Reflectance,Transmittance and Color Gamut of Superimposed Layers of Cholesteric Liquid Crystals

The spectral reflectance curves of superimposed layers of cholesteric liquid crystals (CLC) having the same handedness add as there is little absorption. The peak reflectance of the sum cannot exceed 0.5 and the transmittance cannot be less than 0.5, which is the theoretical limit. The shape of the reflection and transmission band resembles rectangles and thus can be compared with the spectral curves of optimum colors. The chromaticity loci of CLC colors and its luminous reflectance equals 0.5 of the value of the luminous reflectance attached to the optimum colors. The color gamut thus produced can be shown to be greater than that which has been obtained with pigments.

A significant improvement can be obtained using superimposed layers of CLC having opposite handedness. Their spectral curves add and the peak reflectance is not limited by the value of 0.5 but approaches 1.0. Use of CLC's of opposite handedness is not always practical as they have different chemical and physical properties. A similar result can be obtained using two CLC layers of the same handedness and a half-wave retardation plate interposed between them. This plate converts the transmitted through the upper layer right-handed (left-handed) component into a left-handed (right-handed) one, which is then reflected from the lower layer and contributes to the reflected left-handed (right-handed) component from the upper layer. The chromaticity loci and color gamut of such superimposed CLC pairs of coatings now approaches that of optimum colors of the same luminous reflectance.     

Scientific Note

Abstract

Having presented a range of powerful FLC display prototypes (among them a 24′ monochrome, a 21′ colour and several different 15′ screens) Canon Inc. in Tokyo is now manufacturing their first commercial FLC product. It is a colour panel with 15′ (38 cm) diagonal with a resolution of 1240 × 1024 picture elements. Each such element (230 pm × 230 pm) can produce 16 different colours due to its subdivision in four parts. When writing a picture a large number of hues can be simulated (32000 or 26000 are stated for the two different versions marketed) by a graphic so-called error diffusion technique. In general this gives a very good rendition of colour images but in certain cases the differently coloured single dots, which can be seen when the observer is very close to the screen, may be disturbing. The origin of this inconvenience is of course the fact that each subpixel only has two states; it cannot produce a continuous grey scale.     

Influence of heat treatment on the colour of Au and Ag glasses produced by the sol-gel pathway

In this work, coloured glasses were produced based on the synthesis of gold and silver nanoparticles by the sol-gel process having in mind their application in art works. Gold and silver were used separately or as a mixture by varying the mole fractions in order to get a range of colours from yellow to red. The gold and silver nanoparticles were prepared by the reduction of tetrachloroauric acid and silver nitrate with sodium citrate in aqueous solutions which were further introduced in the sol-gel system. Attention was focused on the thermal treatment of the sol-gel samples. Different temperatures were used in order to determine their influence on the obtained colour. The glasses were characterized by UV-Vis absorption spectroscopy and the size of the nanoparticles was examined by transmission electron microscopy (TEM).The range of colours mentioned above is obtained either by preparing nanoparticles of each metal and mixing them or by preparing nanoparticles from solutions containing initially ions of both metals. In the former case, two surface plasmon resonance (SPR) bands were observed for temperatures below 200 °C while higher temperatures promote the formation of alloys between the Ag and Au nanoparticles. In the latter case, only one SPR band is observed and the nanoparticle size distribution is narrower. The results were explained by nanoparticle aggregation promoted by temperature. Glasses containing only Ag did not present the typical yellow colour above 300 °C but it was shown that the colour could be stabilized if Au was added in small amounts (Au/Ag molar ratio 0.1).     

Study of Cholesteric Textures in Phase Change Devices by Laser Light Diffraction

Photographic-recording and photomultiplier-recording laser light diffractometers have been constructed and used to study the cholesteric textures in phase change devices as a function of applied voltage and device parameters. The focal conic texture formed with increasing voltage gives diffraction peaks corresponding to 2π rotations of the director, whereas the texture formed by slow nucleation with decreasing voltage also gives “superlattice” lines corresponding to a 4π repeat, due to the different local structures of the textures combined possibly with tilting of the director. The repeat distances obtained by indexing the diffraction patterns agree well with microscopical and infra-red reflectance observations. Laser light diffraction appears to be a powerful technique for quantitative study of the non-uniform unwinding and re-forming of cholesteric twist in an electric field.     

Primary converters for optical sensors of physical values based on polymer dispersed cholesteric liquid crystal

The investigations of system based on cholesteric liquid crystal (CLC) sensors of physical quantities with an information transfer optical channel are carried out in the paper. The results of experimental investigations of spectral characteristics of polymer dispersed cholesteric liquid crystals (PDCLC) are represented. The application such material as primary transducer of a temperature sensor is proposed. The structure of an optical fiber temperature sensor with successive allocations of primary transducers and shifted areas of selective reflection band has been designed.     

Field induced Increase of Pitch in Planar Cholesteric Liquid Crystals

In cholesteric liquid crystals the application of an electric field parallel to the helix axis can change the wavelength of selective reflection. For decreasing pitch of the studied mixtures, the colour shift becomes more stable against the field-induced transformation of the initial Grandjean structure into a confocal texture. In compounds with a low crossover frequency, a field induced increase of the pitch was observed at the switching between low and high frequency fields. The investigations allow new conclusions on the origin of the colour shift.     

Passive Broadband Reflector: Elaboration and Spectral Properties

Abstract

We present passive broadening obtained by assembling two layers of cholesteric photo-polymerisable monomers having different pitches at the same temperature. A more or less large diffusion occurs between the two layers, which is dependent of various parameters such as degree of cross-linking of the two layers, temperature dependence, time evolution at a given temperature, thickness of the layers… The presented work consider the spectral properties of the sandwich structure and its correlation with the diffusion profile at the interface between the two layers. The band shape of the reflected and transmitted light were studied. Then, the reflected intensity shows a widening of the reflection associated with the existence of a pitch gradient inside the sample, which can be blocked by photo-polymerisation. At short time, the two peaks corresponding to each liquid crystal polymer are still present but the diffusion occurs at the interface. After a longer time, the broadening band still decreases and a single large peak is observed.     

3D Damage Analysis of Laser-irradiated Materials inside a Scanning Electron Microscope Using Ion Beam Slope Cutting Technique

In practical use of high temperature materials extreme thermal loads at materials surfaces often occur which can effect a non-reversible material damage or material failure consequently. These generated damages can be studied using an SEM which is coupled with a pulsed Nd:YAG high power laser. The ion beam slope cutting technique is a very suitable method to produce a highly localized cross section in laser irradiated material. By this the three-dimensional electron microscopic analysis of the damaged bulk material was possible. For the experiments a combination of ion beam slope cutting and conventional ion etching was used. It enables the high quality of microstructure characterization.     

Peculiarities of the phase behavior of liquid-crystal mixtures at the Cholesteric → Smectic (Ch → Sm) transition boundary

It is found that an interphase state, characterized by a large Bragg reflection coefficient, is formed in smectogenic cholesteric liquid-crystal mixtures at the Ch-Sm transition boundary. The interrelation between the mixture composition and the optical and spectral characteristics is studied. A possible structure of the interphase state is proposed.     

Alumina/zirconia composite coated by biomimetic method

In this study, a bioactive zirconia-toughened alumina (ZTA) composite was developed for orthopedic applications. This composite was obtained by slip casting of suspension powder mixtures.Biomimetic processes were used to grow a bone-like apatite layer on composite substrates using sodium silicate solution as a nucleating agent and simulated body fluids. The composites, with or without coating, were characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and scanning electron microscopy (SEM) with energy dispersion spectroscopy (EDS), and their apparent density was determined by the Archimedes method. The composites obtained by this process possessed the expected stiffness and dimensions and their density values were similar to those of the composite’s theoretical density (98.8%TD). The morphology of the hydroxyapatite formed on the composite surface was homogeneous and composed of small globules, characterizing a carbonated hydroxyapatite. The results of the tests indicated that the method employed to produce the composite and its coating was efficient under the conditions of this study.     

The Effect of Rhodamine 6G on Phase Transitions in Cholesteryl Ester Mixtures

Compensated cholesteric mixtures were the subject of a whole range of studies.^1-6 Due to their characteristic properties (the existence of a temperature at which compensation occurs and the dependence of the critical field on temperature), the orientation of guest molecules by compensated cholesteric hosts⁷ shows some particular features in comparison to the orienting mechanisms acting within nematic liquid crystals^8-15 or within chiral nematic mixture.^16,17 When the colour of sample varies due to the influence of the electric field and temperature the guest dye contributes to it as well the compensated cholesteric host.

In this paper we report the modification of compensated cholesteric mixture properties due to the introduction of Rhodamine 6G as guest.     

固相合成Gd2O2S:Pr陶瓷闪烁体粉末

本文采用固相反应合成Gd2O2S：Pr陶瓷闪烁体粉末,确定了固相反应时Gd2O3与S的配比分别约为Gd2O3与S总质量的88％和12％,并往其中成功地掺入了痕量Pr2O3,测量了GOS：Pr的荧光光谱,分析表明,制备出的GOS：Pr粉末的主发射峰位于512nm处,与硅光电二极管光谱感光度匹配。此法制备出的GOS：Pr粉末适于制造GOS：Pr闪烁陶瓷。     

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.     

Chemical trend observed in anisotropic surface reflectance spectra of MOVPE by surface photoabsorption

This paper investigates the origin of the surface reflectance spectrum for the group-V-stabilized III–V surface during MOVPE by using surface photoabsorption. A chemical shift is observed for the stoichiometry sensitive peak in the anisotropic spectra of arsenides and phosphides. The peaks observed in the phosphides are located at higher energies than the arsenides, besides the peak in each compound shows a red-shift as the lattice constant increases. To investigate the possibility of the critical point of the bulk energy state appearing in the reflectance spectrum induced by surface modification, the anisotropic spectrum during InAs-on-GaAs heteroepitaxy are measured. One monolayer InAs growth on GaAs results in a drastic change that a peak sign is reversed, accompanied by a red-shift. This can be interpreted by the optical transition change corresponding to the surface conversion from a two-As-layer c(4 × 4)-like surface in GaAs to a one-As-dimer layer having a bond axis perpendicular to the c(4 × 4) As dimer. The contribution of the GaAs bulk electronic state in the reflectance spectrum is not observed. These results support the model that the anisotropic peak originates from an optical transition of the group-V dimer. The anisotropic spectrum measurement also makes it possible to monitor the P/As surface exchange and the As-atom segregation during the InP-on-InAs heteroepitaxy.     

The Observation of Hydrodynamic Instability in a Cholesteric Liquid Crystal Layer Having a Free Surface

It was for the first time that a hydrodynamic instability has been experimentally observed in a layer of a cholesteric liquid crystal (CLC) having a free surface and being locally heated from below. Steady structures of CLC motion have been obtained in the form of rotating (in the place) spirals consisting of distinct rolls with sharp boundaries; motion in every such roll was note also. It is supposed that the convective instability observed is due to Marangoni effect.     

Controlled light emission from dye-impregnated porous silicon microcavities

The properties of porous silicon Fabry–Perot microcavities impregnated with oxazine, a fluorescent dye, are investigated by photoluminescence and reflectance spectroscopy. The emission spectrum of the dye is noticeably modified by the cavity structures, being both the peak intensity increased and the linewidth narrowed, while the peak position coincides with that measured by UV–Vis reflectance. The results suggest that is possible, by simple methods, to prepare optically active hybrid materials integrating the properties of the inorganic host structure and of the organic guest.     

A Polyester Forming a Thermotropic Cholesteric Phase

Mesomorphic polyesters were synthesized from 4, 4′-dihydroxy-α-methylstilbene and adipic acid (P-6) or (+)-3-methyl adipic acid (P-6M). P-6 forms a thermotropic nematic phase and P6-M a thermotropic cholesteric phase. The nematic phase of P-6 could easily be identified by optical microscopy. For both polymers we observed a biphasic region in which the isotropic and liquid crystalline phases coexist. Bright colors were obtained by increasing the pitch of P6-M by admixture with either a low molecular weight nematogen or with polymer P-6, and also by synthesizing a copolymer containing the two dibasic acids. The copolymeric cholesteric phase, which is stable between 199 and 282[ddot]C, had a predominately planar texture, and these features could be retained in the solid state by quenching to produce a film having a deep blue color at room temperature. The role of the degree of polymerization upon the development of organization, and parameters, of the mesophase is discussed.     

Topological Soft Matter for Optics and Photonics

Liquid crystal colloids are interesting for a variety of mechanisms—including self-assembly, optical-tweezers assisted assembly, topology, and material flow—that can be used to create various complex optical and photonic structures. Here, we present a brief overview of liquid crystal colloidal structures, as recently achieved by numerical modeling and experiments. Central to the structures are complex conformations of topological defects, as they can bind, stabilize, or distort the structure. Using topological and geometrical arguments, we show that the defects can be controllably rewired and imprinted, for example by using optical tweezers. We show that 3D colloidal crystals can be assembled from elastic dipoles of spherical beads in nematic liquid crystals or via inherently inhomogeneous order profiles in bulk and confined cholesteric blue phases. Colloidal crystals are generalized to close-packed colloidal lattices, which we show can serve as natural templates for defect networks. Finally, photonic bands are calculated for selected structures and possible defects in the structure are discussed.     

Electroabsorption studies on amorphous silicon

The electroabsorption spectra of plasma deposited and chemical vapor deposited amorphous silicon films have been studied in comparison with their absorption. In all cases a peak with 350–400 meV halfwidth is observed similar to the results on chalcogenide glasses. The peak occurs at moderate values of the absorption constant near 10⁴ cm^?1 and follows the shift of the absorption edge with variation of the deposition temperature. No response has been obtained in the low absorption region below 1.5 eV. The field modulated reflectance which has been calculated from the electroabsorption data is about two orders of magnitude smaller than the electroabsorption signal. The electroabsorption spectrum is interpreted as resulting from the influence of the external field on the localized states only.