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

A 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.     

The orientational mechanism of nonlinearity and the self-focusing of He-Ne laser radiation in nematic liquid crystal mesophase (theory and experiment)

A giant optical nonlinearity of self-focusing type in the oriented mesophase of nematic liquid crystals (NLC) due to the director reorientation under the action of a light wave field is predicted. Self-focusing of He-Ne laser radiation with power ～10^?2 W and power density ～50 W/cm² in a planar oriented 60 μm thick NLC layer has been carried out experimentally. The measured value of the nonlinearity effective constant ?₂ = 0.07 cm³/erg corresponds to theoretical predictions, and turns out to be larger than the CS₂ nonlinearity by ? 10⁹ times.     

Influence of the molecular structure of components on the induction of mesophases in nematic liquid crystal systems

The dielectric properties of nematic liquid crystals (NLC) are studied in the temperature range of mesophase existence with taking into account the peculiarities of the molecular structure of components. The dependence of the ordering parameter and activation energy on the temperature and composition of an LC system are obtained and investigated by analysis of EPR spectra based on the spin relaxation theory. The values of the local viscosity of the systems are estimated within the framework of the hydrodynamic model. The appearance of induced and reentrant mesophases is established. It is shown that in NLC systems with non-center-symmetric molecules the correlation between the activation energy and thermostability should be inverse to that in the Maier-Saupe model.     

Structure of acoustic domains in a cholesteric liquid crystal under the effect of ultrasound

On the basis of the nonequilibrium hydrodynamics of a cholesteric liquid crystal (CLC), the properties of acoustic domains arising in a CLC layer under a normally incident ultrasonic wave are considered. The geometry and size of domains and the degree of deformation of the CLC structure are analyzed by the perturbation method for sound intensities exceeding the threshold value. Scaling dependences of the domain size and the deformation amplitude on the intensity of external action and the preliminary tension of the layer are plotted. It is shown that, as the effect advances beyond the threshold, the size of domains decreases.     

Investigation of the influence of external electric field on the anisotropy of nematic liquid crystals by X-Ray interferometry

The influence of external electric field on the anisotropy of a layer of nematic liquid crystal (NLC) was investigated by means of X-Ray interferometric method. It was shown that in Xray range of the spectrum NLC E7 is a positive anisotropic medium. The decrements of a layer of NLC subjected to the electric field were calculated. As a result of reorientation of NLC molecules perpendicular to the cell substrate, the decrements prove equal for cases when the optical axis of a sample was initially perpendicular or parallel to the X-ray beam polarization vector.     

Initiation of the formation of chiral strings: Dimension of formation domain,microstructure, and nucleation mechanism

The initiation of the formation of strings in low-concentration chiral solutions was experimentally studied. The domains of initiation were classified according to dimensions: (0–3)D. The dipole-dipole mechanism of the formation of anisotropic nucleation in a mesophase was considered. The role of dispersion interactions in the assembly of supramolecular stacks was estimated at ～0.3–0.4 eV. The threshold values of string formation in chiral mixtures were determined. A phenomenon of the formation of paired strings was established and characterized.     

Experimental verification of the theory of electrohydrodynamic instability in nematic liquid crystals

Williams domains threshold voltage are given as functions of dielectric and conductivity anisotropy. Experimental data confirm the theory of electrohydrodynamic (EHD) instability in nematic liquid crystals (NLC) based on Helfrich's model and taking into account the boundary conditions.     

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.     

Some features of second harmonic generation in the nematic liquid crystal 5CB in the pulsed-periodic electric field

Second harmonic generation (SHG) induced by an external pulsed-periodic electric field in a 5CB nematic liquid crystal (NLC) is investigated. Deformation of the NLC under this field is analyzed, and a possibility of its accumulation is established. The observed SHG features are due to the domains with quite different director orientations emerging in the NLC.     

Identification criterion for the mechanisms of orientation instability of cholesteric liquid crystals in ultrasonic fields

An identification criterion is proposed and experimentally substantiated for the two independent acoustic mechanisms (vortex and relaxation ones) governing the formation of a system of two-dimensional domains in a planar layer of a cholesteric liquid crystal in the frequency range where the sound wavelength is greater than the step of the cholesteric circuit. The results of measuring the critical compression amplitude in mesophase layers with thicknesses from 40 to 240 μm are presented for frequencies lower and higher than the frequency equal to the inverse relaxation time of the orientation order parameter in a cholesteric liquid crystal. The data are obtained for diluted solutions of cholesterilchloride in a nematic liquid crystal with a circuit step varied from 2 to 30 μm. The concept of the binary nature of the acoustic mechanism that causes destabilization of the planar texture of a cholesteric liquid crystal in the frequency range under consideration is experimentally verified.     

Linear domains induced in nematic liquid crystals by an acoustohydrodynamic action

Linear domains formed in a planar nematic liquid crystal layer under the effect of an oscillating hydrodynamic flow induced by audio-frequency shear oscillations are studied for an arbitrary angle between the flow velocity and the director describing the orientation of mesophase molecules in the layer. It is demonstrated that, in addition to the previously observed domains with a spatial period comparable to or greater than the layer thickness, spatially modulated structures with a period much smaller than the layer thickness are formed. The existence of the region of critical angles characterized by the most pronounced influence of the angle value on the distortion pattern of the planar layer is revealed.     

Optical model of transient light scattering in ferroelectric liquid crystals

A static optical model is developed for the effect of field-induced transient scattering on coherent light transmission through ferroelectric liquid crystals. Scattering processes are described by introducing an optically anisotropic medium containing scatterers (transient domains). The results presented in the paper are obtained for a plane parallel layer of ferroelectric liquid crystals with a planar helicoidal structure under normal illumination with a linearly polarized plane wave. An analysis is presented of the coherent transmittance of the layer in static applied electric fields.     

Oscillatory magnetoresistance in the field-induced SDW state in (TMTSF)2 ClO4

The transverse magnetoresistance of (TMTSF)₂ ClO₄ was measured in pulsed high magnetic fields up to 35 T at T = 1.8−-4.2K for studying the field-induced spin density wave (FISDW) phase. In addition to a structure corresponding to the threshold field B_th (_T) of the FISDW phase, we found two temperature dependent structures in higher field range at 15–17 and 22–25 T, respectively. Moreover, Shubnikov-de Haas type oscillations with smaller period were also observed superposing on the structures. It was found that the peak positions of the oscillation are independent of temperature, whereas their amplitude changes discontinuously at the structures.     

Transient times and multiplex behavior of nematic liquid crystals in the electric field

The change of the optical axis in a nematic liquid crystal (NLC) is connected with a macroscopic material flow (backflow), therefore two basic equations (force balance and torque balance equation) have to be discussed to get the transient behavior of the NLC in an electric field. For a homogeneous and a homeotropically aligned NLC the transient times are inversely proportional to the square of the sample thickness and inversely proportional to the normalized voltage (U/U ₀)²?1.U ₀ is the threshold voltage. The agreement between the experimental and the theoretical results is quite good. From the calculated formula the possible duty-cycle for multiplexing a NLC can be estimated.     

Parameters of LC molecules’ movement measured by dielectric spectroscopy in wide temperature range

Dielectric properties of a nematic liquid crystal (NLC) mixture ZhK-1282 were investigated in the frequency range of 10²–10⁶Hz and a temperature range of ?20 to 80?°С. On the basis of the Debye’s relaxation polarization model dielectric spectra of temperature dependence of the orientational relaxation time τ and the dielectric strength δe were numerically approximated at the parallel orientation of a molecular director relative to alternating electric field. Influence of ester components in the mixture plays crucial role in relaxation processes at low temperature and external field frequency. The activation energy of the relaxation process of a rotation of molecules around their short axis was measured in a temperature interval of ?20 to ?+15?°С in which the dispersion of a longitudinal component of the dielectric constant takes place. The energy of potential barrier for polar molecules rotation in the mesophase was calculated. The value of the transition entropy from the nematic to isotropic phase was obtained from this calculation. The values of the coefficient of molecular friction and rotational diffusion were obtained by different methods. The experimental data obtained are in a satisfactory agreement with the existing theoretical models.     

Electrical Freedericksz transitions in nematic liquid crystals containing ferroelectric nanoparticles

A new theoretical approach was elaborated to explain the contradictions reported in many papers about the electric threshold for Freedericksz transition in nematic liquid crystal (NLC) with ferroparticles additives. The free energy density of the mixture was estimated and the contributions of the interaction energy of NLC molecules with ferroparticles surface were calculated. Experimental results for 5CB-BaTiO₃ mixture are given.     

向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

Topological structure in polarization resolved conoscopic patterns for nematic liquid crystal cells

We investigate the polarization structure of coherent light, produced by a convergent light beam transmitted through nematic liquid crystal (NLC) cells with different director configurations. Employing solutions to the transmission problem for the case when plane wave propagates through an anisotropic layer, we analyze the arrangement of the topological elements, such as polarization singularities (C points with circular polarization and L lines with linear polarization), saddle points and extrema of polarization azimuth. We observe transformations of the topological structure under the variation of the incident light ellipticity and represent it by corresponding trajectories of topological elements in three-dimensional space. For the cells with uniform and non-uniform director configuration we describe the processes of creation/annihilation of C point pairs, which can be controlled precisely in the case of the cell with non-uniform director. Our experimental measurements for the homeotropically oriented NLC cells are in good agreement with the theoretical predictions.     

On the threshold characteristics of the flexoelectric domains arising in a homogeneous electric field: The case of anisotropic elasticity

Precise solutions for the threshold voltage U_c and wave number q_c that feature the appearance of longitudinal flexoelectric domains of Vistin’-Pikin-Bobylev at strong anchoring have been derived. Based on the formulated expressions, we present and analyze computer calculations for a planar nematic layer with anisotropic elasticity and both negative and positive dielectric anisotropy under the action of a homogeneous flexoelectrically deforming d.c. electric field. The obtained relations allow a selection of particular values of physical parameters, in order to improve the performance of devices exploiting flexoelectrcity in nematics.