Tunable lasing from a cholesteric liquid crystal film embedded with a liquid crystal nanopore network

Continuous tuning of lasing wavelength is achieved in cholesteric liquid crystal lasers by embedding a network of nanopores with an average size of 10 nm filled with liquid crystals inside a polymerized matrix with helical order. The device possesses both high transparency and a fast response time because the tuning is driven by local reorientation of the liquid crystal molecules in the nanopores.     

VA-TFT用负介电各向异性液晶材料研究进展

综述了VA-TFT(垂直取向薄膜晶体管驱动)用负介电各向异性液晶的结构及性质.重点介绍了近年来开发出的一些具有大的介电各向异性、低黏度、高清亮点的新型液晶,如侧向氟取代萘环、茚环类,轴向氟代环己烷类,含多氟取代类液晶等.     

Lyotropic liquid crystal directed synthesis of nanostructured materials

Abstract

This review introduces and summarizes lyotropic liquid crystal (LLC) directed syntheses of nanostructured materials consisting of porous nanostructures and zero-dimensional (0-D), one-dimensional (1-D) and two-dimensional (2-D) nanostructures. After a brief introduction to the liquid crystals, the LLCs used to prepare mesoporous materials are discussed; in particular, recent advances in controlling mesostructures are summarized. The LLC templates directing the syntheses of nanoparticles, nanorods, nanowires and nanoplates are also presented. Finally, future development in this field is discussed.     

Lyotropic liquid crystal directed synthesis of nanostructured materials

This review introduces and summarizes lyotropic liquid crystal (LLC) directed syntheses of nanostructured materials consisting of porous nanostructures and zero-dimensional (0-D), one-dimensional (1-D) and two-dimensional (2-D) nanostructures. After a brief introduction to the liquid crystals, the LLCs used to prepare mesoporous materials are discussed; in particular, recent advances in controlling mesostructures are summarized. The LLC templates directing the syntheses of nanoparticles, nanorods, nanowires and nanoplates are also presented. Finally, future development in this field is discussed.     

Tunable distributed feedback lasing from leaky waveguides based on gel-glass dispersed liquid crystal thin films

Different amount of nematic liquid crystal (LC) 4-Cyano-4′-pentylbiphenyl(5CB) was encapsulated in sol–gel derived organically modified silica(ORMOSIL) matrices to fabricate dye doped gel-glass dispersed liquid crystal (GDLC) thin films by co-hydrolyzation/condensation of ethyl triethoxisilane (ETES) and tetraethyl orthosilicate (TEOS) precursors. Distributed feedback (DFB) leaky waveguide lasers from GDLC thin films using a dynamic grating were demonstrated. Tunable laser action from 558 to 570 nm was obtained in the sample with lowest LC concentration of 0.375 mol%. The DFB laser performance of GDLC thin films with various LC concentrations was investigated using the same pumping geometry, and it was shown that the encapsulation of 5CB also led to the red shift of lasing peak.     

Unidirectional single-mode lasing on Nd-doped GSGG crystal

The lasing characteristics of a Cr,Nd:GSGG solid state ring laser with different types of monochromatic pumping have been investigated. Unidirectional, single frequency lasing of the ring nonplanar laser was obtained. A high stable automodulation regime of lasing with a high sensitivity to the magnetic field was realized.     

Liquid crystal alignment in electro-responsive nanostructured thermosetting materials based on block copolymer dispersed liquid crystal

Novel well-defined nanostructured thermosetting systems were prepared by modification of a diglicydylether of bisphenol-A epoxy resin (DGEBA) with 10 or 15?wt% amphiphilic poly(styrene-b-ethylene oxide) block copolymer (PSEO) and 30 or 40?wt% low molecular weight liquid crystal 4'-(hexyl)-4-biphenyl-carbonitrile (HBC) using m-xylylenediamine (MXDA) as a curing agent. The competition between well-defined nanostructured materials and the ability for alignment of the liquid crystal phase in the materials obtained has been studied by atomic and electrostatic force microscopy, AFM and EFM, respectively. Based on our knowledge, this is the first time that addition of an adequate amount (10?wt%) of a block copolymer to 40?wt% HBC-(DGEBA/MXDA) leads to a well-organized nanostructured thermosetting system (between a hexagonal and worm-like ordered structure), which is also electro-responsive with high rate contrast. This behavior was confirmed using electrostatic force microscopy (EFM), by means of the response of the HBC liquid crystal phase to the voltage applied to the EFM tip. In contrast, though materials containing 15?wt% PSEO and 30?wt% HBC also form a well-defined nanostructured thermosetting system, they do not show such a high contrast between the uncharged and charged surface.     

Morphological control and polarization switching in polymer dispersed liquid crystal materials and devices

Liquid crystals dispersed in polymer systems constitute novel class of optical materials. The precise control of the liquid crystal droplet morphology in the polymer matrix is essentially required to meet the prerequisites of display device. Experiments have been carried out to investigate and identify the material properties and processing conditions required for the precise control of the droplet morphology of the dispersed liquid crystal systems. Polarization switching has been studied. Aligned liquid crystal dispersed systems showed higher polarization over unaligned ones.     

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.     

Electrooptical properties and microstructure of polymer-dispersed liquid crystal doped with various reinforcing materials

Different types of reinforcing material, such as hydrophilic silica (Aerosil 200), (1-propylmethacrylate)-heptaisobutyl-substituted PSS (POSS-1), octavinyl-substituted PSS (POSS-8), octamethyl-substituted PSS (POSS-octa), and poly(dimethylsiloxane) (PDMS) were incorporated into a conventional polymer-dispersed liquid crystal (PDLC) system to enhance electrooptical properties by increasing phase separation, resulting from increasing the gel content and decreasing the viscosity of the mixture. The mixtures with POSS-1, POSS-octa, and Aerosil 200 show lower viscosity than the neat mixture, caused by the weak interaction of monomer molecules because of inserting these particles into the monomer chains, whereas the mixtures with POSS-8 and PDMS show an increase in viscosity. The PDLC film with POSS-1 represents the lowest off-transmittance value because the gel content is above 94% and the droplet size of the LC is optimal. However, when the gel content is decreased, the droplet size of the LC in the film becomes large because of unreactive monomer flowing into the LC, giving rise to the increase in off-transmittance value.     

Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal

The demand for compact ultraviolet laser devices is increasing, as they are essential in applications such as optical storage, photocatalysis, sterilization, ophthalmic surgery and nanosurgery. Many researchers are devoting considerable effort to finding materials with larger bandgaps than that of GaN. Here we show that hexagonal boron nitride (hBN) is a promising material for such laser devices because it has a direct bandgap in the ultraviolet region. We obtained a pure hBN single crystal under high-pressure and high-temperature conditions, which shows a dominant luminescence peak and a series of s-like exciton absorption bands around 215 nm, proving it to be a direct-bandgap material. Evidence for room-temperature ultraviolet lasing at 215 nm by accelerated electron excitation is provided by the enhancement and narrowing of the longitudinal mode, threshold behaviour of the excitation current dependence of the emission intensity, and a far-field pattern of the transverse mode.     

Sidechain liquid crystal polymers

Research has continued in the field sidechain liquid crystal polymers over recent years, but it is becoming clearer that this research is being directed away from the traditional technology areas of electro-optic devices and researchers are developing new and exciting applications for this novel state of matter.     

Rotatable liquid crystal waveplate

A photoresist well of size 60 × 60 × 15 μm³ has been used to confine a droplet of nematic liquid crystal to create a rotatable waveplate. The optical texture of the droplet between crossed polarisers is consistent with the nematic n-director running substantially along a diameter of the droplet and connecting two nematic defects on the curved edges of the droplet. Electric field induced azimuthal rotation of the axis of the nematic liquid crystal droplet has been demonstrated. At higher temperatures, 30 °C and above, the droplet is more circular in shape and can be switched to arbitrary rotation angles. At lower temperatures, 25 °C and below, the sides of the droplet are straightened by the interaction with the well walls and the switching tends to favour discrete orientations of the optic axis. The shape of the time–voltage switching response curve for rotation by an angle of 40° also depends on the temperature of the droplet. A switching time that is inversely proportional to the voltage squared results when the droplet is nearest to circular in shape.     

Graphene-based liquid crystal device

Graphene is only one atom thick, optically transparent, chemically inert, and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter, we demonstrate liquid crystal devices with electrodes made of graphene that show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally used metal oxides in terms of low resistivity, high transparency and chemical stability.     

New directions in liquid crystal science

While we are all familiar with liquid crystal displays, an industry currently worth more than US dollars 60 billion yr(-1) and growing rapidly, fewer people are aware of the breadth of the subject of liquid crystals--one that represents the study of the fourth state of matter. Liquid crystals are found as essential elements in biological systems, soaps and detergents, sensor technologies and in the manipulation of electromagnetic radiation of various wavelengths. This meeting was designed to highlight both the truly multidisciplinary nature of liquid crystal science and to feature those areas away from electro-optic displays; these issues are developed and summarized in more detail.     

Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.     

Polymer-stabilized liquid crystal blue phases

Blue phases are types of liquid crystal phases that appear in a temperature range between a chiral nematic phase and an isotropic liquid phase. Because blue phases have a three-dimensional cubic structure with lattice periods of several hundred nanometres, they exhibit selective Bragg reflections in the range of visible light corresponding to the cubic lattice. From the viewpoint of applications, although blue phases are of interest for fast light modulators or tunable photonic crystals, the very narrow temperature range, usually less than a few kelvin, within which blue phases exist has always been a problem. Here we show the stabilization of blue phases over a temperature range of more than 60 K including room temperature (260-326 K). Furthermore, we demonstrate an electro-optical switching with a response time of the order of 10(-4) s for the stabilized blue phases at room temperature.     

Substrate-induced crystal plastic phase of a discotic liquid crystal

A new phase of a known discotic liquid crystal is observed at the interface with a rigid substrate. The structure of the substrate-induced phase has been characterized by atomic force microscopy, specular X-ray diffraction, and small-angle and wide-angle grazing incidence X-ray diffraction. The substrate-induced phase, which has a thickness of ～30 nm and a tetragonal symmetry, differs notably from the bulk phase. The occurrence of such phase casts a new light on alignment of discotic liquid crystals.