Nematic Liquid Crystal Mode Research Articles

Structural transitions in nematic liquid crystals doped with magnetite functionalized single walled carbon nanotubes

The 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) nematic liquid crystal was doped with single walled carbon nanotubes (SWNTs) and with magnetite labeled single walled carbon nanotubes (SWNTs/Fe3O4). Prepared samples were characterized by infrared spectroscopy, transmission electron microscopy, optical microscopy and by magnetic measurements. Electric and magnetic Freedericksz transitions were measured for the pure liquid crystal, and for the liquid crystal doped with SWNTs, as well as with SWNTs/Fe3O4 (volume concentration ϕ=10−4). The density of anchoring energy at the nematic—magnetic particle boundary was determined. The value of the anchoring energy density for SWNTs/Fe3O4 is lower than for SWNTs. The critical voltage of the Freedericksz transition depends significantly on the bias magnetic field.

Electro-optical characteristics of omnidirectional liquid crystal domain mode using doughnut-shaped slit electrodes

This paper proposes a liquid crystal (LC) display (LCD) mode, characterized by an azimuthally continuous nematic domain, driven by patterned electrodes with circular- and doughnut-shaped slits producing conelike fields, as a vertically aligned (VA) nematic LC mode. This proposed mode is focused on achieving a high transmittance display with omnidirectionally uniform optical characteristics by utilizing the proposed electrode structure. Consequently, the experimental results of the proposed LCD mode show high brightness and wide viewing angles that correlate well to numerical calculations. Other electro-optics characteristics of this mode correspond to the patterned VA LC mode.

Optically induced orientational transitions in nematic liquid crystals with planar alignment

A theoretical study of dynamical phenomena induced by linearly polarized light incident perpendicularly on a planar aligned nematic layer with the light intensity as the control parameter is reported. We find the threshold of the optically induced twist Fréedericksz transition as a function of the problem parameters. The critical light intensity was found substantially lower than predicted earlier by Santamato [Phys. Rev. A 36, 2389 (1987)]. We have demonstrated by a rigorous stability analysis that the bifurcation is stationary only for a thickness of the nematic layer smaller than a certain critical value and becomes a Hopf bifurcation in the opposite case. Above the Hopf bifurcation an oscillatory state appears. It becomes unstable at a secondary threshold through a heteroclinic bifurcation, and the director settles to a new stationary distorted state.

Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell

A fast response nematic liquid crystal (LC) modulator using dual fringe field switching (DFFS) mode is demonstrated. Both top and bottom substrates have pixel and common electrodes to generate complementary fringing fields. The cell consists of a vertically aligned LC layer whose dielectric anisotropy is positive. In a voltage-on state, self-imposed thin LC walls near the center of the slits and electrodes are formed and the surrounding LC molecules exert a strong restoring force. As a result, submillisecond gray-to-gray response time is achieved. Two DFFS cells are used as examples to illustrate the design principles for display and photonic applications.

Structural transition of nematic liquid crystal in cylindrical capillary as a result of the annihilation of two point defects

We study the annihilation of hedgehog-antihedgehog defects in confined nematic liquid crystals using Brownian molecular dynamics simulations. After the collision, merging of defects, and building a loop disclination structure, system can experience a structural transition into another nematic structure, triggered by a nucleation of loop disclination structure. In our rough theoretical approach we calculate the size of the emerged loop structure as the function of the typical size of the confining cavity. Attention is paid also to the dynamics of the loop structure after collision.

On the numerical study of Frederick transition in nematic liquid crystals

A numerical study of Frederick transition, as a static instability and its dynamical behavior in various situations is presented. Dependence of the threshold field on deformation and geometry of the cell is discussed. Dynamical features of Frederick transition are investigated by studying the time evolution of director and velocity fields. This study is based on a stable and simple numerical method that gives accurate results in a short time, compared to the existing methods. The method is also used to study the effects of backflow on the relaxation process. Although the backflow effect is only a perturbation to the relaxation, it is found that it has maximum influences at the middle of the cell and center reorientation.

Fréedericksz transitions in nematic liquid crystals as boundary eigenvalue problems

Fréedericksz transitions between stationary planar deformation states, induced by cover surface interactions and by static external electric or magnetic field in a flat-parallel nematic liquid crystal cell, are studied theoretically in one-dimensional approximation. There is demonstrated that for initial stages of transition, when the deviation of director field from a boundary-induced state is small, the system of Euler-Lagrange equations for free-energy functional can be linearised and the threshold external field magnitudes can be deduced from eigenvalues of differential operator of second derivative for boundary eigenvalue problems with different boundary conditions. Different combinations of electric or magnetic field, parallel or perpendicular to the nematics layer boundaries, and different boundary conditions (including both strong and weak anchoring of nematics molecules) are analysed by using the same formalism. The considerations are based on Frank theory of static deformations of nematic liquid crystals. The formulae for the threshold fields, as depending on nematics and nematics-substrate coupling parameters, are derived. There are stated possible applications of these formulae to determining material parameters by exploiting threshold field magnitudes experimentally found.

Air tube formation at the freezing transition in nematic liquid crystals

A phenomenon is presented, which changes the shape of gas bubbles in liquid crystals and also creates long gas tubes. The system consists of air bubbles which are injected into a nematic liquid crystal host. The shape of these air bubbles changes from spherical to ellipsoidal by initiating freezing of the sample. Furthermore, long gas tubes are formed from the air which was formerly dissolved in the liquid crystal. The gas tubes are created by the progression of the crystalline-liquid interface. Their length can reach up to 40 times their diameter. The diameter of the tubes depends on the pressure applied to the system, as well as on the interface velocity.

Light depolarization effects during the Fréedericksz transition in nematic liquid crystals

This work is aimed to the photopolarimetric characterization of the disorder evolution occurring in homeotropically aligned nematic liquid crystal films during the electrically induced Fréedericksz transition. The molecular director dynamics and the transversal reorientation modes are investigated by the analysis of the depolarization of the light beam emerging from the sample. Our measurements reveal unexpected depolarization effects at the transition, which we interpret in terms of director field unhomogeneity and defects creation.

Influence of shear flow on the Fréedericksz transition in nematic liquid crystals

Within the framework of Ericksen-Leslie continuum theory we analyze the influence of shear flow on the magnetic-field-induced Fréedericksz transition in nematic liquid crystal with rodlike molecules. We consider three basic orientational configurations of a nematic planar layer in the uniform magnetic field. Conditions of rigid director coupling on the boundaries of the layer and constant shear flow gradient inside the layer are used. We exhibit some flow aligning effects for nematic liquid crystals with various ratio of rotary viscosities and investigate how unequal elastic constants (elastic anisotropy) alter the magnetic Fréedericksz transition in sheared nematics. Our calculations predict that surface boundary effects in nematic films and magnetic field action lead to existence of stationary flow regimes in the so-called nonflow aligning nematics, otherwise, surface and magnetic forces extend the range of viscous coefficient values corresponding to the flow aligning regimes. We show that imposing of shear flow on the Fréedericksz transition leads to a threshold behavior or to a "smoothing" of the transition. It depends on the orientation of the nematic layer in magnetic field and magnitudes of rotary viscous coefficients.

Optical-induced Freedericksz transition of nematic liquid crystal doped with porphyrin (ZnTPP)

In this Letter, we studied the optical-induced Freedericksz transition of 5CB doped with 1% volume fraction of bio-molecule, porphyrinozinc(II) (ZnTPP). Excited by Ti:sappire laser with the 82 MHz repetition rate and 100 fs pulse duration, the optical Freedericksz threshold of a 23.6 μm-thickness planar alignment sample occurred at an intensity level of 0.35 mW/mm 2 in contrast to the normally observed 83 mW/mm 2 value for pure 5CB.

Assembly technology for the manufacture of LCOS panels

— The technological issues related to the fabrication of high-performance LCOS displays are described. The considered devices use the vertically aligned nematic liquid-crystal mode because of its superior performance characteristics. The issues addressed mostly stem from this choice and the ensuing need for inorganic alignment layers. In particular, the compatibility of the inorganic alignment layers with the sealant material are treated in detail.

Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Phase diagrams of multi‐wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C60‐I h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic–nematic phase transition temperature (T NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney‐type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ∼0.1–0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C60‐I h)/E7 blend showed no discernible change of T NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.

Dynamical signature of two “ideal glass transitions” in nematic liquid crystals

A temperature scaling analysis using the same mode coupling theory (MCT) scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from ∼1 ps to 100 ns and a range of temperatures from ∼50 K above the isotropic to nematic phase transition temperature TNI down to ∼TNI. The slowest exponential component of the data obeys the Landau–de Gennes (LdG) theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature TC as found for supercooled liquids, in the liquid crystals there are two scaling temperatures TCL (L for low temperature) and TCH (H for high temperature). TCH is very close to T*, which results from LdG scaling, just below the isotropic to nematic phase transition temperature, TNI, but is 30–50 K higher than TCL. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve TCL, but the exponential decay time constant (long time dynamics) scales with an expression that involves TCH. The existence of two scaling temperatures can be interpreted as a signature of two “glass transitions” in liquid crystals. In ideal MCT developed for spheres, TC is the “ideal glass transition temperature,” although it is found experimentally to be ∼20%–30% above the experimental glass transition temperature, Tg. The transition in nematic liquid crystals at TCL corresponds to the conventional ideal MCT glass transition, while the transition at TCH can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order.

Dielectric shape dispersion and biaxial transitions in nematic liquid crystals

Using two order tensors, we propose a mean-field model to describe the uniaxial and biaxial phases of nematogenic molecules presenting a shape dispersion of their biaxial dielectric susceptibility. We recover the classical isotropic-uniaxial-biaxial sequence of phases. The phase diagram exhibits a tricritical point, a feature that cannot be retraced in the other mean-field models established for molecules without shape dispersion.

Observation of cancellation and second light-induced Fréedericksz transition in nematic liquid crystals.

We describe the observation of a new phenomenon: the cancellation of molecular reorientation that has been achieved in nematic liquid crystals acted on by two competing (pulsed) light beams. The effect is unstable: Starting from the cancellation condition, we have observed patterns related to the presence of a nonlinear phase shift in both beams when their intensity was increased simultaneously. We characterize the behavior of the induced phase shift as a function of the impinging power.

Observation of surface and bulk phase transitions in nematic liquid crystals.

The behaviour of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena, and surface-induced ordering of the LC molecules is integral to the operation of LC displays. Here we report the observation of a pure isotropic-nematic (IN) surface phase transition-clearly separated from the bulk IN transition-in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected, but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. We have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which we then study independently as a function of substrate and applied magnetic field. We find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. We show that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. We attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.

P‐75: Fast Nonchiral Smectic C Liquid Crystal Display Mode in a Transverse Electrode Configuration

AbstractWe demonstrated a fast liquid crystal (LC) display mode based on a nonchiral smectic C (NSC) LC with analog gray scales in a transverse electrode configuration and analyzed the display performances by means of two‐dimensional numerical simulation; In this configuration, wide viewing properties were also achieved. The analog gray scales in the NSC LC mode are obtained in a dielectrically driving scheme as those in the nematic LC mode.

High resolution LCOS microdisplay for single-, double- or triple-panel projection systems

A new liquid-crystal-on-silicon display device is described. The 0.9 in. SXGA resolution device runs at speeds up to 450 fields/s, making this microdisplay ideal for single-panel or dual-panel projection applications. Low F-number scrolling-type systems can be used with this panel, making substantial increases in the achievable lumen throughput while maintaining high contrast. The nematic liquid crystal mode switches in 0.2 ms. Contrast ratios at F#/2.8 of 1500:1 have been achieved in projection engines using full visible spectrum UHP illumination. Preliminary measurement indicates contrast of 700:1 at F#/1.0. Photometric reflectivity is >70%.

Design, fabrication and evaluation of a high-performance XGA VAN-LCOS microdisplay

The design, fabrication and evaluation of a high-performance 0.9 in. diagonal XGA microdisplay is described. The DRAM type active matrix comprises square pixels with a 17.6 μm pitch. Bi-directional row and column drivers are integrated on-chip. A 4-metal 0.35 μm CMOS technology enabled the design of a very effective light shield. The peripheral interface electronics supports a programmable look-up table with 10-bit voltage values, as well as programmable threshold voltage and steepness settings, all accessible through a graphic user interface. A vertically aligned nematic liquid crystal mode with an inorganic SiO 2 homeotropic alignment layer yields contrast ratios exceeding 1900:1 and video-compatible response times.