Doped Liquid Research Articles

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.

Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids

Electrochemical impedance spectroscopy in a thin cell (10μm) was used to infer conductivity, permittivity and the differential double-layer capacitance of solutions of dodecane doped with OLOA 11000 [poly(isobutylene) succinimide] for concentrations of dopant between 0.1% and 10% by weight. All spectra (frequencies between 1Hz and 100kHz) were well fit by an equivalent circuit having four elements including a constant-phase element representing the double-layer capacitance. Using Gouy–Chapman theory for small zeta potentials and assuming univalent charge carriers, the double-layer capacitances were converted into charge carrier concentration which was found to be directly proportional to the weight percent of dopant with a 1wt% solution having 87 carriers/μm3 (the concentration of either positive or negative charges). This is only 17ppm of the total monomer concentration calculated from the average molecule weight of the dopant. Dividing the measured conductivities by the charge carrier concentration, we inferred the mobility and hydrodynamic diameters for the charged micelles. The hydrodynamic diameters of carriers were significantly larger than the average diameter of all micelles measured independently by dynamic light scattering. This suggests that only large micelles become charged.

Coordination and interactions in a Li-salt doped ionic liquid

We report on the coordination and interactions in the LiTFSI doped ionic liquid PyR14TFSI over a large concentration range, 0.01≤x≤0.4, using Raman spectroscopy. We find that the concentration dependence of the average number of TFSI anions coordinating to one Li-ion (NTFSI/Li) can be divided into three regimes. For low concentrations, x≤0.05, we find that a large number TFSI anions coordinate each Li-ion, N>2. The number decreases with increasing salt concentration and the interaction between the Li-ion and the TFSI anions is rather weak in this concentration range. At intermediate concentrations, 0.1≤x≤0.2, the number of TFSI anions coordinating each Li-ion (NTFSI/Li) is ~2 pointing towards the formation of [Li(TFSI)2]− ionic clusters. At higher concentrations, x>0.2, NTFSI/Li decreases further indicating the transition to more complex structures with Li-ions bridging TFSI anions. We also show that the evolution of the microscopic structure as a function of Li-salt concentration is mirrored in the behaviour of macroscopic properties such as the ionic conductivity and the glass transition temperature, which also show a crossover in the same concentration range.

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.

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.

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.

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.

Investigation of photoinduced change of dielectric and electrical properties of indium (III) phthalocyanine and fullerene doped nematic liquid crystal

In this work, we have studied photoinduced dielectric and electrical properties of E63 nematic liquid crystal (E63-LC) materials doped with indium (III) metallo phthalocyanine (In-MPc) and fullerene (C60) in dark and under UV illumination. Doping concentration of In-MPc in E63-LC was chosen to be 5wt%, whereas a trace amount of C60 was used in investigated samples. Real and imaginary parts of dielectric permittivity were investigated in the frequency range of 100Hz–10MHz by using dielectric spectroscopy technique (DST). It was observed that doping agents and UV illumination enhanced the real part of dielectric permittivity at low frequencies, which was thought to originate from photoinduced charge transfer between In-MPc and C60 caused by extra dipole strength in LC medium. In addition to that, critical frequency (fc) and relaxation time (τ) were obtained and analyzed for all investigated samples. Photoelectrical characteristics of hybrid LC structures were carried out by current–voltage measurements. A major increase in electrical conductivity was observed in In-MPc and C60 doped LC structures under UV irradiation.

Electrical Parameters of Different Concentrations of Methyl Red in Fullerene Doped Liquid Crystal

Dielectric anisotropy and relaxation time of a liquid crystal (LC) system, containing fullerene and methyl red (MR) dye, were studied via impedance spectroscopy technique under a bias voltage. Dye concentration is tried to be 0.1%, 0.5%, and 1% in host nematic liquid crystal coded E7. Dielectric permittivity and dielectric anisotropy values of the samples were investigated between the frequency ranges of 100–10 MHz. It is seen that dielectric anisotropy is strongly affected by doping ratio and this value decreases with increasing dye. Also, relaxation time τ and critical frequency values were calculated depending on voltage of different dye ratios.

Electro-Optical Characteristics of ZrO2 Nanoparticle Doped Liquid Crystal on Ion-Beam Irradiated Polyimide Layer

It is well known that doping liquid crystals (LCs) with nanoparticles can readily change the physical and electro-optical properties of LC mixture. In this paper, we report on how the electro-optical properties and thermal stability of an LC system were enhanced by dispersing zirconia (ZrO2) nanoparticles in nematic LCs on ion-beam irradiated polyimide layers. Homogeneous LC alignment was achieved and ZrO2/LC mixture was applied in twisted-nematic (TN) mode. The addition of ZrO2 nanoparticles contributed to improvement of electro-optical properties in the TN LC cell by lowering voltage operation and decreasing response time. The TN LC cells with a ZrO2 nanoparticle concentration of 2.0 wt% showed the lowest threshold voltage of 2.0 V and the fastest response time of 15.3 ms. This enhanced electro-optical performance was likely due to van-der waals interactions and the screening effect of the ZrO2 nanoparticles in the LC medium. The thermal stability of the ZrO2/LC mixture was also improved compared to a pristine LC system.

Changes in the electro-optical behaviour of ferroelectric liquid crystal mixture via silica nanoparticles doping

We investigated the dynamic properties of a ferroelectric liquid crystal mixture doped with silica nanoparticles using polarization current reversal method. The effect of temperature and bias field on the switching responses of silica–ferroelectric liquid crystal composites with different silica concentrations (0.01, 0.02, 0.05 and 0.1wt%) were studied and compared. Various electro-optic properties of ferroelectric liquid crystal mixture changed according to silica nanoparticles concentration. We observed about 14% decrease in spontaneous polarization and up to 17% increase in switching response for 0.01wt% silica dispersion, whereas a minor increase in the isotropic temperature of composites. The rotational viscosity and optical contrast increased with increasing silica concentration. At a 0.1wt% silica nanoparticle doping concentration, the ferroelectric liquid crystal cell showed the best electro-optic properties, such as low threshold and driving voltage, better optical contrast and low tilt angle. Our experimental results show that silica doped ferroelectric liquid crystal composites are promising materials for better optical contrast in electro-optical applications.

All-Optical and Thermal Tuning of a Bragg Grating Based on Photosensitive Composite Structures Containing Liquid Crystals

We report on the fabrication and characterization of a tunable filtering effect, observed in a waveguide grating made of alternated strips of photocurable polymer and a mixture of azo-dye doped liquid crystal. The grating is sandwiched between two borosilicate glasses, one of which includes an ion-exchanged channel waveguide, which confines the optical signal to be filtered. We analyze the effects of a low power visible, pump light beam that hits the sample and temperature effects. Both effects are investigated experimentally. The application of the pump light allowing the filtered wavelength to be tuned over a 6.6 nm range, while a temperature variation of 4°C, implies a tuning of 14 nm.

Photo-induced birefringence in a nematic liquid crystal mixture doped with light-switchable mesogenic azobenzene derivatives

We report on photoinduced effects observed in azo-mesogene doped nematic liquid crystal mixture using birefringence measurement method. In Optical Kerr Effect configuration the photoinduced birefringence observed in the studied planarly aligned liquid crystal cell is equivalent to very high n2≈13cm2/W nonlinear optical coefficient. Experimental results are explained on a basis of light-induced modification of the surface conditions due to trans-cis isomerizations that affect the bulk effective electric field acting on molecules. The measurements are supported by a simple model of the phenomenon and Jones matrix calculus. The response of the system is composed of two dynamically different contributions: the fast and the slow one.

Phase behaviour, transport properties, and interactions in Li-salt doped ionic liquids

We report on the influence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) doping on the glass transition temperature (Tg), the ionic conductivity, and Li-ion coordination of two dicationic ionic liquids (DILs) based on the TFSI anion. The results are compared to the behaviour of traditional mono-cationic ionic liquids. The cations of the DILs contain two imidazolium rings, connected by a decane hydrocarbon chain. Homogeneous mixtures of these ILs and LiTFSI can be obtained in a large concentration range. With increasing Li-salt concentration the ionic conductivity decreases whereas the glass transition temperature increases in both systems. However, the influence of the salt doping on the ionic conductivity and the glass transition temperature is low compared to typical mono-cationic ionic liquids, based on for example the pyrrolidinium cation and the TFSI anion. This behaviour is mirrored in the average coordination number of TFSI anions around Li-ions, determined by Raman spectroscopy. The coordination number is systematically lower in the DILs, suggesting a connection between the difference in the Li-ion environment and the behaviour of the glass transition and the ionic conductivity. A Tg-scaled Arrhenius plot of the ionic conductivity shows that the ionic conductivity for all LiTFSI concentrations has the same temperature dependence, i.e., the fragility of the liquid is the same. This implies that the conduction process is dominated by the viscous properties of the liquids over the entire concentration range. This provides further support for linking the local environment of the Li-ions to the glass transition and conduction process in the ionic liquid/salt mixtures.

Irreversible visual sensing of humidity using a cholesteric liquid crystal

Irreversible optical sensing of humidity by a doped cholesteric liquid crystal is achieved by using a thin film of nematic host E7 with a binaphthylorthosilicate ester as dopant (guest). The film changes its color from blue (to green to orange to red) to colorless when exposed to humidity as the dopant is hydrolyzed.

Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films

Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films