Experimental comparison of a liquid-crystal point-diffraction interferometer (LCPDI) and a commercial phase-shifting interferometer and methods to improve LCPDI accuracy

We compare the phase measurements of a fused-silica witness sample made with a liquid-crystal point-diffraction interferometer (LCPDI) with measurements made with a Zygo Mark IV xp phase-shifting interferometer and find close agreement. Two phase-shift-error sources in the LCPDI that contribute to measurement discrepancies are frame-to-frame intensity changes caused by the dichroism of the dye and alignment distortions of the host liquid crystal. An empirical model of the phase-shift error caused by the host alignment distortions is presented and used to investigate the performance of two different phase-detection algorithms. It is suggested that by proper choice of LCPDI fabrication parameters and phase-acquisition methods, the device's accuracy can be significantly improved.     

Analysis of dichroic dye-doped polymer-dispersed liquid crystal materials for display devices

Dichroic polymer-dispersed liquid crystals (DPDLCs) based on nematic liquid crystal materials with azo dye were investigated in detail for the application of display devices. Polarizing optical microscopy, differential scanning calorimeter and electro-optic experiments all have shown that the DPDLC containing low concentration of dyes modifies the basic properties of these materials like optical transmission, threshold voltage, contrast ratio and absorbance factor. A minimum amount of dye needs to be added to the liquid crystal with the polymer matrix for its effective phase separation and to minimize the transmittance in the OFF state and therefore gives rise to an overall improvement in contrast ratio of the devices. Molecular orientation and dynamics in droplet sizes are readily controlled in these DPDLC materials. These findings imply that the value of the threshold electric field E_th is approximately 8 V/μm for pure polymer-dispersed liquid crystal (without dye) where the threshold electric field E_th values are approximately 4.0 V/μm, 2.0 V/μm, 1.7 V/μm 1.0 V/μm, for 0.0625%, 0.125%, 0.25% and 0.5% with azo dye in DPDLCs, respectively. From the results we can also infer that the maximum contrast is approximately 2.55 times the minimum contrast observed in the experiment for DPDLCs. The results show that the DPDLC with proscribed dye concentration will be possibly suitable and promising functional electronic materials for green technology flexible liquid crystal display.     

Dye-doped cholesteric-liquid-crystal room-temperature single-photon source

Abstract

Fluorescence antibunching from single terrylene molecules embedded in a cholesteric-liquid-crystal host is used to demonstrate operation of a room-temperature single-photon source. One-dimensional (1-D) photonic-band-gap microcavities in planar-aligned cholesteric liquid crystals with band gaps from visible to near-infrared spectral regions are fabricated. Liquid-crystal hosts (including liquid crystal oligomers and polymers) increase the source efficiency, firstly, by aligning the dye molecules along the direction preferable for maximum excitation efficiency (deterministic molecular alignment provides deterministically polarized output photons), secondly, by tuning the 1-D photonic-band-gap microcavity to the dye fluorescence band and thirdly, by protecting the dye molecules from quenchers, such as oxygen. In our present experiments, using oxygen-depleted liquid-crystal hosts, dye bleaching is avoided for periods exceeding one hour of continuous 532 nm excitation.     

Optical properties of permanent gratings in liquid crystal doped with dye and carbon nanotube

Permanent gratings were written in planar nematic liquid crystal cells containing E7 doped with the azo dye Methyl Red and single-wall carbon nanotubes. An order parameter of S = 0.605 shows that the dye molecules align themselves very well with the liquid crystal’s director axis. A maximum absolute diffraction efficiency of 0.39% was found for cells doped only with 0.6% MR while a maximum of 2.6% was observed for cells doped with 0.6% MR and 0.002% CNTs. The data are consistent with permanent grating formation based on trans-cis photoisomerism of the dye molecules, followed by surface adsorption of the cis-isomer that leads to homeotropic alignment in the bright regions of the light illumination pattern. The robust permanent gratings were written without the aid of an external electric field and have remained stable for over 2 years.     

Nano-engineering of the anchoring of liquid crystals on solid surfaces

The solid surface in the conventional liquid crystal displays, being responsible for the liquid crystal alignment in the absence of external fields, is playing only a passive role in the switching of the liquid crystal layer. Recently, we have shown that the anchoring strength and the easy axis, two important parameters characterizing the liquid crystal alignment, can effectively be controlled by light via photo-induced nano-scale changes of the solid surface properties. We have also introduced two novel concepts of commanded anchoring, the electrically commanded surfaces (for mediating switching of the liquid crystal) and the high performance alignment layers (for facilitating the switching of the liquid crystal) realized practically by means of a proper nano-engineering of the alignment layer. Electrically commanded anchoring concepts are strong candidates for implementation in a new generation of advanced liquid crystal displays and devices.     

Carbon nanotube enhanced diffraction efficiency in dye-doped liquid crystal

We report on the optical properties of nematic liquid crystal cells containing the liquid crystal E7 doped with the azo dye Methyl Red at percentages ranging from 0.4% to 0.8% and 0.002% single-wall carbon nanotubes. From polarized absorption measurements an order parameter of S = 0.605 was obtained, showing that the dye molecules align themselves very well with the liquid crystal's director axis. Diffraction efficiencies of 532-nm pump and 670-nm probe beams were measured. Efficiencies and rise times were found to increase with dye concentration. A maximum efficiency of 5.8% was found for cells doped with carbon nanotubes, while cells without carbon nanotubes had a maximum efficiency of 3.2%. Therefore the presence of carbon nanotubes enhanced the diffraction efficiency by a factor of 1.8. The nonlinear index coefficient, n₂, was calculated to be 18 × 10^?3 cm²/W. The data are consistent with a grating formation based on trans-cis photoisomerism of the dye molecules that leads to a reorientation of the liquid crystal phase. These photonic devices are functional without the application of any external field, are easily prepared, and have lifetimes in excess of two years without any indication of degradation when stored at ambient temperatures.     

Optical properties of disperse-red-1-doped nematic liquid crystal

We report on the optical properties of nematic liquid crystal cells containing the commercially available liquid crystal E7 doped with the azo dye Disperse Red 1 at concentrations ranging from 0.3% to 1.0%. From polarized absorption measurements an order parameter of S=0.569 was obtained, thus showing that the dye molecules align themselves very well with the liquid crystal's director axis. Optical index gratings were written in the material by overlapping two coherent laser beams at 532 nm. Self-diffraction of the writing beams and diffraction of a probe beam at 670 nm were measured. First-order self-diffraction efficiencies and rise times were found to be linearly dependent upon the dye concentration with a maximum efficiency of 3.29% and a minimum rise time of 10.6 ms. The non-linear index coefficient, n ₂, was calculated to be 7.6 × 10^-3cm²W^-1. The data are consistent with a grating formation based on trans–cis photoisomerism of the dye molecules that leads to a reorientation of the liquid crystal phase. These photonic devices are functional without the application of any external field, they are easily prepared, and have lifetimes in excess of two years without any indication of degradation when stored at ambient temperature.     

All-optical switchable holographic Fresnel lens based on azo-dye-doped polymer-dispersed liquid crystals

Fabrication of an all-optical switchable holographic liquid crystal (LC) Fresnel lens based on azo-dye-doped polymer-dispersed LCs is reported using a Michelson interferometer. It is found that, upon circularly polarized photoirradiation, the diffraction efficiency of the fabricated Fresnel lens was increased significantly in a reversible manner. We believe this is due to the anisotropy induced by reorientation of the LC molecules coupled with azo-dye molecule orientation due to trans-cis-trans photoisomerization, which modulates the refractive index of the LC-rich regions. We also studied the effect of azo dye on the polarization dependency of the fabricated lens.     

Polymer-stabilized liquid crystal indicator used in thermometry

Research on the biological effects and on critical-temperature alarm indicators for fire safety often requires the use of non-metallic and reliable temperature sensors. For such applications, we investigated the temperature dependence of the spectral response of a pure cholesteric liquid-crystal temperature indicator and of a polymer/liquid crystal mixture. We observed a modulation of the intensity of transmitted monochromatic light, in accordance with the temperature changes. We emphasize that the modulation occurs over a wide temperature range outside the active temperature of the thermochromic liquid-crystal indicator. Such behavior suggests the possibility of using these liquid crystals in a wider temperature range, under appropriate conditions. The spectral response of the liquid crystal in its active temperature range shows that a thermochromic liquid crystal indicator may be practically used as a precision thermometer.     

液晶显示材料用偶氮染料的研究进展

二向色性偶氮染料分子结构上的特性和可修饰性,使其在液晶彩色显示中得到重要的应用.与液晶掺杂后这类染料表现出许多良好的光电特性,如具有宽液晶显示的视角、增加显示亮度等.简要介绍与归纳了近几年来发现和新合成的偶氮染料与液晶共混后产生的吸光性质、二向色性以及偶氮染料在液晶中的溶解性和稳定性,并展望了这类染料的合成方向.     

Photo-rewritable flexible LCD using indium zinc oxide/polycarbonate substrates

This work presents a photo-rewritable transmissive flexible-LCD based on the alignment effect of the photo-induced adsorption of azo dye on flexible indium zinc oxide/polycarbonate (IZO/PC) substrates. Images can be displayed without applying an external field and rewritten by changing the direction of the writing laser beam while the cell temperature is controlled. By using IZO/PC substrates, the writing and erasing processes can be achieved within 1 min with a high contrast.     

Self-assembled liquid-crystalline gels designed from the bottom up

Liquid crystals are often combined with polymers to influence the liquid crystals' orientation and mechanical properties, but at the expense of reorientation speed or uniformity of alignment. We introduce a new method to create self-assembled nematic liquid-crystal gels using an ABA triblock copolymer with a side-group liquid-crystalline midblock and liquid-crystal-phobic endblocks. In contrast to in situ polymerized networks, these physical gels are homogeneous systems with a solubilized polymer network giving them exceptional optical uniformity and well-defined crosslink density. Furthermore, the unusually high-molecular-weight polymers used allow gels to form at lower concentrations than previously accessible. This enables these gels to be aligned by surface anchoring, shear, or magnetic fields. The high content of small-molecule liquid crystal (>/=95%) allows access to a regime of fast reorientation dynamics.     

High transmittance in-plane switching liquid crystal mode using double-exposed UV alignment method

In general, the super in-plane switching (S-IPS) liquid crystal (LC) mode is widely used for LCD applications because of its wide-viewing properties. However, it can also diminish the aperture ratio in active areas due to the zigzag pattern. In this paper, we proposed an IPS LC mode that has wide-viewing properties even if the stripe-patterned electrodes, which can provide a high transmittance property, are applied using the double-exposed UV alignment method to the upper/lower domain of the active area. The initial alignment direction of the upper/lower domain was optimized with simple experiments and we achieved both the superior dark level in the initial mode and wide-viewing properties in the applied voltage mode. As a result, we obtained an increased aperture ratio of about 10% and a higher optical transmittance of 15% compared to the conventional S-IPS LC cell because of the high aperture ratio without any optical loss of the initial dark state and viewing angle property.     

Luminance addition of a stack of multidomain liquid-crystal displays and capability for depth-fused three-dimensional display application

A stack of liquid-crystal displays is expected to reduce visual fatigue caused by a flat-panel three-dimensional (3D) display. We previously developed a compact depth-fused 3D (DFD) display by using a stack of two twisted-nematic (liquid-crystal) LC panels, but its viewing-angle characteristics and color reproducibility were not so good. Therefore recent wide-view LC panels should be used. We report calculated and measured luminance addition characteristics, which are essential for evaluating 3D display characteristics, of a stack of two multidomain LC panels. We found that LC panels with super in-plane switching, patterned vertical alignment, multiple vertical alignment, and continuous pinwheel alignment modes are suitable for DFD display application.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

Langmuir Blodgett films of arachidic acid and a nematic liquid crystal: Characterization and use in homeotropic alignment

We study the behavior of a mixed Langmuir monolayer consisting of a fatty acid and a nematic liquid crystal. We demonstrate that the mixed monolayer successfully transfers as a Langmuir-Blodgett film and characterize the transferred film using UV spectroscopy. An important application of Langmuir-Blodgett films is in the alignment of liquid crystals for electro-optical applications, such as displays. We show that including the liquid crystal in the Langmuir-Blodgett film produces homeotropic alignment for a system which fails to align by other standard techniques.     

Orientation of methyl red dye molecules in a polymer matrix

We have studied the optical anisotropy of waveguide layers formed upon introduction of Methyl Red azo dye molecules into a polycarbonate matrix. The observed anisotropy of absorption in the dyed layers is related to ordering of the azo dye molecules in the course of waveguide layer formation. The order parameter is estimated and the possible mechanism of orientation is proposed.     

Refractive index of nematic liquid crystals in the submillimeter wave region

Large electro-optic effects of liquid-crystal materials are attractive in applications to various optical devices in a wider wavelength region. Fundamental optical properties in the submillimeter wave region, such as refractive indices and transmission losses for some cyanobiphenyl nematic liquid crystals, have been investigated for the first time, to our knowledge, with a submillimeter laser. Refractive indices of the liquid crystal materials for ordinary and extraordinary rays are a little larger than those in the visible region, and a larger birefringence comparable with the visible region can also be obtained. Although the loss level is larger by ~2 orders of magnitude than that of quartz plate, which is an excellent window in the submillimeter wave region, the transmission of the liquid crystal cell is high enough.     

Feasibility of using liquid crystals for the development of molecularly selective fiber-optic chemical sensors

A new type of fiber-optic chemical sensor has been developed for the determination and differentiation of geometric isomers. The operation of the sensor is based upon the molecular-geometric-selective absorption of polynuclear aromatic hydrocarbons (PAH) on a liquid crystal. The selective interaction of a PAH with a liquid-crystal substrate causes quenching of the liquid-crystal fluorescence. Detection limits of such a device for PAH compounds approach 10(-10) mol/cm3; the sensor response time is about 2 min. Because the sensor is based upon physical absorption, it is reusable and reversible.     

液晶垂直取向膜材料的研究

叙述了近几年来液晶垂直取向材料领域的发展。对目前常见的几种具有应用前景的垂直取向技术分别给予了关注,同时初步探讨了其各自的取向机理,并展望了该技术在未来几年可能获得的发展。