Optics of Twisted Nematic Liquid Crystal Layers

Calculations have been made of the optical properties of various liquid crystal displays (LCDs) based on twisted nematic liquid crystal layers. It is shown that the steepness of the electro-optical characteristics of TN LCDs can be reduced and consequently the gray-scale capability can be improved by decreasing the twist angle. The twist reduction permits the realization of displays which, due to their outstanding gray-scale capability and low inherent coloration, are particularly suited for active matrix TV-applications. In addition to calculations made on displays based on weakly twisted layers, a comparison is made of the optical properties of displays with a high multiplex capability, which utilize highly twisted nematic liquid crystal layers. It is shown that the brightness of the OMI LCD can be improved without deterioration of the weak inherent coloration. This renders OMI LCDs particularly well suited for applications as time-multiplexed, full colour, high information content panels.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Dependence of the Electrically Controlled Birefringence on Applied Frequency in a Nematic Liquid Crystal

Detailed studies on the dependence of the ECB effect on the applied frequency have been made in a nematic liquid crystal MBBA. It is found experimentally that, when the applied voltage is just above the threshold voltage, there are strong dependences of both the delay time and the mean deformation angle of the molecular director subjected to a constant voltage setup on the applied frequency. Those applied frequency dependences of the ECB effect can be interpreted in terms of the dielectric alignment in an electrically conducting nematic liquid crystal. We have measured the capacitance of the deformed liquid crystal cell varying the applied frequency. The experimental results are compared with the Gruler and Cheung theoretical prediction, and good agreements are obtained between them when the applied voltage is just above the threshold voltage.     

利用水平沉积法制作PS微球二元胶体薄膜及其它复杂结构

本文采用水平沉积方法,制作出各种胶体晶体薄膜.以聚苯乙烯微球为构建单元,制作了多种二元胶体晶体和非单一平面的胶体晶体.为实现太赫兹光子晶体波导,还采用这一方法将线缺陷植入胶体晶体.结果显示通过该方法可以获得很好的有序结构,说明水平沉积方法可以广泛于各种胶体晶体制作.     

二维异质结光子晶体弯曲波导的传输特性研究

应用时域有限差分(FDTD)方法,通过数值模拟研究光在二维异质结光子晶体L型弯曲波导的传输特性.结果表明:在异质结光子晶体波导中能更好地对光进行控制;并且异质结光子晶体L型弯曲波导的传输效率较同质结光子晶体波导提高了6;~7;.计算结果为高度集成化的光子晶体波导器件,提供了一条新的设计思路和依据.     

Growing of bulk crystals and structuring waveguides of fluoride materials for laser applications

We report on recent progress in the synthesis, the crystal growth and the epitaxial growth of fluoride and other laser materials. Results on the fabrication of single crystalline waveguides for dielectric down - and upconversion lasers pumped by semiconductor diode lasers are summarized. Epitaxial growth (molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), pulsed laser deposition (PLD)) and surface modifying techniques (high energy ion implantation, ion diffusion) have been applied in several laboratories. Progress in techniques fabricating optical waveguides from glassy media is addressed as well. Particular emphasis is given on the structuring (wet etching, chemical polishing, ion beam etching) of fluoride crystals for the purpose of obtaining 2-D and 1-D optical waveguides. Results on the structuring of LiYF₄ by wet and ion beam etching are reported. With respect to laser action, the generation of short wavelength light by upconversion (UC) processes, stimulated Raman scattering (SRS) and second harmonic generation (SHG) is discussed. Reports on the first crystalline waveguide lasers of fluoride crystals LiYF₄ and LaF₃, both doped with neodymium, are presented.     

Analyses of Crystal Shape Monitoring of LEC-Grown InP Crystals by using a Disc Approximation Approach

This paper analyzes crystal shape monitoring for automatic diameter control (ADC) during the liquid encapsulated Czochralski (LEC) growth of InP crystals. The crystal diameter (shape) is monitored numerically (diagrammatically) by using a disc approximation approach based on precise weight and pulling length measurements. The error in the diameter calculation based on the approximation is estimated to be sufficiently small for practical use. The monitoring accuracy was investigated for crystal bodies with nearly flat growth interfaces, and for their shoulder portions with largely convex growth interfaces. For the straight body portions, the accuracy depended on diameter, d, and improved from ±15% for d=10 mm to ±3% for d=50mm. For the shoulder portions, the diameter was monitored with nearly the same accuracy. This method has therefore made it possible to monitor the growing crystal visually in real time, and was applied to the growth of <111> InP crystals with a cone angle of less than 39° and a smooth appearance to avoid twinning.     

Acid mediated tunability of stimulated laser emission from dye doped chiral microdroplets

Abstract

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60?nm. This effect could find applications for the development of spectroscopy based sensors.     

Oriented action of some solid substrates to liquid crystal layer

The orienting action of three types of layers on liquid crystal is investigated. Homogeneous orientation of the liquid crystal layer is obtained under the action of a transparent conductive SnO₂ film and carbon film. Such an orientation is achieved with gelatin films as well. Homeotropic orientation of the liquid crystal can be also obtained by exposing the latter to ion bombardment with Ar ions.     

Advanced electro-optic properties of nematic liquid crystal doped with high birefringence liquid crystal molecules

The study of fast response nematic liquid crystal display material mixture and synthesis about compounds has always been an important subject in recent years. In this article, one type of high birefringence liquid crystal based on phenyl-diacetylenes was synthesized by sonogashira coupling reaction. A research is conducted on the effect of electro-optic properties of a widely used nematic liquid crystal mixture doped with the prepared high birefringence liquid crystal molecule which concentrations ranging from 1% to 5 wt%. While the results show that the steepness of electro-optic curves first increase and then decrease with the prepared high birefringence liquid crystal molecule content increasing, when the content of high birefringence liquid crystal molecule reaches 3%, electro-optic properties of the mixture liquid crystals are excellent.     

Sapphire waveguides and fibers for terahertz applications

Sapphire shaped crystals are considered as a favorable material platform of the terahertz (THz) waveguide and fiber optics. Unique physical properties of sapphire, along with advantages of the Edge-defined Film-fed Growth (EFG) technique, yield fabrication of the THz waveguides and fibers with a complex cross-section geometry directly from the Al₂O₃-melt, where no labour-intensive mechanical processing is required. Wide variability of the as-grown sapphire shaped crystal geometries yields different physical mechanisms of electromagnetic waveguidance. In this review, recent advantages in the THz waveguides and fibers based on the EFG-grown sapphire shaped crystals are discussed. While possessing moderate THz-wave absorbtion and quite high dispersion, flexible sapphire fibers with a simple step-index cross-section geometry yield strong confinement of guided modes in a fiber core due to a high refractive index of sapphire in the THz range. This effect opens novel opportunities of sapphire fibers in high-resolution THz imaging, using the principles of either scanning-probe near-field optical microscopy or optical fiber bundles. In turn, antiresonant and photonic crystal hard hollow-core waveguides demonstrate advanced optical performance, along with wide capabilities in THz endoscopy and sensing in harsh environments. This review highlights that the EFG-grown sapphire shaped crystals hold strong potential in different branches of THz optics.     

Liquid Crystals and Polymer-Based Photonic Crystal Fibers

Experimental results of polymer photonic liquid crystal fibers based on commercially available (Kiriama) PMMA and cyclo-olefin polymer (Zeonex 480R) microstructured polymer fibers infiltrated with nematic liquid crystals (2CHBT/8CHBT and PCB) are presented and thermally-tuned photonic band-gap propagation mechanism is observed. These preliminary results suggest, that polymers binding to liquid crystals much easier than silica, can offer new opportunities while using polymer-based photonic crystal fibers.     

Compensation for the spectral dispersion of light polarization states in electro-optic modulators based on chiral liquid crystals

The effect of compensation for the spectral dispersion of light polarization states at the output of a single-domain layer of a chiral liquid crystal (CLC) is experimentally studied. It is shown that such dispersion can be decreased significantly with the aid of phase plates of two types that have different signs of the spectral dispersion of birefringence. The dispersion compensation allows one to significantly increase the operating spectral range of fast light modulators based on chiral nematic liquid crystals (NLCs).     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Nanostructures on the Basis of Porous Alumina with Intercalated with Cholesteric Liquid Crystal

The introduction of liquid crystal materials into the porous material host will help us to get optically active nanocomposites. Nanocomposite based on aluminum oxide host with cholesteric liquid crystal is characterized by a shift of bandwidth wavelength minimum. In case of intercalation of cholesteric liquid crystals into the pores of aluminum oxide host the shift of transmittance minimum into the short wave region are observed. We analyzed the ways, which indicate the deformation of the pitch of the cholesteric helix and showed the role of capillary forces in the deformation of the pitch of cholesteric liquid crystal.     

Captured Orientational Order in Polymer Network Assemblies

Abstract

The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.     

Synthesis and Mesomorphic Properties of New Columnar Liquid Crystal Containing 1,3,5-triiminebenzene with Pendant 1,3,4-thiadiazole Group

A novel discotic liquid crystal series based on 1,3,5-benzenetrisazomethine derivatives with three pendant 2-amino-5-(4′-n-alkoxy)phenyl-1,3,4-thiadiazole has been synthesized, which is the first columnar molecules containing 1,3,4-thiadiazole moiety exhibiting a discotic liquid crystal. The molecular structure of compounds was confirmed by FT-IR, ¹H-NMR, and mass spectroscopy and elemental analysis. The electron excitation properties of these compounds were investigated by UV-vis absorption spectroscopy. Their liquid crystalline properties were studied by polarizing optical microscopy and differential scanning calorimetry. The formation of a columnar mesophase was found to be dependent on the number of methylene unit in alkoxy side chains.     

Tunable Optofluidic Polymer Photonic Liquid Crystal Fibers

New trends are presented in optofluidics based on microstructured photonic crystal fibers filled with liquid crystals. It significantly enhanced optical properties of the fibers and introduced new levels of tunability to photonic crystal fibers. The paper discusses basic research directions in fiber-based optofluidics and, in particular, in polycarbonate-based photonic liquid crystal fibers. Experimental and theoretical results are presented on polymer crystal fibers filled with nematic liquid crystal with two different types of dielectric anisotropies.     

Two Frequency Addressing of a DTN-Cell

The authors have already reported that a DTN-cell (depolarization in a twisted nematic-cell) had several advantages to a matrix display such as a sharp threshold and wide viewing angle. But if the usual amplitude selection method is applied to a large scale matrix display using the DTN-mode, multiplex capability is limited by the cutoff frequency F_c, of a liquid crystal. Therefore, the authors investigated a two frequency addressing method, that is, simultaneous application of a constant low-frequency voltage and a variable high-frequency voltage. In this method, a liquid crystal with lower-F_c can be used. In addition, some advantages of extremely sharp threshold, high contrast and relatively fast response and recovery can be obtained by using a liquid crystal with large negative dielectric anisotropy. These advantages are useful for a large scale matrix display.     

X-ray Measurements of the Bulk Modulus of the Myofilament Liquid Crystal in Striated Muscle

Low-angle X-ray diffraction measurements of the distance between myosin filaments comprising the smectic B₁ liquid crystal found in skinned crayfish striated muscle were made. The interfilament distanceas a function of applied compressive osmotic pressure wasdetermined and the bulk modulus of this myofilament liquid crystal ascertained. Under compression the behavior of this myosin filament lattice is shown to be predicted well by the Murnaghan equation of state. The isothermal bulk modulus, B _o, of this myosin filament liquid crystal is 1.51 × 10⁵ dynes/cm² at 2°C and at 46% actin overlap.