Nematic Liquid Crystal Cell Research Articles

Colorimetry characterization of molecular reorientation transition in thin nematic cells.

The characterization of equilibria and their transition is fundamental in dynamic systems. Experimentally, the characterization of transitions is complex due to time scales separation, the effect of thermal fluctuations, and inherent experimental imperfections. Liquid crystal devices are derived from the manipulation of the molecular reorientation and transition between them by employing external electrical and magnetic fields. Here, we investigate and determine the Fréedericksz transition using hue measurements of the transmitted light in thin nematic liquid crystal cells. Based on birefringent retardation experienced by transmitted light due to molecular reorientation, the color adjustment of the nematic liquid crystal cells under white light illumination is characterized. By monitoring the hue of the transmitted light, the bifurcation diagram is determined. As a function of the voltage frequency, the critical transition voltage is characterized. The critical voltage increases with the applied frequency.

Characterization of surface anchoring energy of nematic liquid crystals via electrohydrodynamic instability.

Herein, a method is proposed to determine the azimuthal anchoring energies of surface liquid crystals (LCs), as they gradually change orientation from a vertical to a horizontal state owing to an increase in the voltage applied to each LC cell. The LC cells are characterized using the direction of the Williams roll pattern related to the midplane LC director of the conduction regime of the electrohydrodynamic convection patterns of LCs. The application of the midplane LC directions, obtained from the direction of the roll patterns, to the Ericksen-Leslie equation produces the precise values of the surface anchoring strength. The hybrid type 90°-twisted nematic LC cell, composed of homeotropic and homogeneous LC alignment layers on the top and bottom substrates, respectively, was used to find the azimuthal anchoring energy of the surface LCs, indicated by voltages at the initially vertically aligned LC state. It was observed that the surface azimuthal anchoring energy on the homeotropic layer increased with an increase in voltage. We expect that the proposed technique may be excellent in terms of ease of use, simplicity, and accuracy because the azimuthal anchoring energy can be visually evaluated through the roll pattern.

Field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell.

In this paper, based on the numerical calculation of total energy utilizing the Green's function method, we investigate how a field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell is influenced by the direction of the applied field, surface anchoring feature, and nematic's dielectric properties. The new equilibrium position of the translational movement is decided via a competition between the buoyant force and the effective force built on the microparticle by the elastic energy gradient along the vertical direction. The threshold value of external field depends on thickness L and Frank elastic constant K and slightly on the microparticle size and density, in a Fréedericksz-like manner, but by a factor. For a nematic liquid crystal cell with planar surface alignment, a bistable equilibrium structure for the transition is found when the direction of the applied electric field is (a) perpendicular to the two plates of the cell with positive molecular dielectric anisotropy or (b) parallel to the two plates and the anchoring direction of the cell with negative molecular dielectric anisotropy. When the electric field applied is parallel to both plates and perpendicular to the anchoring direction, the microparticle suspended in the nematic liquid crystal tends to be trapped in the midplane, regardless of the sign of the molecular dielectric anisotropy. Such a phenomenon also occurs for negative molecular dielectric anisotropy if the external field is applied perpendicular to the two plates. Explicit formulas proposed for the critical electric field agree extremely well with the numerical calculation.

Accurate determination on the pre-tilt angle of liquid crystal cell by combining optical and electrical measurement

ABSTRACT In order to accurately determine the pre-tilt angle, we proposed a systematic method by combining optical and electrical measurement. First, the pre-tilt angles of the parallel and vertical aligned nematic liquid crystal (LC) cell with a large thickness (>10 μm) were measured by the crystal rotation method. Then, the material parameters of E7, including the dielectric constant and the splay and bend elastic constant, were obtained by using the dual-cell capacitance model and comparing the capacitance-voltage (C-U) curves of the thick cells. Finally, the pre-tilt angles of two thin LC cells to be tested were determined by theoretical and experimental fitting of their normalised C-U curve, which are 1.80°±0.04° and 3.50°±0.02°, respectively. Our method not only has relatively high accuracy, but also is not limited by the thickness and mode of LC cell.

Transition from escaped to decomposed nematic defects, and vice versa.

An escaped radial director profile in a nematic liquid crystal cell can be transformed into a pair of strength m = +1/2 surface defects (and their associated disclination lines) at a threshold electric field. Analogously, a half-integer defect pair can be transformed at a threshold electric field into a director profile that escapes into the third dimension. These transitions were demonstrated experimentally and numerically, and are discussed in terms of topologically discontinuous and continuous pathways that connect the two states. Additionally, we note that the pair of disclination lines associated with the m = +1/2 surface defects were observed to co-rotate around a common point for a sufficiently large electric field at a sufficiently low frequency.

Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy.

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

Polarization properties of nematic liquid crystal cell with tapered optical fiber

Abstract In the paper, an extended analysis of the polarization properties of a liquid crystal cell with a biconically tapered single-mode telecommunication optical fiber was presented. These properties are a result of a sample geometry and used LC materials. They were analyzed by using two theoretical models based on the matrix decomposition methods, i.e., polar and singular-value one. By measuring Mueller matrices, information about losses, depolarization, dichroism and birefringence was obtained. In the experiment two types of tested samples filled with well-known 6CHBT and E7 liquid crystals were prepared and all optical parameters were shown as the voltage dependence. The tested samples have dichroic properties and for both models calculated PDL is similar and it increases from 2.6 to 6.6 dB for E7 and from 0.4 to 2.7 dB for 6CHBT with voltage changes within the range of 40 – 190 V. Optical losses simultaneously decrease from 30 dB to 27 dB and from 36 dB to 28 dB, respectively. The birefringence properties cannot be directly comparable due to differences between both applied models but voltage fluctuations of these parameters are not significant. These results confirm expected dichroic properties of designed device and complete knowledge about its working principles. Moreover, presented analysis validates usefulness of the singular-value decomposition model applied to dichroic optical fiber elements.

Chiroptical Differentiation of Twisted Chiral and Achiral Polymer Crystals

Charles Mauguin, in his groundbreaking 1911 paper on mechanically twisted, nematic liquid crystal cells, speculated on the possibility of measuring the effects on polarized transmitted light along a sufficiently plastic crystalline material with a preserved twist. This experiment is realized in crystalline, spherulitic polymers wherein twisted, fibrous lamellae emanating from the cores are directed along the wave vector of the incident light. Twisted lamellae of synthetic polymers are found in optically banded spherulites of both chiral and achiral polymers. The material’s chirality (the mesoscopic twist sense) is established at the cores where crystals nucleate. However, determining the twist sense directly by microstructural analyses (e.g., scanning probe or electron microscopy) is possible only in favorable cases. An optical assay, of the kind suggested by Mauguin, would be universal. The differential transmission of circularly polarized light in the cores of polymer spherulites was imaged by complete ...

Fréedericksz-like positional transition

In this paper, a Freedericksz-like positional transition is found for a spherical micro-droplet suspended in a nematic liquid crystal cell in the presence of an external electric field. Based on the numerical calculation of elastic energy using Green function method, the equilibrium position of micro-droplet is decided through a competition between the buoyant force and the effective force built by the elastic energy gradient existing inside the nematic liquid crystal(NLC) cell. It is shown that the elastic energy dominates the kinetics of micro-droplet until the external field applied reaches a critical value large enough to flatten the elastic energy contour in the central region, which enables the asymmetric buoyant force to drive the liquid droplet abruptly from the cell midplane to a new equilibrium position. It is also found that such a threshold value of external field, which triggers positional transition, depends on thickness $L$ and Frank elastic constant $K$, in a Freedericksz-like manner, but multiplied by a factor of $3\sqrt{\pi}$. An explicit formula proposed for the critical electric field agrees extremely well with the numerical calculation.

Random lasing in dye-doped chiral nematic liquid crystal

Random laser action is observed in the PM597 dye-doped chiral nematic liquid crystal (DDCLC) cells. A focal conic texture of chiral nematic liquid crystal (CLC) with many micro-domains is achieved in the cell without rubbing treatment. An Nd:YAG pulsed solid-state laser with a wavelength of 532 nm is employed as a pump source. Discrete and sharp random lasers with a line-width of 0.3 nm and wavelength range of 570–590 nm are obtained. The pump threshold energy of a random laser significantly depends on the cell thickness. The DDCLC cell with a thickness of 80 μm shows the lowest threshold value Eth of 7.6μJ, whereas the 10 μm DDCLC cell presents the highest Eth of 19.6μJ. The lower Eth is attributed to a denser micro-domain distribution that provides a stronger photon scattering at a certain pump depth. The random laser emission is shut down when the temperature reaches 61 °C.

Electrically Tunable Fresnel Lens in Twisted-Nematic Liquid Crystals Fabricated by a Sagnac Interferometer.

This paper presents an electrically tunable Fresnel lens in a twisted nematic liquid crystal cell fabricated by using a Sagnac interferometer. When the Fresnel-patterned green beam, formed by the Sagnac interferometer, is irradiated on the azo-dye doped liquid crystal mixture, the azo-dye molecules undergo trans–cis photoisomerization and then generate the photo-alignment effect in the bright (odd) zones. The director of the liquid crystal molecules in the odd zones reorients the direction perpendicular to the polarization direction of the linearly polarized green beam. The various structures of liquid crystals in the odd and even zones will result in a phase difference and thus, a Fresnel lens can be generated. The experimental results show that the proposed Fresnel lens has a high diffraction efficiency of 31.5% under an applied alternating-currents (AC) voltage. The focal length of the Fresnel lens can also be tuned by thermally erasing the photo-alignment effect of the azo dyes and rewriting by a different Fresnel-like pattern.

Temperature dependence of voltage-controlled polarization plane rotation in a magnetic and electro-optic heterostructure

We propose a theoretical study of the influence of temperature on the state of polarization of a near-infrared light beam reflected from or transmitted through a voltage-controlled nematic liquid crystal (LC) cell deposited on a magnetic film. In both cases, the change in polarization with respect to that of the incoming beam originates in the linear Kerr or Faraday effects taking place in the magnetic layer, but the overall value of the polarization plane rotation depends on the reflectivity and transmittivity of the whole heterostructure. Temperature manifests itself in all layers of the structure through both thermal expansion and the thermo-optic effect when it varies between room temperature and the temperature of nematic-isotropic phase transition in the LC. The thermo-optic effect is strongest in the LC cell, where it is shown to induce noticeable variations of the polarization plane rotation of both reflected and transmitted light beams in that temperature range. Such temperature-dependent changes of the magneto-optical response can be compensated for by adjusting the dc voltage applied to the LC. On the other hand, we show that they may be exploited for sensitive temperature monitoring.

Electrically tunable liquid-crystal optical and imaging devices operating in the infrared wavelength range of 10 μm band

Homogeneously aligned nematic liquid-crystal (LC) cells and twisted nematic LC cells are fabricated using a Si wafer as a substrate and electrode. Their optical properties are investigated in an infrared (IR) wavelength range of 10 μm band. Transmittance of LC materials is high, and the effective birefringence value is enough to apply the LC to the IR optical devices although it is smaller than that in the visible region. The LC lens cell is fabricated using the Si wafer as a lower electrode-substrate. A flat plate of 1.0 mm thick polyolefin resin on which a hole-patterned aluminum film is deposited is used as an upper substrate and electrode, in order to reduce an absorption loss by the electrode. An IR imaging system composed of the LC lens and a thermography camera is implemented, and a focused thermal image is obtained using a wire-grid IR polarizer. A polarizer-free IR imaging system is also reported.

Improvement of the electro-optical properties of nematic liquid crystals doped with strontium titanate nanoparticles at various doping concentrations

ABSTRACTIn this study, we doped homogenous aligned nematic liquid crystal (NLC) systems with strontium titanate (SrTiO3) nanoparticles (NPs), and investigated the impact of doping concentration on the NLC’s electro-optical (EO) properties. SrTiO3 NP-doped NLC cells maintained a high optical transmittance of 77.51% to 78.41% compared to pure NLC cells (78.09%). At a 0.1 wt.% SrTiO3 NP doping concentration, twisted-nematic (TN)-LC cells exhibited enhanced EO performance, with a reduced threshold voltage from 1.70V to 1.61V and a shorter response time from 17.03 ms to 10.66 ms without optical defects and degradations. We also observed an improvement in thermal endurance for doping concentrations of 0.05 and 0.1 wt.%.

Opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) film-coated tandem-90°-twisted nematic liquid crystal cell

The opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) (PVK) film-coated tandem-90°-twisted nematic liquid crystal (TNLC) cell is investigated. The tandem-90°-TNLC cell assembled by two common 45°-TNLC cells can be adopted for incident linearly polarized lights at any linear polarization direction. The variations of diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell are demonstrated opto-electrically and electro-optically. The differences in the degree of linear polarization between the tandem-90°-TNLC and single-90°-TNLC cells for demonstrating diaphragm apertures are investigated. The dynamic transmittance, related to orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric fields generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is studied, and the possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is also proposed. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the developments of other PVK film-coated LC cells. Moreover, the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter if the above ion-induced issues are overcome.

Electro-optical and dielectric properties of TiO2 nanoparticles in nematic liquid crystals with high dielectric anisotropy

The electrical, optical and dielectric performance of high-dielectric-constant TiO2 nanoparticles (NPs) dispersed in nematic liquid crystals (NLCs) with high dielectric anisotropy is demonstrated. TiO2 doping decreases the pretilt angle of the NLC cell because of the increase in Van der Waals interaction between the NLC molecule and alignment layer. Moreover, TiO2 doping strengthens the electric field in the NLC bulk and suppresses the ionic screen effect on the substrate surface, thus decreasing the threshold and driving voltages of the NLC cell. The measured field-off response time and estimated conductivity initially increase and then decrease with increased TiO2 concentrations. This phenomenon is due to the antagonism between the entrained ion impurities during fabrication and the ion harvest effect of TiO2 NPs. Dielectric spectral measurements reveal that TiO2 doping changes the relative permittivity and dielectric anisotropy of NLCs because of the changed space–charge polarisation and order parameters in the NLC bulk. The physical mechanisms are discussed in detail accordingly.

Alignment of liquid crystals with 200 nm-sized V-shaped groove structure fabricated by nano-imprint lithography

We have investigated an alignment of liquid crystals with 200 nm-sized V-shaped groove structure using nano-imprint lithography (NIL) and anisotropic wet-etching of crystal silicon. A periodical V-shaped groove structure was well fabricated on a silicon substrate using polymethylmethacrylate as a stamping material, silicon as a wet-etching material, and KOH solution as an etchant. Smoother V-shaped pattern was obtained on the silicon substrate and a resultant periodical trapezoid groove structure was obtained after second NIL. Azimuthal anchoring energy was 1.09 × 10−4 J/m2 measured by the torque-balance method. A contrast ratio of a twisted nematic liquid crystal cell with the V-shaped groove structure was 44:1.

Dual-mode liquid crystal grating based on photo- and nanoparticle-induced alignment effects.

A silica-nanoparticle-and-azo-dye-doped liquid crystal (LC) phase grating with multistable and dynamic modes based on photo- and nanoparticle-induced alignments was demonstrated. The photoalignment suppressed the electrophoretic movement of the silica nanoparticles in the hybrid aligned nematic (HAN) LC cell to maintain the vertical orientation of LCs during the electrical operation process by means of the azo dyes adsorbed on the silica networks in the homogeneously aligned side of the sample, and contributed two LC structures to form a grating. Through the excitation of a DC pulse with proper polarity and an AC voltage, the multistable and dynamic diffraction efficiencies of the grating were achieved, respectively, by electrophoretically controlled silica nanoparticles and the electrically controlled birefringence effect of LCs.

Shaping Airy beams by using tunable polarization holograms

Optical beam shaping has emerged as an important tool in optics and photonics applications. Here we present the generation of Airy beams by means of the spatial-light-modulator-assisted polarization holography technique. The polarization holograms (PHs) are recorded in a polarization-sensitive nematic liquid crystal cell. By exploiting the advantages of the PH’s diffraction properties, such as the achromatic behavior, the high diffraction efficiency, and the polarization dependence, we experimentally generated Airy beams with diffraction efficiency of 91% at a wavelength of 633 nm.

Twisted Nematic Liquid-Crystal-Based Terahertz Phase Shifter using Pristine PEDOT: PSS Transparent Conducting Electrodes

For this study, we demonstrated three different types of twisted nematic (TN) liquid crystal (LC) terahertz (THz) phase shifters using pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) thin films as transparent conducting electrodes (TCEs). The transmittance of spin-coated pristine PEDOT: PSS thin film was as high as 92% in the frequency range of 0.2–1.2 THz. This is among the highest reported. Several TN-LC cells were constructed in a comparative study, which confirmed the reliability of pristine PEDOT: PSS as a TCE layer for THz phase shifter applications. The highest phase shift, required root-mean-square (RMS) driving voltage, and threshold voltage achieved by devices tested were 95.2° at 1 THz, 7.2 VRMS, and 0.5 VRMS, respectively. The thickness of the LC layer for the phase shifter was 250 µm, approximately half as thick as previous designs. In addition, the pristine PEDOT: PSS-based TN-LC phase shifter exhibited a figure-of-merit (FOM) value of approximately 6.65 degree·dB−1·V−1. This compared favorably with previously reported homogeneously aligned phase shifters with an FOM of 2.19 degree·dB−1·V−1. Our results indicated that a twisted nematic LC cell with pristine PEDOT: PSS thin films as electrodes is a good combination for a THz phase shifter and wave plates as well as other LC-based THz devices.