Homogeneous Nematic Liquid Crystal Research Articles

Short-focus nematic liquid crystal microlens array with a dielectric layer

ABSTRACTA short-focus microlens array using dielectric layer and inhomogeneous electric field over a homogeneous nematic liquid crystal (LC) layer is proposed. The top substrate has a planar indium tin oxide (ITO) electrode which is coated on the inner surface. The bottom substrate has strip ITO electrodes which are embedded in the dielectric layers. The inhomogeneous electric field generates a required gradient refractive index profile within the LC layer which, in turn, causes the focusing effect. Due to the thinner LC layer (15 μm), the spherical aberration should be negligible. Moreover, the fabrication process of the proposed microlens array can be easily carried out because of the layer-by-layer configuration. The simulation results show that the focal length of the LC microlens can be continuously tuned from infinity to 0.988 mm with the change of applied voltage.

The interaction of colloidal particles with weak homeotropic anchoring energy in homogeneous nematic liquid crystal cells

We have investigated interactions of colloidal particles with weak homeotropic anchoring energy in homogeneous nematic liquid crystal cells. Particle-wall and inter-particle interactions were observed experimentally and analyzed using typical dipole-dipole and quadrupole-quadrupole interactions, including substrate effects as the image charges. Both experimental results matched well with the calculated results for the effective particle radius reflecting the weak anchoring. The effective radius is reduced by the amount of extrapolation length than the actual particle radius. The effective radii of polyethylene micro-particles were reduced to a coefficient ζ (0.78 ≥ ζ ≥ 0.52) times the actual radius with anchoring coefficients in the range of 3.8 × 10(-6) to 1.4 × 10(-6) J m(-2). The anchoring energy of the particles is, therefore, a key component for explaining liquid crystal colloidal systems.

Liquid crystal phase shifters with a twist

The phase-shift per unit-length of microwave radiation passing through a liquid crystal cell is calculated. By using twisted nematics, as compared to the standard homogeneous nematic liquid crystal, the phase-shift per unit-length is increased over the transition region. This has the potential to lower the operating voltage and decrease the size of such phase shifters. In particular, the phase shifts for microwave radiation produced by the liquid crystals K15, E7, and E44 are calculated. It is found that liquid crystals with large values of the elastic constant ratio K22/K11 have their performance most improved when a twist is introduced.

Static alignment states in a bistable azimuthal nematic device with blazed grating sidewalls

Bistable azimuthal alignment has been produced in channels of homogeneous nematic liquid crystal with periodic grating sidewalls. The grating morphologies included a symmetric triangular profile (blaze/pitch (b/p) = 0), an asymmetric highly blazed sawtooth profile (b/p = 0.5) and profiles with different amounts of blaze asymmetry between these two extremes. The observed optical textures and the trend in the relative frequency of occurrence of the two stable states as a function of the asymmetry were in agreement with the predictions of n-director-based Frank–Oseen nematic continuum theory. A sidewall grating morphology with an intermediate degree of blaze asymmetry, b/p = 0.3, gave the highest optical contrast between the bistable states.

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.

Control of the orientation of nematic liquid crystal on iron surfaces: Application to the self-alignment of iron particles in anisotropic matrices

We report on the interactions between nematic liquid crystal (NLC) and iron surfaces. Our aim is to apply, to iron particles, the new colloidal interactions evidenced by Poulin et al. [P. Poulin, H. Stark, T.C. Lubensky, D.A. Weitz, Novel colloidal interactions in anisotropic fluids, Science 275 (1997) 1770–1773]. This approach makes it possible to self-align water droplets suspended in NLCs. For so new type of interactions, the driving forces are related to the NLC distortions induced by the anchoring at the droplet surface. So, to generate self-alignment of iron particles, first, it is essential to control the NLC anchoring at iron surfaces. The anchoring of NLCs on various surfaces has been largely described; however, few results concern iron surfaces. In a first step, cleaned and reproducible iron surfaces were well characterized using reflection adsorption infrared (RAIR) and X-ray photoelectron spectroscopy (XPS). A reproducible hydrated native oxide was formed on iron plates. In a second step, these plates were modified with organic molecules. We describe planar and normal anchorings easily and reversibly obtained from adsorbed molecules such as lecithin and methanol. We also report on very stable planar and normal anchorings induce by the grafting of alkyltriethoxysilane derivatives. The experimental conditions used to form stable polymerised films on the plate surface were successfully adapted to particles. The formation, around isolated particles suspended in homogeneous NLC, of a hyperbolic hedgehog (a point defect) with normal boundary conditions, and the formation of boojums with planar boundary conditions, were evidenced. All this goes to show that these planar and normal anchorings were strong enough to self-align iron particles in homogeneous NLC. For homeotropic anchoring beautiful alignments of particles were evidenced; these chains of particles present the original property of being constituted of particles separated by a distance related to their diameters.

Crucial effects of the anisotropy on optical field induced pattern formation in nematic liquid crystal films

We present here the crucial effects of material anisotropy on optical field induced pattern formation in the one-feedback-mirror arrangement which utilizes the nematic liquid crystal film as the nonlinear medium. By using the quasi-static electric-field-biased planar-aligned homogeneous nematic liquid crystal (NLC) films, we observe both the hexagon and the roll patterns which can be switched optically due to the intrinsic anisotropic distribution of the threshold intensity. The anisotropy comes from the anisotropic nonlinear response of the NLC film and is the crucial factor for such a one-feedback-mirror system to form both the roll and hexagon patterns. The observed phenomena can be explained from the linear stability analysis of the governing diffusion-like equation. The experimental results indicate that the stable roll patterns are formed at low input light power and the stable hexagon patterns formed at high input power.

Influence of flexoelectricity above the nematic Fréedericksz transition.

Continuum theory is used to demonstrate that the presence of flexoelectricity significantly alters the response to an applied voltage of a homogeneous nematic liquid crystal cell above the ac Fréedericksz threshold voltage. In such a system there is a fitting degeneracy: we obtain very good fits between theory and experimental permittivity data using any value of the sum of flexoelectric coefficients, e(11)+e(33), between 0.0 C/m and 1.5 x 10(-11) C/m. The corresponding values of the nematic bend elastic constant show an inverse parabolic relationship with e(11)+e(33), with K33 being reduced down to 90% of its value when flexoelectricity is neglected.

Intermediate periodic "saddle-splay" nematic phase in the vicinity of a nematic-smectic-A transition.

We consider possible spontaneous modulations of the nematic director induced by the elastic saddle-splay K24 term when the value of the elastic constant K24 does not satisfy the Ericksen stability condition for the homogeneous ground state. According to the standard formula expressing K24 in terms of the twist elastic constant K22, this can be expected close to the nematic-smectic-A transition where K22 becomes very large. It is predicted that in a planar nematic layer (or, more generally, if the surface director alignment is sufficiently close to a planar one), a modulated phase with observable long wavelength period can occur in samples considerably thicker than the anchoring extrapolation length. The modulated nematic phase is expected to persist into the smectic phase so that its temperature of the transition to smectic phase has to be lower than that for the homogeneous nematic liquid crystal. Low amplitude short wavelength modulations are predicted for any thickness if the surface director is sufficiently far from a pure homeotropic alignment. At the expense of this mode the temperature of a nematic-smectic-A transition in a planar cell with isotropic surfaces has to be lower than that for a homeotropic cell even if the periodic structure is not accessible for the direct observation.

Pulsed beams in a nonlinear nematic fiber

A model for a cylindrical fiber whose cladding is an initially homogeneous nematic liquid crystal is presented. We construct a narrow wavepacket of transverse magnetic modes propagating through the fiber taking into account the dispersion, self-focusing and diffraction in the nematic. Then a nonlinear Schrödinger equation for the time evolution of its envelope is derived. The speed, time- and length-scales and nonlinear index of refraction of the optical solitons are estimated by using typical experimental values for some of the parameters.

Optical Characterization of Homogeneous Nematic Liquid Crystal Cells Based on the Liquid Crystal Continuum Model

We formulated and applied a new algorithm based on the continuum theory of nematic liquid crystals to the characterization of the electrooptic response of homogeneous nematic liquid crystal cells. Appropriate selection of the device parameters ensures predictions of the electrooptic response which are in agreement with results of optical measurements to a precision of better than 1°. The effect of induced molecular twist and the limitations of decoupling between orthogonal polarizations are discussed.

Propagation of a Gaussian-profile laser beam in nematic liquid crystals and the structure of its nonlinear diffraction rings

We present experimental results on and a theoretical explanation of the laser- and electric-field-induced diffraction effect in both homeotropic and homogeneous nematic liquid-crystal (NLC) films. The diffraction effect is due to the Gaussian profile of a laser beam propagating in the liquid crystal. Many diffraction ring patterns under different electric and optical fields are compared and are analyzed. Some novel phenomena have been discovered. We have found that the diffraction ring pattern at a high electric field is much different from that at a low electric field. Rings in a pattern should be asymmetrical to one axis, and the outer ring interval should be wider. For the homogeneous NLC film it is possible that a second set of diffraction rings may occur when the electric field or the maximum optical field is larger than the tricritical field. The numerical result for the deformed wave-front profile of a Gaussian-profile laser beam passing through the NLC film is calculated. The results can explain these novel phenomena of diffraction ring structures.