Doped Liquid Crystal Research Articles

Ordering Behavior and Linear Dichroism of Pure and Doped Liquid Crystals

In the present paper, C7 liquid crystals' refractive indices have been measured throughout the nematic and isotropic phase, in order to determine the order parameter by using Vuks method and characterization of anisotropic behaviors. The temperature dependence of refractive indices, birefringence and order parameters in the anisotropic phase, has been investigated for this liquid crystal. Furthermore, the dichroic ratios of doped C7 by linear azo dye, obtained on the basis of polarized absorption measurements. Then, the orientational ordering of the doped liquid crystal is analyzed by determining the order parameters, in different dye concentrations. The ordering behavior of a doped liquid crystal mixture is studied. It is found that even in very low compositional percentage of the dyes, the obtained order parameters are strongly affected by increasing the dye concentration.

Fast Response Beam Coupling in Dye Doped Liquid Crystal Cells Sandwiched with ZnSe Coated Substrates

ABSTRACTIn this paper we present fast response processes in dye doped liquid crystal cell with ZnSe layers on electrodes. Two-wave mixing and grating monitoring experiments were performed to characterize this liquid crystal system. With the help of photoconductive layers, the response time was reduced to a few milliseconds, which improved further the response rate in liquid crystal photorefractive system. A peak in response time versus applied voltage and unexpected oscillations in diffraction efficiency dynamics were observed, which implies unconventionally complex charge dynamics associated with SPPs excited at one of the photoconductive layer/liquid crystal interfaces.

Influence of methyl red as a dopant on the electrical properties and device performance of liquid crystals.

The ionic effect in nematic liquid-crystal (LC) cells containing the azo dye methyl red was investigated by means of dielectric spectroscopy, measurements of voltage holding ratio (VHR) and ultraviolet/visible absorption spectroscopy. The experimental results indicated that incorporating a minute amount of the methyl red (< 0.03 wt%) in the LC host leads to the suppression of the ionic effect caused by impurity ions. Practically, the doped LC cells with a dye content of 0.02 wt% showed improved VHR and promoted lifetime by 15% and 180%, respectively, in virtually no expense of the optical transmittance.

Control over nanostructures and associated mesomorphic properties of doped self-assembled triarylamine liquid crystals.

We have synthesized a series of triarylamine-cored molecules equipped with an adjacent amide moiety and dendritic peripheral tails in a variety of modes. We show by (1) H NMR and UV/Vis spectroscopy that their supramolecular self-assembly can be promoted in solution upon light stimulation and radical initiation. In addition, we have probed their molecular arrangements and mesomorphic properties in the bulk by integrated studies on their film state by using differential scanning calorimetry (DSC), variable-temperature polarizing optical microscopy (VT-POM), variable-temperature X-ray diffraction (VT-XRD), and atomic force microscopy (AFM). Differences in the number and the disposition of the peripheral tails significantly affect their mesomorphic properties associated with their lamellar- or columnar-packed nanostructures, which are based on segregated stacks of the triphenylamine cores and the lipophilic/lipophobic periphery. Such structural tuning is of interest for implementation of these soft self-assemblies as electroactive materials from solution to mesophases.

Optical induction of Bessel-like lattices in methyl-red doped liquid crystal cells

The optical induction of annular photonic lattices by a traveling Bessel beam has been investigated in Methyl-red (MR) doped nematic liquid crystal (LC). Non-diffracting Bessel beams were formed by an axicon. The induced Bessel-like lattice had a ~15µm period in the radial direction. The lattice was tested by measuring the forward diffracted power of the recording Bessel beam. The dependency on the angle between the polarization of the laser beam and the director of the LC and on the axial position of the LC cell had been investigated. A diffraction efficiency of 14% had been obtained. Investigations have been performed for different MR dye doping concentrations. An erasure time of the lattice of 60s has been determined by a 532nm probe Gaussian beam of 2mW in a LC cell with MR dye concentration of 1.15 wt%. The induced periodically varying refractive index in the LC medium is analogous to microstructured fibers and allows the study of light localization and soliton behavior in highly nonlinear waveguide arrays.

The effect of Sudan dyes concentration in the linear dichroism of the nematic liquid crystals

In this work the absorption spectrum of the azo-dye doped nematic liquid crystals in the visible region of parallel aligned samples were investigated. Dichroic ratio R and order parameter S of the samples are calculated and the effect of concentration on Dichroic ratio R and order parameter S is studied. The results indicate that Dichroic ratio R and order parameter S increase with the increase in dye concentration.

Influence of particle size on the ion effect of TiO2 nanoparticle doped nematic liquid crystal cell

In this study, the transient currents of nematic liquid crystal cells doped with different sizes of titanium dioxide (TiO2) nanoparticles were measured. The experimental results illustrate that doping TiO2 nanoparticles into nematic liquid crystals leads to a reduction in the moving-ion density and the amount of reduced impurity ions is related to the size of doped TiO2 nanoparticles. Under the same doping concentration, TiO2 nanoparticles with a smaller average particle size are more effective in trapping the impurity ions. The measurements of voltage–transmittance curves and time evolution characteristics of the doped liquid crystal cells further confirm this phenomenon. This study suggests that TiO2 nanoparticles with a smaller size are better candidates as nanodopants for liquid crystal display applications.

Molecular Alignment Structure of DNA Doped Liquid Crystals

We focus on liquid crystals (LCs) doped with deoxyribonucleic acid (DNA). We have researched the effect of the single strand DNA doping on the LC molecular alignment structure in terms of the kind of base (adenine, thymine, guanine, and cytosine), the concentration, and the molecular weight of DNA. As a result, it is found that the DNA doping can induce a twist deformation of LC alignment. Furthermore, the twisted structure strongly depends on the kind of DNA base.

Nonlinear and quantum optics with liquid crystals

Thermotropic liquid crystals' usual application is display technology. This paper describes experiments on light interaction with pure and doped liquid crystals under for these materials unconventional incident light powers: (1) under high-power laser irradiation, and (2) at the single-photon level. In (1), I will outline several nonlinear optical effects under high-power, nanosecond laser irradiation which should be taken into account in the design of lasers with liquid crystal components and in fabrication of optical power limiters based on liquid crystals: (1.1) athermal helical pitch dilation and unwinding of cholesteric mirrors (both in free space and inside laser resonators); (1.2) some pitfalls in measurements of refractive nonlinearity using z-scan technique under two-photon or linear absorption of liquids; (1.3) the first observation of thermal lens effects in liquid crystals under several-nanosecond, low-pulse-repetition rate (2-10 Hz) laser irradiation in the presence of two-photon absorption; (1.4) feedback-free kaleidoscope of patterns (hexagons, stripes, etc.) in dye-doped liquid crystals. In (2), at the single-photon level, it will be shown that with a proper selection of liquid crystals and a single-emitter dopant spectral range, liquid crystal structures can be used to control emitted single photons (both polarization and count rate). The application of the latter research is absolutely secure quantum communication with polarization coding of information. In particular, in (2.1), definite handedness, circular polarized cholesteric microcavity resonance in quantum dot fluorescence is reported. In (2.2), definite linear polarization of single (antibunched) photons from single-dye-molecules in planar-aligned nematic host is discussed. In (2.3), some results on photon antibunching from NV-color center in nanodiamond in liquid crystal host and circularly polarized fluorescence of definite handedness from nanocrystals doped with trivalent ions of rare-earths dispersed in liquid crystal host are presented.

High diffraction efficiency in permanent optical memories based on Methyl Red doped liquid crystal

This paper proposes a method for enhancing diffraction efficiency in an existing grating by re-illumination of the sample with a secondary pump beam. Though this single-armed pump beam is typically used to erase the grating, in specific conditions, as in our case for Methyl Red doped liquid crystal, it increases the diffraction efficiency. Describing physical background of this enhancement based on the molecules reorientation, high diffraction efficiencies (around 11.5 %) are reported which are not obtainable during the grating formation, even after long exposure times. Concerning the permanency of the obtained efficiency, this system may find various applications in read-only optical data storage devices.

Nonvolatile memory effects in an orthoconic smectic liquid crystal mixture doped with polymer-capped gold nanoparticles.

Promising applications of liquid crystal nanocomposites have driven extensive efforts to achieve non-volatile memory effects for the realization of electronic storage devices. In this context, non-volatile memory effects in an orthoconic smectic liquid crystal mixture, with and without polymer capped gold nanoparticles, were investigated. The dielectric spectroscopy technique was performed by applying a d.c. bias during the measurement or a d.c. potential before the start of the measurement in order to obtain pre-conditioning of the sample. Both techniques showed the presence of non-volatile memory effects in the pure orthoconic smectic liquid crystal mixture similar to the doped one. The results demonstrate that the addition of gold nanoparticles enhances the memory effect making it permanent. Our experimental evidence underlines the importance of the structure of the host liquid crystal and clearly suggests that the prolonged time memory effect, observed in the doped liquid crystal, is due to the electric field inducing charge transfer from the liquid crystal molecules to the gold nanoparticles, thanks to the polymer-capping which acts as an ionic charge trapper. Such an ionic trap effect is also responsible for strong reduction of total conductivity of the doped system.

Photo-stimulated phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals

We report concurring phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals. The transitions are induced by photo-stimulation and stable against light and thermal treatments. Photochromic trans- to cis-isomerization of azo-dye induces an augmented dipole moment and strong dipole-dipole interaction of the cis-isomers, resulting in the formation of nano-sized dye-aggregates. Consequent phase separation of the aggregates of a chiral azo-dye induces phase transition from a chiral to nonchiral nematic phase. In addition, the deposition of dye-aggregates at the surfaces brings about anchoring transition of LC molecules. The stability and irreversibility of the transition, together with no need of pretreatments for LC alignment, provide fascinating opportunity for liquid crystal device applications.

The effect of initial alignment on the optical properties of Fe3O4 nanoparticles doped in nematic liquid crystals

Recently the nonlinear effects of the different materials doped liquid crystals are more interesting. In all previous works, nonlinearity of samples with the homeotropic alignment is investigated because of the larger component of the refractive index in this direction. Here, there are spherical Fe3O4 nanoparticles that have both parallel and perpendicular components. We were looking for the effect of initial alignment on the nonlinearity of pure and doped nematic liquid crystals (NLCs). The experimental results emphasize, even the same compositional percentage of nanoparticles prepared by two different alignment configurations are showing different results when dispersed in the same NLCs. Comparing nonlinear studies, the magnitude of nonlinear refraction index, n2 and nonlinear absorption coefficient, β increase 102 and 101 times, respectively, in homeotropic alignment samples and the sign of these parameters is changed rather than homogeneous ones.

Enhanced electro-optical behaviour of a liquid crystal system via multi-walled carbon nanotube doping

We propose a multi-walled carbon nanotube (MWCNT)-doped liquid crystal (LC) device with enhanced electro-optical properties. MWCNT-doped LC cells were successfully operated as a twisted nematic LC device with high performance. The LC cells fabricated with MWCNT-doped LCs and a polyimide layer exhibited the best electro-optical properties, such as a low threshold voltage (1.60 V) and fast response time (13.95 ms). MWCNT doping decreased the pre-tilt angles of LC mixtures. Additionally, UV-visible transmittances and photomicrograph images of LC cells indicated LCD application potential with clear and uniform LC alignment.

Electrowetting pixels with improved transmittance using dye doped liquid crystals

Electrowetting display pixels have been created using a dye doped liquid crystal as the dielectric liquid in a simple electrowetting architecture. In addition to electrowetting, the dye doped liquid crystal reorients, giving two mechanisms to modulate the light. We show that realignment of the liquid crystal, due to the electric field, occurs both before and during electrowetting. The transmission of the pixel has been compared to the transmission of a pixel containing an isotropic liquid, using a simple mathematical model, and we show that electrical realignment of the LC improves the transmission of the pixel. We show a 6.8% gain in the transmission during electrowetting, and before electrowetting occurs.

Guest–host interaction of some chloroanthraquinone dyes doped in liquid crystalline matrix and effect of addition of carbon nanotube

Liquid crystals (LCs) are commonly used in displays. For practical guest–host applications, it is important to choose dyes with a high ability of orientation in the liquid crystal matrix. In this work, two different nematic liquid crystals (E7 and ZLI-1132) were separately doped (1% m/m) with four different dyes (1-chloro, 2-chloro, 1,5-dichloro and 1,8-dichloro anthraquinone). Their solubilities, textures, phase transition temperatures and order parameters were determined. At the second stage, carbon nanotubes (CNTs) were added (0.05% m/m) to these solutions and the same parameters except solubilities were studied again. Although the solubilities of dyes in the liquid crystal ZLI-1132 were lower than those of E7, the order parameters were much higher. Addition of carbon nanotubes as dopant resulted in an increase in order parameter. Nematic/isotropic microphase separation in doped liquid crystal samples was detected by polarised microscopy. Determination of both phase transition temperatures and phase type was done by temperature dependent texture investigations between crossed polarisers. The dyes and CNTs did not significantly destabilise the mesomorphic phase of nematic hosts. When passing a phase transition, characteristic textures and structures changed with the temperature increasing and decreasing. The nematic–isotropic transition occurred in several pictures (textures). An appreciable change in textures was not observed with the addition of dopant(s) when compared with pure liquid crystal textures.

Efficient one-step novel synthesis of ZnO nanospikes to nanoflakes doped OAFLCs (W-182) host: Optical and dielectric response

We are reporting a one-step solvo-chemical to hydrothermal technique under high temperature and pressures with varying concentration. Without exchange of any chemical compounds compared to the previous treatment, we obtained a significant novel uniform nanoflakes synthetic approach. The proportionality ratio of solvent, reaction time and temperature, leads to new prominent growth of ZnO nanoflakes structure having diameter of the order of 180–185nm through hydrothermal approach. A comparative study with pure and doped orthoconic antiferroelectric liquid crystals (W-182) revealed that, the optical and electrical behaviors drastically differed. This fabrication strategy mainly nano doped technique are phenomenologically unique and has promising applications to dc switching devices. Additionally, the uniform growth mechanism of these nano objects doped OAFLCs have been clarified.

Electro-optical Properties of Twisted Nematic Liquid Crystal Displays Fabricated with TIPS-pentacene Doping

This paper introduces 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) doped liquid crystal (LC) alignment properties on a rubbed polyimide (PI) layer as a function of the doping concentration of TIPS pentacene. Pretilt angles, photomicrographs, and electro-optical properties of TIPS pentacene doped LCs were comparable to those of pure LCs. However, TIPS pentacene in a LC medium supported twisted nematic-liquid crystal displays (TN-LCDs) to improve electro-optical properties. The threshold voltages observed in the TN cells decreased as the TIPS pentacene concentration increased. In addition, suitable response times were observed in TN cells.

Electro-Optical Tuning in Photonic Crystals – Dispersed Liquid Crystalline Metamaterials

Two different metamaterial-liquid crystal structures are fabricated with the metamaterials as liquid crystal (LC) and semiconductor nanorods doped LC alignment layers. E-beam lithography was used to pattern the electron-sensitive polymer one-dimensional photonic crystal (PC) structures. The nanostructured aligning surfaces have been characterized by scanning electron microscopy. The PCs-Dispersed Liquid Crystalline Metamaterials have been investigated through polarized optical microscopy. The threshold voltage and response times for the undoped and doped nematic LCs in the glass cells are measured as a function of the applied external electric field.

Photo-switchable bistable twisted nematic liquid crystal optical switch

This work demonstrates a photo-switchable bistable optical switch that is based on an azo-chiral doped liquid crystal (ACDLC). The photo-induced isomerization of the azo-chiral dopant can change the chirality of twisted nematic liquid crystal and the gap/pitch ratio of an ACDLC device, enabling switching between 0° and 180° twist states in a homogeneous aligned cell. The bistable 180° and 0° twist states of the azo-chiral doped liquid crystal between crossed polarizers correspond to the ON and OFF states of a light shutter, respectively, and they can be maintained stably for tens of hours. Rapid switching between 180° and 0° twist states can be carried out using 408 and 532 nm addressing light. Such a photo-controllable optical switch requires no specific asymmetric alignment layer or precise control of the cell gap/pitch ratio, so it is easily fabricated and has the potential for use in optical systems.