field due to the periodicity of the anchoring energy of nematic with waveguide surface. Metal and dielectric waveguides have been studied. The transmission of TE-modes and its reflection from the periodic structure of the nematic layer have been investigated. The intensity of a signal- mode at the output of the waveguide has been calculated. The dependence of intensity on the anchoring energy amplitude and modulation period, dimensions of the nematic layer and an external electric field has been analyzed. It was shown that the frequency interval of a TE-mode reflection expands with increasing of the electric field. The primary maximum of the reflected mode intensity, as function of the nematic layer length, increases monotonously with growing s number of anchoring energy periods placed on the layer length. The primary peak reaches the largest value of 1 for s ∼ 10÷100 (metal waveguide) and for s ∼ 1000 (dielectric waveguide). The same dependence for the signal-mode, which propagates through the NLC layer, has nonmonotonous form and a complete energy exchange is reached only for certain values of s and the length of the nematic layer.

Focus control of a nematic liquid crystal cell using ultrasound vibration

Nematic liquid crystals used in optical devices such as liquid crystal displays are controlled by the electric fields. Although indium tin oxide (ITO) is generally employed as the transparent electrode material to apply the electric field, it includes the rare metal indium and needs complicated processes such as sputtering deposition method. Our group has been investigating control techniques of molecular orientation of liquid crystals using ultrasound. In this study, we have applied these techniques to optical variable-focus lenses and the optical characteristics were examined. The ultrasonic liquid crystal cell has a simple structure with no transparent electrodes; it consists of has an annular ultrasound PZT transducer (inner diameter: 30 mm; thickness: 2 mm) and a liquid crystal cell in which a nematic liquid crystal layer (thickness: 25 μm) was formed between two glass substrates (thickness: 0.7 mm) and orientational films. The concentric flexural vibration modes could be generated on the cell at the several resonance frequencies, and the liquid crystal molecular orientation was changed by the acoustic radiation force. The transmitted light through the liquid crystal cell was focused, and the focal length could be controlled by the input signal.

On the nature of the orientational effect of ultrasound on nematic liquid crystal

Experimental substantiation of the validity of the model of orientational distortion in a homeotropic layer of nematic liquid crystal under an ultrasonic beam with a sharp boundary is presented for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics, taking into account the processes of structural relaxation of the mesophase. It establishes the relationship between the characteristics specifying the homeotropic structure deformation (layer thickness, ultrasound frequency, parameters of the molecular micromodel of liquid crystal, and its material constants) and the layer transparency for a linearly polarized light beam. The calculation results are compared with the experimental data in the frequency range of 0.1–3 MHz.

Optical textures: characterizing spatiotemporal chaos.

Macroscopic systems subjected to injection and dissipation of energy can exhibit complex spatiotemporal behaviors as result of dissipative self-organization. Here, we report a one- and two-dimensional pattern forming setup, which exhibits a transition from stationary patterns to spatiotemporal chaotic textures, based on a nematic liquid crystal layer with spatially modulated input beam and optical feedback. Using an adequate projection of spatiotemporal diagrams, we determine the largest Lyapunov exponent. Jointly, this exponent and Fourier transform allow us to distinguish between spatiotemporal chaos and amplitude turbulence concepts, which are usually merged.

Novel Plasmonic Data Storage Based on Nematic Liquid Crystal Layers

In this paper, compact plasmonic data storage with high-speed sequential writing/reading rate has been proposed and numerically simulated using two-dimensional finite difference time domain method. The proposed storage is composed of a combination of two nematic liquid crystal (NLC) layers and nano-silver sheet perforated with two rectangular nano-holes. The suggested storage has been designed for storing visible light in one of the two nano-holes based on the biasing states of NLC layers. Tuning the dimensions of the proposed storage results in storing three different binary states “01,” “10,” and “11” with 0.4 power absorption efficiency, 13 dB crosstalk and 1 Gbps sequential writing/reading rate. It is expected that such proposed two-bit binary storage can be used as a basic unit for constructing large scale storage device with high storage capacity and ultra-high bit rate.

Modulation of localized surface plasmon resonance for an array of Ag nanostructures layered with nematic liquid crystals

Sensitivity of the localized surface plasmon resonance (LSPR) for an array of Ag (silver) nanostructures layered with nematic liquid crystals (NLC) is investigated. Calculations are made by using finite-difference time-domain (FDTD) method under different geometrical and environmental parameters. Results show that the LSPR wavelength in this array can be controlled and tuned to infrared wavelength range by the rotation of the NLC optical-axis. The rotation of the array and the modifications to height of the NLC layer, the size and periods of the array can affect the sensitivity of the LSPR. The sensitivity is higher when the optical-axis is in xoz plane, than that for the optical-axis in xoy plane. An improved sensitivity has been obtained in the simulation.

Switchable circular beam deflectors

In this work, we report two types of electrically tunable photonic devices with circularly symmetric polarization independent beam steering performance (beam condensing resp. beam broadening). The devices consist of circular micro grating structures combined with nematic liquid crystal (LC) layers with anti-parallel alignment. A single beam deflector converts a polarized and monochromatic green laser beam ( nm) into a diffraction pattern, with the peak intensity appearing at the third order when is applied and at the zeroth order (no deflection) for voltages above . Depending on the shape of the grating structure (non-inverted or inverted), the deflection is inwards or outwards. Both grating types can be made starting from the same diamond-tooled master mold. A polarized white light beam is symmetrically condensed resp. broadened over 2° in the off state and is passed through unchanged in the on state. By stacking two such devices with mutually orthogonal LC alignment layers, polarization independent switchable circular beam deflectors are realized with a high transmittance (>80%), and with the same beam steering performance as the polarization dependent single devices.

Flow-induced delayed Freedericksz transition.

We demonstrate that a compact manometer experiment allows direct observation of a delay to the classical electric-field-induced Freedericksz transition produced by flow in a highly dispersive nematic liquid crystal layer. The Ericksen-Leslie equations are used to show that a flow aligning torque generated in the nematic layer under Poiseuille flow competes with the orthogonal electric-field reorientation torque. This model fully reproduces the experimental results using only self-consistently determined viscosity values, and predicts a more generally applicable expression for the dependence of the delay E(c)∝sqrt[ζ/Δχ(e)] on the shear rate ζ and on the electric susceptibility anisotropy Δχ(e).

Orientational instability of a lyotropic nematic liquid crystal layer

The problem of periodic domain initiation in a thin lyotropic nematic liquid crystal layer is studied. This layer has a planar director initial orientation, but the anchoring energy is minimized by the homeotropic one. The periodic structures whose wave vector is perpendicular to the director exist during the director reorientation process from the planar orientation to the homeotropic one when the reorientation wave front appears. It is shown that the divergent terms of the Prank orientation elasticity energy plays an important role in this effect. The saddle-splay Prank constant and the anisotropic anchoring energy coefficient are estimated.

Electrically Tunable Quasi-3-D Mushroom Plasmonic Crystal

This paper reports an electrically tunable plasmonic crystal incorporating a nematic liquid crystal (LC) layer on the top surface of quasi-3-D mushroom plasmonic nanostructures. The presented plasmonic crystal is formed by an array of polymeric mushroom nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The coupling between surface plasmon polariton (SPP) and Rayleigh anomaly (RA) is observed in experiments with quasi-3-D plasmonic crystals, and verified by simulations. The coupled SPP-RA resonance mode has its electric field vector prominently normal to the surface of the plasmonic nanostructures, and extends into the surrounding medium. This feature makes the coupled resonance sensitive to molecular reorientation in LC, and thus, is useful for designing index modulation-based tunable plasmonic crystal devices. Therefore, by applying external voltages across the LC layer, the SPP-RA resonance mode shows a redshift of 8 nm with a 35% change in the amplitude.

Orientation instability of the layer of a nematic liquid crystal

The origin of periodic structures in a layer of a lyotropic nematic liquid crystal observed in the director (vector, describing the anisotropic properties of the medium) reorientation experiment is studied. Such perturbations with the wavevector perpendicular to the initial orientation can develop in a liquid crystal layer in the unstable equilibrium state when the director is parallel to the walls under the condition that its orthogonality to the boundary corresponds to the minimum anchoring energy. It is shown that the linear dependence of the domain period on the layer thickness observed experimentally can be theoretically described when the Frank orientation elasticity energy is considered in the most general form taking the divergence terms into account and the anchoring energy of orientation is small as compared with the bulk energy. A relation between the coefficient of the divergence terms (saddlesplay elastic constant) and two other coefficients in the Frank energy is obtained.

Substrate-induced gliding in a nematic liquid crystal layer.

We consider the interaction between nematic liquid crystals (NLCs) and polymer substrates. Such substrates can interact with NLCs, exhibiting a phenomenon known as director gliding: the preferred orientation of the NLC molecules at the interface changes on time scales that are slow relative to the elastic relaxation time scale of the NLC. We present two models for gliding, inspired by experiments that investigate the interaction between the NLC and a polymer substrate. These models, though simple, lead to nontrivial results, including loss of bistability under gliding. Perhaps surprisingly, we find that externally imposed switching between the steady states of a bistable system may reverse the effect of gliding, preventing loss of bistability if switching is sufficiently frequent. Our findings may be of relevance to a variety of technological applications involving liquid crystal devices, and particularly to a new generation of flexible liquid crystal displays that implement polymeric substrates.

Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy

A photopolymerizable itaconic acid-based amphiphile (abbreviated as Ita3C12) consisting of a hydrophilic carboxylic acid, three alkyl tails, and a reactive vinyl function was newly designed and synthesized for the formation of automatic and robust vertical alignment (VA) layer of nematic liquid crystals (NLC). Since a hydrophilic carboxylic acid was chemically attached to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium were migrated toward the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer directly interacted with host NLC molecules and made them to automatically align vertically. Because of the reactive vinyl functions of Ita3C12 amphiphiles, it was possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers. The polymer-stabilized robust Ita3C12 VA layer exhibited a strong surface anchoring energy without generating any light scatte...

Modulation of defect modes intensity by controlled light scattering in photonic crystal with liquid crystal domain structure

A method to modulate the defect modes intensity in a multilayer photonic crystal with a nematic liquid crystal layer arranged midmost has been proposed. The various electrohydrodynamic domain structures (Williams domains, oblique rolls and grid pattern) were formed in the nematic layer under the action of ac electric field. The domains cause a polarization-sensitive light scattering which leads to an anisotropic reduction of the defect modes intensity. Thus by varying the applied voltage, we can tune gradually the transmittance spectrum of photonic crystal. In addition, the spectrum strongly depends on the light polarization direction above threshold voltage.

Low-voltage and wide-band tuning of lasing in a dye-doped liquid-crystal sandwich structure.

This paper reports the electrical tuning of a lasing in a liquid crystal (LC) sandwich structure. A dye-doped nematic LC (NLC) layer is sandwiched between two CLC layers to act as a phase retarder with the CLC layers acting as cavity mirrors, for the selective reflection of light in the photonic band with the same sense of helix handedness as that of the CLC layers. The transmittance spectrum of the sandwich cell provides a large range of modulation due to the wavelength dependent nature of phase retardation between the optical eigenmodes in the NLC layer. Lasing occurs at wavelengths corresponding to the maximum transmittance within the reflection band of the CLC layers. The application of voltage to the NLC layer makes it possible to shift the wavelengths of maximum transmittance, thereby tuning the wavelength of lasing. In these experiments, an applied voltage of 1.25 V was sufficient to shift the lasing peak wavelength by approximately 47 nm.

Control of photoluminescence of CdSe/ZnS quantum dots in a nematic liquid crystal by an electric field

Photoluminescence (PL) spectra of a nematic liquid crystal suspension doped with 10 wt % CdSe/ZnS semiconductor quantum dots (QDs) with a core size of 3.5 nm in a 20-µm thick plane-parallel cell have been investigated. The spectrum of a homogeneously oriented suspension layer exhibits a significant shift of the PL peak by ~83 nm to a wavelength of 676 nm as compared with the QD spectrum in toluene. The application of a dc electric field with a strength of 0.25 V/µm to the cell leads to a slight hypsochromic shift of this peak, the position of which does not change with a further increase in the field strength up to 2 V/µm. The PL intensity changes exponentially and decreases by a factor of two at the field strength of 1 V/µm. The mechanism of PL quenching of QDs in a homogeneously oriented layer of nematic liquid crystal is discussed.

Acoustical analog of the Fréedericksz transition in liquid crystals

A nonlinear relaxation model describing thresholdlike variation in the symmetry of a homeotropic orientational structure of a nematic liquid crystal (NLC) layer in an ultrasonic wave field has been constructed in the nonlinear hydrodynamics framework based on a molecular micromodel representing all processes in the mesophase proceeding from the behavior of a separate LC molecule. The relationship between the threshold characteristics and the ultrasound frequency, NLC mesophase layer thickness and material parameters, temperature, and parameters of the molecular micromodel is determined. The results of calculations for the frequency range containing the relaxation frequency of the NLC orientational order parameter are compared to the obtained experimental data.

Polarizer-free liquid crystal display with double microlens array layers and polarization-controlling liquid crystal layer.

We propose a polarizer-free liquid crystal display (LCD) consisting of two microlens array (MLA) layers, a twisted nematic (TN) LC layer, and two light-blocking masks. By changing the polarization state, focal length of the LCD can be controlled. Since two light-blocking masks have a circular stop pattern and a complementary open pattern, entire gray-scale spectrum may be realized by controlling the intensity of light passing through masks. Ultimately, fast response time characteristics could be achieved due to the alignment of LC molecules on the flat MLA surface.

Design and analysis of a tunable liquid crystal switch/filter with metallic nano-slits

We present a tunable device with wavelength switching/filtering features using periodic metallic nano-slits integrated with a nematic liquid crystal layer. Electrical tunability is a result of the presence of nematic liquid crystal in the cavity of a device. Resonant wavelength can be fine-tuned in 94-nm intervals by controlling the bias voltage. The present study shows that the performance of the device is dependent strongly on the geometry of nano-slits. Through the optimization of parameters, 23 dB ON–OFF ratio is achieved, which is better compared to other solutions. These characteristics can be used in switching/filtering applications at near-infrared optical communication systems. In addition, the dependence of reflectance spectrum on incident angle suggests that an application is a tunable optical filter for this device.