Freedericksz Transition Threshold Research Articles

Enhancement in electrical response of liquid crystals with carbon nanotubes doping

ABSTRACT The Freedericksz transition threshold is investigated in a homogeneously aligned carbon nanotube (CNT) doped liquid crystal (LC) planar cell. By neglecting the aggregate and bundle formation of CNTs in a low concentration of CNT doped LC cell, the reduction in the Freedericksz transition threshold is calculated theoretically. Flexoelectric polarisation tensorial expansion in terms of the order parameter and the generalised Helfrich theory is used to estimate the flexoelectric coefficients in the mixture. Dipolar and quadrupolar polarisations are studied in the flexoelectric coefficients of the mixture. It is shown that doping CNTs changes the flexoelectric coefficients through the order parameter of LCs. According to these calculations, for the LC molecules with a dipole moment parallel to their molecular axis, up to 4-fold increase in flexoelectric coefficient is expected, which is in good accordance with experimental reports. Also, it is shown that in the absence of dipolar flexoelectricity, the quadrupolar polarisation is responsible for the changes of the flexoelectric coefficients.

Electric Properties of Multiwalled Carbon Nanotubes Dispersed in Liquid Crystals and Their Influence on Freedericksz Transitions.

Liquid crystal composites with multiwalled carbon nanotubes present dielectric properties considerably different from those of pure liquid crystal (LC). Using a proper dispersion of nanotubes in the LC-sample and a theoretical model in agreement with the experimental configuration, the dielectric permittivities of multiwalled carbon nanotubes are calculated. The influence of dielectric properties on the Freedericksz transition threshold is discussed. Theoretical values for dielectric permittivities of multiwalled carbon nanotubes are calculated for different temperatures

Ferroelectric Particles in Nematic Liquid Crystals with Soft Anchoring.

A theoretical evaluation of the electric Freedericksz transition threshold and saturation field is proposed for a liquid crystals composite with ferroelectric particles. Existing models consider a strong anchoring of nematic molecules on the glass support of the cell, but in this paper a soft molecular anchoring of molecules on the glass support and also on the ferroelectric nanoparticle’s surface is assumed. Thus, a finite saturation field was obtained in agreement with real systems. Calculations are made for planar configuration of positive dielectric anisotropy liquid crystals. The results are compared with data obtained on similar systems from different publications and the differences are discussed.

Electrically tuned whispering gallery mode microresonator based on Kagomé photonic crystal fibers infiltrated with nematic liquid crystals.

In this study, an electrically tunable whispering gallery mode (WGM) microresonator based on Kagomé hollow-core photonic crystal fiber (HC-PCF) is proposed by infiltrating the cross-sectional air holes with nematic liquid crystals (NLCs). Experimental results indicate that the LC molecules turn out in planar alignment. As the applied electric field strength gradually increases, the WGM resonance wavelengths shift toward the longer-wavelength region. The Freedericksz transition threshold of the proposed Kagomé HC-PCF microresonator is experimentally proved to be around 1.2 V μm-1. The electrically tunable microresonator, integrated with NLCs, is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.

Magnetic Freedericksz transition in a ferronematic liquid crystal doped with spindle magnetic particles

In this work we studied new composite ferronematic system based on the liquid crystal 4-n-hexyl-4′-cyanobiphenyl (6CB) doped with hematite spindle-like magnetic particles. We experimentally showed that magnetic moments of the elongated spindle particles are oriented perpendicularly to their main axes. It is a fundamental difference of our systems from ferronematics, which were synthesized before by many authors. For a research of magnetic and orientational properties of new composite materials, the ferronematic samples were prepared with different length of the particles and with the volume concentrations of solid impurity ϕ1=10−3, ϕ2=10−4 and ϕ3=10−5. The structural Freedericksz transition in ferronematic samples were observed by capacitance measurements in a capacitor made of ITO-coated glass electrodes. Experimental results showed a decrease of the critical field in the ferronematic samples in comparison with the pure 6CB, and an increase of the threshold shift with a growth of both the volume fraction of solid impurity and the length of particles. The response of capacitance to low external magnetic field - far below the magnetic Freedericksz transition threshold of prepared samples was also observed experimentally. For a theoretical description of this magnetically induced behavior of composite systems, the continual model of ferronematics with bistable coupling between colloidal particles and the nematic matrix was used. This theory permits the equilibrium tilted orientation of the particles with respect to the nematic director. We showed that it creates a so-called “pretilt angle effect” which explains the behavior of the ferronematic systems in a low field regime as well as decrease of the threshold field and the dependence of this field on the volume fraction of solid impurity and the length of the particles.

Investigation of Nematic Liquid Crystals Doped with Spherical Multiferroic Nanoparticles in the Presence of a Magnetic Field

Recently, multiferroic nanoparticles (MFNPs) have attracted significant attention as dopant in the nematic liquid crystal (NLC) medium. Here, behavior of the NLC molecules and MFNPs in the presence of a magnetic field is theoretically investigated in the framework of continuum theory, based on our knowledge. The electric field of the MFNPs is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of spherical nanoparticles is determined. Dependence of the FT threshold to the diameter and volume fraction of the nanoparticles was studied. Also, the NLC director and nanoparticle reorientations for the magnetic field larger than the FT threshold field are studied. It was shown that the FT threshold of NLC is decreased by doping of spherical MFNPs. Migration of the nanoparticles to the central layers of the cell (segregation effect) is investigated.

Weak Coupling Effect on the Magnetic Freedericksz Transition in a Ferronematic Liquid Crystal

The magnetic Freedericksz transition in a ferronematic representing a diluted suspension of magnetic nanoparticles in an N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystal was experimentally studied. The transition point was determined by means of dielectric capacitance measurements in cells of different thickness for samples with various volume fractions of ferroparticles. The effect of weak coupling between the magnetic and liquid-crystal subsystems was studied. Low concentrations of quasispherical magnetic particles were shown to decrease the magnetic Freedericksz transition threshold in comparison with a pure liquid crystal. The obtained results were theoretically substantiated. The phase diagram of the suspension was plotted, and the method of estimating the energy of coupling between magnetic particles and a liquid-crystal matrix was proposed.

Electrically tunable photo-aligned hybrid double-frequency liquid crystal polarisation grating

ABSTRACTA polarisation grating (PG) based on the hybrid photo-aligned double-frequency liquid crystal has been demonstrated and fabricated in this paper. The single-order diffraction efficiency is above 95% for circular polarisation incident light. The PG can be driven by 2.5 V/µm electric field with alternating frequency and has no Freedericksz transition threshold. In response time, the rise time and decay time are 650 and 950 , respectively; the sub-millisecond can be achieved under a low driving voltage.

Tuning of Polarization-Dependent Whispering Gallery Modes in Grapefruit Microstructured Optical Fibers Infiltrated With Negative Dielectric Anisotropy Liquid Crystals

An electrically tunable whispering gallery mode microresonator based on microstructured optical fibers (MOFs) infiltrated with negative dielectric anisotropy liquid crystals (LCs) is proposed and experimentally demonstrated. Experimental results indicate that the second radial order mode of the MOF microresonator has stronger electric field response than the first radial order mode and the resonance dip for TE polarization mode is more sensitive to the applied electric field intensity in comparison with the TM polarization mode resonance dip. The Freedericksz transition threshold of the proposed MOF microresonator is experimentally found to be about $\text{2.0}\;{\text{V}}\mu {{\text{m}}^{ - 1}}$ . The electrically tunable microresonator integrated with negative dielectric anisotropy LCs is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.

Behaviour of nematic liquid crystals doped with ferroelectric nanoparticles in the presence of an electric field

ABSTRACTA planar nematic liquid crystal cell (NLC) doped with spherical ferroelectric nanoparticles is considered. Polarisation of the nanoparticles are assumed to be along the NLC molecules parallel and antiparallel to the director with equal probability. The NLC molecules anchoring to the cell walls are considered to be strong, while soft anchoring at the nanoparticles surface is supposed. Behaviour of the NLC molecules and nanoparticles in the presence of a perpendicular electric field to the NLC cell is theoretically investigated. The electric field of the nanoparticles is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of nanoparticles is found. Then, the director and particles reorientations for the electric fields larger than the threshold field are studied. Measuring the onset of the nanoparticles reorientation is proposed as a new method for the FT threshold measurement.

Влияние слабого сцепления на магнитный переход Фредерикса в ферронематическом жидком кристалле

AbstractThe magnetic Freedericksz transition in a ferronematic representing a diluted suspension of magnetic nanoparticles in an N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystal was experimentally studied. The transition point was determined by means of dielectric capacitance measurements in cells of different thickness for samples with various volume fractions of ferroparticles. The effect of weak coupling between the magnetic and liquid-crystal subsystems was studied. Low concentrations of quasispherical magnetic particles were shown to decrease the magnetic Freedericksz transition threshold in comparison with a pure liquid crystal. The obtained results were theoretically substantiated. The phase diagram of the suspension was plotted, and the method of estimating the energy of coupling between magnetic particles and a liquid-crystal matrix was proposed.

Analytical description of 2D magnetic Freedericksz transition in a rectangular cell of a nematic liquid crystal.

We study the Freedericksz transition induced by a magnetic field in a rectangular cell filled with a nematic liquid crystal. In the initial state the director of the nematic liquid crystal is uniformly aligned in the cross section plane of the cell with rigid anchoring of the director at cell walls: planar on the top and bottom walls, and homeotropic on the left and right ones. The magnetic field is directed perpendicular to the cell cross section plane. We consider two-dimensional (2D) orientational deformations of the nematic liquid crystal in the rectangular cell and determine the critical value of the Freedericksz transition field above which these orientational deformations occur. The 2D expression for the director alignment profile above the threshold of Freedericksz transition is analytically found and the profile shapes as functions of cell sizes, values of the Frank elastic constants of the nematic liquid crystal and the magnetic field are studied.

Effects of chirality on optical and electro-optic behavior of nematic liquid crystals doped with functionalized silver nanoparticles

A comprehensive study based on the impact of chirality on the molecular organization in nematic liquid crystal matrix doped with dodecanethiol-capped silver nanoparticles (Ag NPs) was conducted. The temperature dependent morphological behavior was investigated by polarized optical microscopy. The thermo-optic and electro-optic switching behavior of the liquid crystalline helix was confirmed with an active chiral dopant. An improvement in the dielectric behavior in the frequency range of 50Hz to 1MHz was recorded by the induction of chirality in the matrix of the Ag NPs doped nematic medium confined in planar boundary conditions. A lowering of the threshold of Freedericksz transition and a nearly tenfold rise in dielectric constant with temperature was recorded. A pronounced surge in photoluminescence in addition to an irreversible memory was also observed in the chiral medium. Accredited with such prominent optical and electro-optical properties, silver nanoparticles doped chiral liquid crystals find a promising future as the materials of choice for optical sensors and electro-optic liquid crystal devices with memory.

Voltage-controllable liquid crystal waveguide

This study presents an optical waveguide formed by a self-focusing pump beam in a nematic liquid crystal thin film. The pump beam is a polarized high power Gaussian laser beam. Applying a voltage near the Freedericksz transition threshold enhances the optical field of pump beam to reorientate liquid crystal molecules. The reorientation of LC molecules forms the refractive index as a Gaussian-like distribution in the pump beam region. By the assistance of applying electric field, the pump beam can form an optic waveguide by self-focusing effect. A polarized lower power probe beam near the waveguide can be coupled to the waveguide. The coupling efficiency can be controlled by applying voltage.

Freedericksz transition in a nematic cell with periodic anchoring energy in electric field

The threshold value and shape of the spatially inhomogeneous threshold distribution of the director in an electric field in a planar nematic cell with periodic polar nematic anchoring energy with the cell surface have been determined. It is shown that the threshold value depends nonmonotonically on the number of anchoring energy periods per cell length. An analytical expression for the Freedericksz transition threshold at small anchoring energy modulation amplitudes is obtained, and a numerical calculation of threshold values at large amplitudes is performed. The spatial period of the director structure arising at Freedericksz transition depends strongly on the ratio of Frank elastic constants (K2/K1) and the number of anchoring-energy modulation periods per cell length, but is hardly affected by the anchoring energy modulation amplitude.

Light-controlled electric Freedericksz threshold in dye doped liquid crystals

We report the results of measurements of the threshold of Freedericksz transition in a nematic liquid crystal doped by Methyl-red. We show that in case of dc field the threshold voltage can decrease or increase depending on the light dose, due to the light-induced desorption and adsorption of charge complexes from and on the irradiated surface, that has been recently demonstrated. This effect has the potential to be exploited in optical devices such as liquid crystal microlenses and spatial light modulators.

Light-induced orientation of the molecules of nematic liquid crystals doped with comb-shaped polymers with different spatial distributions of chromophores

The orienting effect of light on nematic liquid crystals (NLCs) doped with comb-shaped polymers with different spatial distributions of side absorbing azobenzene fragments, i.e., a homopolymer (containing only azofragments), a block copolymer (containing additionally a block of non-absorbing fragments), and a statistical copolymer (containing randomly arranged absorbing and non-absorbing fragments) is experimentally studied. The light-induced Freedericksz transition threshold for the block copolymer is two times lower than that for the homopolymer. For NLC with statistical copolymer dopant, the first-order orientation transition with an extremely wide optical bistability region is observed.

Influence of the anchoring energy on hysteresis at the Freedericksz transition in confined light beams in a nematic cell

The influence of the finite value of a nematic director anchoring energy on the hysteresis of the light-induced Freedericksz transition induced by a normal incident linearly polarized one-dimensional bounded light beam is considered. The intensity distribution throughout the cross section of the beam is described by a Heaviside function. Values of the Freedericksz transition threshold and the director deviation angle jumps with increasing and decreasing incident light intensity are numerically obtained and are dependent on the director anchoring energy magnitude and the light beam cross section. Allowable regions of the light beam widths, anchoring energy values and values of the parameters , at which the Freedericksz transition is accompanied by hysteresis are determined.

Decrease in the threshold of electric Freedericksz transition in nematic liquid crystals doped with ferroelectric nanoparticles

A phenomenological theory of electric Freedericksz transition in planar-oriented nematic liquid crystals doped with spherical ferroelectric nanoparticles is considered. It is shown that even spherical nanoparticles can critically reduce the threshold of Freedericksz transition, due to their ability to strongly increase the electric field in the medium. At higher values of polarization and concentration of nanoparticles a Freedericksz transition without threshold can occur. In this case only an insignificant electric field needs for orienting the nanoparticles.

Light- and electric-field-induced first-order orientation transitions in a dendrimer-doped nematic liquid crystal

Interaction of light and ac electric fields with a nematic liquid crystal (NLC) doped with nanosized second-generation carbosilane codendrimers containing terminal azobenzene fragments has been studied. A first-order Freedericksz transition in the linearly polarized light, accompanied by an intrinsic bistability in a wide region, was observed. An additional ac electric field decreases the light-induced Freedericksz transition threshold and narrows the bistability region. Light illumination transforms the second-order electric-field-induced Freedericksz transition to a first-order one. The width of the bistability region increases with the light wave intensity. The theory of the interaction of light and ac electric fields with the dendrimer-doped NLCs is developed taking into account an additional (with respect to the undoped nematic host) dependence of the optical torque on the angle between the director and the light field.