Response Of Nematic Liquid Crystal Research Articles

Control of liquid crystals combining surface acoustic waves, nematic flows, and microfluidic confinement.

The optical properties of liquid crystals serve as the basis for display, diagnostic, and sensing technologies. Such properties are generally controlled by relying on electric fields. In this work, we investigate the effects of microfluidic flows and acoustic fields on the molecular orientation and the corresponding optical response of nematic liquid crystals. Several previously unknown structures are identified, which are rationalized in terms of a state diagram as a function of the strengths of the flow and the acoustic field. The new structures are interpreted by relying on calculations with a free energy functional expressed in terms of the tensorial order parameter, using continuum theory simulations in the Landau-de Gennes framework. Taken together, the findings presented here offer promise for the development of new systems based on combinations of sound, flow, and confinement.

Interplay of reorientational and thermal solitons: unveiling the dynamic coexistence and enhanced nonlinear response in nematic liquid crystals

Spatial solitons in nematic liquid crystals (NLC) are optical beams that maintain their transverse profile due to the self-focusing effect of light. The mechanism leading to the formation of solitons can be either light-induced reorientation of the elongated molecules or thermal changes in the refractive index induced by a light beam. The specific optical properties of nematic liquid crystals cause the reorientational and thermal solitons to coexist in NLC, and their mutual competition can lead to a reduction or enhancement of the overall nonlinear response of the NLC. The study of the interplay between reorientational and thermal solitons allows a better understanding of the conditions under which the solitons can coexist, compete, or enhance each other. Here, we show that the simultaneous existence of both soliton types can lead to a more robust and versatile nonlinear response and can be used in competitive and synergistic configurations by carefully controlling the temperature, intensity, and polarization of the propagating light. As a result of our experimental findings, it may be possible to shape the nature of the interplay between reorientational and thermal solitons and better understand the behavior of reorientational solitons under varying thermal conditions. Furthermore, understanding this mutual interplay may provide insights into the potential of advanced NLC-based photonic devices that exploit the joint effect of reorientational and thermal nonlinearities.

Impact of strontium titanate nanoparticles on the dielectric, electro-optical and electrical response of a nematic liquid crystal

The impact of ferroelectric nanoparticles (FNPs), namely strontium titanate (SrTiO3) on the dielectric, electro-optical and electrical response of a room temperature nematic liquid crystal (NLC), 5CB (4-pentyl-4′-cyanobiphenyl) is investigated through dielectric, electro-optical and electrical techniques. The dielectric studies clearly show that low frequency dielectric relaxation shifts towards higher frequency as concentration of FNPs increases but decreases to the value even lower than pure sample at 2 wt% concentration. Afterwards, the short axis molecular relaxation observed in homeotropic cell obtained from homogeneous aligned cell by applying bias voltage higher than threshold shifts towards higher frequency by ∼ 28 % in composites but remains independent of concentration. Interestingly, after the removal of bias voltage, dielectric spectra does not return to its original state even after ∼ 22 min in the composites unlike pure sample and could be considered as apparent dielectric memory (i.e. memory effect). However, the voltage dependent optical textures confirm the short term memory in the composites. The electrical response recorded as a function of frequency in the composite is also significantly different than pure sample which further confirms the impact of FNPs on NLC. We believe that the observed effects of FNPs on dielectric, electro-optical and electrical behavior could be due to the plausible favorable interaction among FNPs and NLC molecules. Such investigations would certainly be pivotal in tuning the dielectric and electro-optical properties of NLCs using FNPs and helpful in designing the next-generation tunable electro-optical and soft memory based devices.

Ultralow-VπL Silicon Electro-Optic Directional Coupler Switch With a Liquid Crystal Cladding

An ultralow-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L photonic switch device is demonstrated utilizing the high optical and electrical field confinement in silicon slot waveguides coupled with the strong electro-optic response of nematic liquid crystals. A silicon photonic directional coupler switch with a modulation efficiency of 0.0195 V·mm and a loss-efficiency product of 0.0624 V·dB is achieved. The 1.5 mm long device is based on two-mode interference within a single slot waveguide resulting in a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> of 0.013 V and an extinction ratio of ~ 9dB at 1550 nm wavelength. The power consumption of the photonic switch is estimated to be below 0.6 nW and it possesses a response time of <; 1.5 ms. A comparative performance study between the directional coupler switch and a Mach-Zehnder Interferometer (MZI) switch is performed. The directional coupler switch is projected to have a smaller footprint and lower optical loss compared to a similar design MZI switch making it a strong candidate for switch matrix designs and applications.

Study The Influence of Doping Electric and Magnetic Nanoparticles on The Nonlinear Optical Properties of Nematic Liquid Crystals

The nonlinear optical properties response of nematic liquid crystal (6CHBT) and the impact of doping with two kinds of nanoparticles; Fe3O4 magnetic nanoparticles and SbSI ferroelectric nanoparticles have been studied using the non-linear dynamic method through z-scan measurement technique. This was achieved utilizing CW He-Ne laser. The pure LC and magnetic LC nanoparticle composite samples had a maximum absorption while the ferroelectric LC nanoparticle composite had a minimum absorption of the incident light. The nonlinear refractive index was positive for the pure LC and the rod-like ferronematic LC composite samples, while it was negative for the ferroelectric LC composite. The studying of the nonlinear optical properties of these types of liquid will be very helpful in the field of liquid crystal applications like display screen and spatial light modulators.

Enhanced electro-optical response of nematic liquid crystal doped with functionalised silver nanoparticles in twisted nematic configuration

ABSTRACT The dipolar and surface plasmon properties of metallic nanoparticles, especially gold and silver NPs made them suitable candidates for doping with LC system, which provides faster switching time, low power consumption in the display devices. The doping of NPs in the LC system influences the physical properties such as dielectric anisotropy, optical anisotropy, elastic constants, viscosity, and conductivity. In this work, we report the effect of doping of hexane thiol functionalized silver nanoparticles (f-AgNps) of average diameter 3-5nm on the electro-optical switching properties of nematic LC 4-trans-pentyl-cyclohexyl cyanobenzene (5PCH) in the twisted nematic geometry. The threshold as well as switching voltages have also reducedin the nanocomposites of 5PCH compared to pure 5PCH. The steepness of the V-T graph necessary for better multiplexing performance and high contrast has increased by 230% in the 0.025 wt% of f-AgNPs doped 5PCH compared to pure 5PCH. The ion current for 0.025 wt% f-AgNPs doped nanocomposites of 5PCH has reduced by 60% compared to pure 5PCH. We have observed faster switching time in f-AgNPs nanocomposites compared to pure 5PCH due to the decrease of visco-elastic ratio. The total response time of 0.05 wt% f-AgNPs nanocomposites is reduced by 19% compared to pure 5PCH.

Enhancing the thermo-optical response of nematic liquid crystal with a polymer network

As nematic liquid crystals are heated, the change in the nematic order parameter affects the physical properties of the phase such as dielectric permittivity, indices of refraction, and elastic constants. This enables nematic liquid crystals to be used as temperature sensors. Of particular interest is their use as an uncooled thermal imager. In this paper, an optical sensor for this application is considered where the change in birefringence of the liquid crystal with temperature is probed by a visible wavelength. One problem with this type of sensor is the large change in birefringence as the liquid crystal that undergoes a first-order nematic-isotropic phase transition cannot be used. To solve this problem, confinement of the liquid crystal to obtain a second-order phase transition is considered. Confinement of the liquid crystal is accomplished with a polymer network. Through numerical Landau-de Gennes calculations and experimental measurements, it is demonstrated that the average domain size of an appropriate polymer network is ∼250 nm. This confinement results in a doubling of the thermo-optical response of the nematic liquid crystal. Additionally, it is demonstrated that the polymer network suppresses twist fluctuations of the nematic director, which halves the noise of the sensor. The results of this work allow for improved uncooled thermal imagers.

Temperature control of nematicon trajectories.

Using modulation theory, we develop a simple [(2+1)-dimensional] model to describe the synergy between the thermo-optical and reorientational responses of nematic liquid crystals to light beams to describe the routing of spatial optical solitary waves (nematicons) in such a uniaxial environment. Introducing several approximations based on the nonlocal physics of the material, we are able to predict the trajectories of nematicons and their angular steering with temperature, accounting for the energy exchange between the input beam and the medium through one-photon absorption. The theoretical results are then compared to experimental data from previous studies, showing excellent agreement.

Optimizing low frequency electro-optic response of nematic liquid crystals

Optimized enhanced electro-optic (EO) response of nematic liquid crystals (NLC) driven by low modulation frequency AM electric signal is investigated. A perturbed non-uniform transmitted output signal was observed at modulating frequencies (fm) lower than 80 Hz due to the effect of free charges, and optimized uniform response is achieved by adding a small holding voltage to the AM driving signal. The minimum applied holding voltage required for attaining uniform response at frequencies <80 Hz decreases as the modulating signal frequency increase. As the frequency increases from 10 Hz to 70 Hz, the total response time decreases from 8.8 ms to 4.7 ms at field strength 1.2 V/μm and at holding voltage (Vh); 1.95 V and 0.7 V (peak voltage), respectively. By such way the response times is effectively stabilized and enhanced for NLC low frequency EO applications.

Localised opto-thermal response of nematic liquid crystal to laser light

ABSTRACTWe report on the thermal response of a thin nematic liquid crystal layer to a strong laser pulse, which is partially absorbed by the Indium Tin Oxide electrodes of the measuring LC cell. We measure local increase and temperature profile of the heated region and we measure the time response of the birefringence of this nematic layer. It is found that local increase of the temperature due to absorption of a 90 Mw laser light of 10 MS duration can easily exceed °C and depends on the localisation and focusing of the laser beam. Local thermal response of the heated nematic cell is measured by monitoring the time dependence of birefringence during the cooling and we found that nematic layer can be cooled down at an extremely fast rate of 10000 K/s. Optical retardation of 200 nm can be switched in 100 μs by laser pulse absorption and heating/cooling.

Colossal nonlinear optical response of liquid crystals

We briefly review the main steps related to the discovery of the extraordinarily large nonlinear optical response of nematic liquid crystals doped by Methyl Red leading to detection of n2>103 cm2/W in 1μm thin films, that is the strongest nonlinear response ever reported in liquid crystals. This investigation started from the researches of Professor Yuri Reznikov on light-induced anchoring effects in these materials.

Role of specific and nonspecific intermolecular interaction in electro-optical response of doped 6CHBT nematic liquid crystal with azo dyes

Investigations done so far have shown that dopant material have a considerable effect on the electro-optical responses of dye-doped liquid crystal. But the type of inter or intra-molecular interaction roles in electro-optical properties and nonlinearity of doped liquid crystalline systems has uncovered. In this work, to study of the kind of molecular interaction effects on electro-optical properties of doped LC, two dyes with dissimilar interactional and functional groups were used as LC dopants. Therefore, doping these dyes into liquid crystals allows us to understand and analyze the effect of interaction type on electro-optical responses of the pure and doped samples. The electro-optical Kerr behavior, the pre-transition temperature and third order nonlinear susceptibility of pure and doped LC by two dyes with different interactional groups were having studied. The obtained results revealed that the electro-optical response of nematic liquid crystal can be strongly enhanced or decreased, depending on dopant's nature, concentration and type of specific and nonspecific interaction between doped dye and liquid crystal molecules. In other words, it will be shown that dopants molecular interactions with each other and liquid crystal molecules are important factors for controlling of electro-optical responses of dye doped liquid crystal system.

Reduction of power-dependent walk-off in bias-free nematic liquid crystals

We investigate numerically how to control and reduce power-dependent walk-off by using a three-dimensional model for beam propagation in highly nonlocal bias-free nematic liquid crystals (NLCs). We consider beam propagation with different initial momenta and rest angles. Owing to the highly nonlocal character of the dielectric response of NLC, we demonstrate various beam trajectories. We show that it is possible to practically eliminate walk-off in real NLCs by properly choosing the angle of incidence.

Highly sensitive color-indicating and quantitative biosensor based on cholesteric liquid crystal.

Liquid crystal (LC)-based biosensors employ highly sensitive interfaces between the alignment layers and LCs to detect biomolecules and their interactions. Present techniques based on optical texture observation of the homeotropic-to-planar response of nematic LCs are limited by their quantitative reproducibility of results, indicating that both the accuracy and reliability of LC-based detection require further improvements. Here we show that cholesteric LC (CLC) can be used as a novel sensing element in the design of an alternative LC-based biosensing device. The chirality of the vertically anchored (VA) CLC was exploited in the detection of bovine serum albumin (BSA), a protein standard commonly used in protein quantitation. The color appearance and the corresponding transmission spectrum of the cholesteric phase changed with the concentration of BSA, by which a detection limit of 1 fg/ml was observed. The optical response of the VA CLC interface offers a simple and inexpensive platform for highly sensitive and naked-eye color-indicating detection of biomolecules, and, thus, may facilitate the development of point-of-care devices for the detection of disease-related biomarkers.

Response Speed of Polymer-Stabilized Liquid Crystals

Polymer stabilization is an attractive technique because the physical property of liquid crystals can be improved and furthermore the new characteristics may occur. In this study, we try to obtain a high-speed response of nematic liquid crystals by using polymer stabilization technique for realization of the field sequential color. We have researched the dependence of the electrooptical characteristics such as response time and threshold voltage on UV irradiation energy, polymer and liquid crystal materials, and polymer concentration. As a result, it is found that the electrooptical characteristics strongly depend on their conditions.

Application of Ferroelectric Liquid Crystals to Optical Devices

Liquid crystals (LCs) are attractive for optical devices, such as lenses because of their large birefringence and high response to external field. However, the low-speed response of nematic LCs often becomes an obstacle to the progress of LC devices. In this study, we research the lens effect of ferroelectric LC by using a voltage-gradient cell. As a result, it is found that in order to obtain the lens effect in FLC, the wide cell gap, the large tilt angle and/or the large birefringence of FLC materials are necessary to increase the birefringence variation under the gradient voltage in addition to the monostabilization of FLC media.

Study of dynamics and relaxation optical response of nematic liquid crystals doped with CdSe/ZnS quantum dots

This study is devoted to the dynamics of the electro-optical response and relaxation time of LC cells with a nematic liquid crystal doped with CdSe/ZnS semiconductor quantum dots (QDs) and homogeneously oriented by rubbed polyimide. The influence of the nanoparticle concentration as well as the parameters of the alternating electric field has been studied. Experimental dependence of the response and relaxation time on the concentration of QDs was obtained. The increase of concentration of QDs in the range of 0.5–1.5 mg/ml results in a reduction of the optical response time. The threshold voltage reduced twice with increasing QD concentration up to 1.5 mg/ml as compared with the pure LC. For the first time it was shown that applying of unipolar square wave electric field to the LC cell reduces the optical response time even twice regardless to the frequency of the alternating electric field compared to bipolar square wave voltage.

Magneto-optic and converse magnetoelectric effects in a ferromagnetic liquid crystal.

We have studied the response of ferromagnetic liquid crystals to external magnetic and electric fields, and compared it to the usual response of nematic liquid crystals (NLCs). We have observed effects, which are not present in a pure NLC and are a consequence of the coupling between the nematic director and the magnetization. The electro-optic effect, which is in the ferromagnetic phase the same as in the pure NLC, is accompanied by a converse magnetoelectric effect. The magneto-optic effect differs completely from the one observed in the pure NLC, where it is a quadratic effect and it only appears when a magnetic field larger than a critical field is applied perpendicular to the director. In the ferromagnetic NLC in addition to the response to the perpendicular field, there is also a qualitatively different response to the parallel field. Contrary to the pure NLC no critical field needs to be exceeded for the system to respond to a perpendicular field, but a critical field needs to be exceeded to observe a response to the field parallel to the director and antiparallel to the magnetization. The critical field is in this case two orders of magnitude smaller than the critical field of the magnetic Frederiks transition in the pure NLC. The experimental observations are well described by a simple macroscopic theory.

Non-Debye relaxation in the dielectric response of nematic liquid crystals: Surface and memory effects in the adsorption-desorption process of ionic impurities

We demonstrate theoretically that the presence of ions in insulating materials such as nematic liquid crystals may be responsible for the dielectric spectroscopy behavior observed experimentally. It is shown that, at low frequencies, an essentially non-Debye relaxation process takes place due to surface effects. This is accomplished by investigating the effects of the adsorption-desorption process on the electrical response of an electrolytic cell when the generation and recombination of ions is present. The adsorption-desorption is governed by a non-usual kinetic equation in order to incorporate memory effects related to a non-Debye relaxation and the roughness of the surface. The analysis is carried out by searching for solutions to the drift-diffusion equation that satisfy the Poisson equation relating the effective electric field to the net charge density. We also discuss the effect of the mobility of the ions, i.e., situations with equal and different diffusion coefficients for positive and negative ions, on the impedance and obtain an exact expression for the admittance. The model is compared with experimental results measured for the impedance of a nematic liquid crystal sample and a very good agreement is obtained.

Three-dimensional molecular dynamics simulations of reorientation process and backflow generation in nematic liquid crystals under application of electric fields

The dynamic responses of nematic liquid crystals in a parallel-plate cell under the application of electric fields were investigated using three-dimensional molecular dynamics simulations, which should provide more precise dynamics as compared to those in two-dimensional molecular dynamics simulations as in our previous work [Sunarso et al., Appl. Phys. Lett. 93, 244106 (2008)]. The study is focused on the reorientation process and the generation of backflow, which should be important in the development of liquid crystalline actuators. It is shown that bulk reorientation is coupled with the generation of backflow owing to the conversion of electric-field-induced molecular rotation into bulk translational motion. The increase in electric torque due to the increase in electric field strength results in a faster change in the bulk orientation, thus accelerating the development of the flow field and increasing the magnitude of the generated velocity field. Different initial orientation angles result in similar dynamics, although the magnitude of the generated velocity decreases with increasing initial orientation angle. The development of velocity profiles confirms the results of the experiment and the simulation using a macroscopic continuum approach. Simulations under various molecular aspect ratios show that with an increase in the aspect ratio, the reorientation process becomes slower due to the increase in moment of inertia and elastic torque, whereas the magnitudes of the velocity show the trade-off between the speed of the orientation change and the effectiveness of the molecular motion conversion. Furthermore, the simulation results show the spatial variation in the reorientation process as the result of interplay between electric torque, elastic torque, and backflow.