Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

Liquid crystal material with gold nanoparticles as optical sensors active medium for the amino acids detection

Abstract

In the article, the possibility of using the cholesteric liquid crystal mixture doped by the gold nanoparticles as an active medium of optical amino acid sensors is shown. The BLO-62 and the 5CB were used as a cholesteric liquid crystal and a nematic liquid crystal correspondingly. This mixture was doped by the gold nanoparticles. The method of gold nanoparticles synthesis is considered and their parameters are determined. Studies on the spectral characteristics of the investigated materials have shown that the addition of gold nanoparticles to the system leads to a significant decreasing in the spectral characteristic steepness and give the possibility of expanding the measurement range of amino acid concentration.     

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.     

Tuning of a Dye Laser By a Liquid Crystal†

We describe how a cholesteric liquid crystal device can be used in a laser cavity to induce tuning of the laser line. Tuning of the order of 10Å is obtained by applying low voltages to the liquid crystals film. The problems, the advantages and the feasibility of the method is discussed, reporting several experimental data.     

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.     

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.     

Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.     

Light Scattering by Cholesteric Spherulites

Abstract

In this work, we analyze the scattering of light by the so-called spherulites or skyrmions in cholesteric liquid crystals. These are quasi-planar localized excitations of the director configuration. We compute the cross section of the polarization conversion for polarized incident light in the Born approximation, considering the anisotropic optical properties of the liquid crystal and the shape of the spherulite. We compare the results obtained by the analytical and the numerical skyrmion solutions found for several values of the model parameters. Finally, possible practical employments and theoretical extensions in the study of the phenomenon are foreseen.     

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.     

Experimental Solution of the Local Field Problem in Uniaxial Liquid Crystals†

A method of experimental determination of the Lorentz-factor tensor in uniaxial liquid crystals is suggested. The specific features of the local field tensor of the light wave in a nematic liquid crystal with low optical anisotropy have been investigated experimentally. Anisotropy of the local field and Lorentz-factor tensors has been found to decrease with the decreasing optical and molecular-optical anisotropies. These results appear to contradict the existing polarization theories of liquid crystals. The local field problem in cholesteric liquid crystals has been considered. A new approach to the local field problem in uniaxial liquid crystals with arbitrary optical anisotropy is developed and the experimental results are explained.     

Formation of the Liquid Crystal of a Surface-active Dye in Aqueous Methanol Solutions

The formation of the liquid crystal phase of a surface-active dye, p-octylphenol yellow amine poly(ethylene oxide), in aqueous methanol solutions has been examined by optical microscopy. Rodlike swarms appear at relatively low dye concentrations only slightly higher than the second critical micelle concentration. They develop into liquid crystal phase when the dye concentration is further increased. It takes some hours for the formation of a stripe-like texture characteristic of nematic liquid crystals, depending on the methanol content and dye concentration. The combination of the surface active-part and the azobenzene moiety would be responsible for the formation of the liquid crystal.     

Flexible Structures Based on a Short Pitch Cholesteric Liquid Crystals

We report on the realization and characterization of electro-responsive and pressure sensitive polydimethylsiloxane (PDMS) conductive photonic structures combined with the reconfigurable properties of short pitch cholesteric liquid crystals (aligned in Grandjean configuration). By combining ion-implantation process and surface chemistry functionalization, we have overcome the insulating properties of PDMS and induced long range organization of cholesteric liquid crystals, thus controlling both diffraction and selective Bragg reflection of light by means of external perturbations (electric field, pressure). We have characterized our devices in terms of morphological, optical and electro-optical properties.     

Optical Field Induced Scattering in Polymer Dispersed Liquid Crystal Films

Abstract

Polymer dispersed liquid crystals are composite materials consisting of inclusions of liquid crystalline materials dispersed in a polymer binder. If the refractive indices of the constituent liquid crystal and polymer are appropriately matched, then films of these materials may be switched between an optically scattering state and a non–scattering transparent state^1?2 by the application of electric fields which reorient the liquid crystal in the inclusions. In this paper we discuss the response of these materials to intense laser radiation, and examine the mechanisms associated with optical field induced reorientation.     

Influence of the Orientational Order on the Optical Properties of Cholesteric Liquid Crystals

An orientational order parameter for cholesteric liquid crystals is defined in analogy to nematic mesophases. The influence of this parameter on the selective reflexion and the optical rotatory dispersion is described. Its relative change with temperature is evaluated from ORD measurements for an induced cholesteric soIution and is found to be similar to that known for nematic liquid crystals.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.     

Liquid Crystal Light Valves with Organic Polymeric Photoconductors

Abstract

It has been suggested to use organic polymerle photoconductors in the liquid crystal light valves instead of widely used inorganic ones. Light valves with photoconductive sensibilized polyimides with various types of electro-optical effects in liquid crystals were constructed and tested. The influences of the read-on light regimes and control voltages were analysed. High resolution up to 1500 mm^?1 at diffraction efficiency 0.1%, and 40% at space frequencies up to 100 mm^?1 were obtained in pulse regimes. Thin holograms can be effectively read on such modulators in reversible mode in real time. The results obtained permit to come to a conclusion about prospects of the organic polymeric photoconductor usage in liquid crystal light valves and applications of the latter for input and output of the optical information.     

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.     

Polarisation‐independent liquid crystal devices

Liquid crystal (LC) devices can be used as amplitude modulators and phase modulators. Most LC devices are polarisation‐dependent and require at least one polariser. As a result, the optical efficiency is greatly reduced. In this paper, we review some of our recently developed polarisation‐independent LC devices. For amplitude modulation, we report two polariser‐free devices which combine light scattering with dye absorption: dye‐doped polymer‐dispersed liquid crystal (PDLC) and dye‐doped LC gels. For phase modulation, we also present two examples: residual phase type, e.g., voltage‐biased PDLC, Polymer‐Stabilised Cholesteric Texture (PSCT), and homeotropic LC gels; and double‐layered structure, such as thin polymer film‐separated orthogonal LC layers, and double‐layered LC gels. Potential applications of these polarisation‐independent LC devices for displays, laser beam steering and adaptive optics are emphasised.     

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.     

Laterally‐Graded SiGe Crystals for High Resolution Synchrotron Optics

A novel type of optical element for synchrotron radiation applications in the X‐ray region: namely laterally‐graded aperiodic crystals on the basis of Si_1‐xGe_x mixed crystal is presented. The lattice parameter of such gradient crystals containing up to some at. % Ge changes nearly linearly along the plane of diffraction. Thus the variation of the Bragg angle of divergent incident light on the crystal can be compensated. This opens up the possibility to operate a crystal monochromator in nearly crystal limited resolution in the whole energy range above 2 keV at the full vertical synchrotron source divergence without a collimating pre‐mirror. Simultaneously the reflected spectral intensity can be increased considerably as compared with a conventional Si‐crystal monochromator.