Retrieval of order parameters of a monolayer of liquid-crystal droplets with weak anchoring

An optical method is proposed for extracting the order parameters of single-layer polymer-dispersed liquid-crystal film (monolayer) containing bipolar liquid-crystal droplets under weak anchoring conditions. The method is based on an analysis of the coherent transmittance of a monolayer irradiated with a normally incident linearly polarized plane wave. The method is used to retrieve the order parameter of a spherical liquid-crystal droplet and the order parameter of a monolayer consisting of such droplets as functions of the applied electric field. The effect of inaccuracy of the input values of monolayer parameters on the error of retrieval of the order parameters is examined. The method can be used to determine the refractive index of the polymer binder, as well as other parameters of the film. It provides a tool for solving the inverse scattering problem with field-dependent or field-independent droplet order parameter.     

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal

We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θ _s ≤ 8°).     

Study on the algorithm of computational ghost imaging based on discrete fourier transform measurement matrix

We have developed a model and realized an algorithm for the calculation of the coefficient of coherent (direct) transmission of light through a layer of liquid crystal (LC) droplets in a polymer matrix. The model is based on the Hulst anomalous diffraction approximation for describing the scattering by an individual particle and the Foldy-Twersky approximation for a coherent field. It allows one to investigate polymer dispersed LC (PDLC) materials with homogeneous and inhomogeneous interphase surface anchoring on the droplet surface. In order to calculate the configuration of the field of the local director in the droplet, the relaxation method of solving the problem of minimization of the free energy volume density has been used. We have verified the model by comparison with experiment under the inverse regime of the ionic modification of the LC-polymer interphase boundary. The model makes it possible to solve problems of optimization of the optical response of PDLC films in relation to their thickness and optical characteristics of the polymer matrix, sizes, polydispersity, concentration, and anisometry parameters of droplets. Based on this model, we have proposed a technique for estimating the size of LC droplets from the data on the dependence of the transmission coefficient on the applied voltage.     

Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

Phase change of a plane wave propagating through polymer dispersed liquid crystal film with nanosized droplets

The coherent field propagation in a polymer dispersed liquid crystal layer is described using a method based on the Foldy-Twersky integral equation for the vector case. Expressions for a polarization-independent phase shift and the coherent transmission coefficient of such a layer containing nanodimensional nematic liquid crystal droplets are obtained. Theoretical results for the phase shift are compared to the available experimental data.     

Coherent transmittance quenching effect in thin films of polymer-dispersed ferroelectric liquid crystals

The conditions for vanishing of the coherent component of radiation transmitted through a monolayer of polymer-dispersed ferroelectric liquid-crystal droplets are found. The influence of polydispersity and disorientation of axes of semispherical droplets on the conditions of interference quenching is studied. It is shown that these factors cause a change in the average size and concentration of droplets for which the quenching effect is realized. Data on the contrast ratio and the modulation depth of a monolayer, determined from the coherent component of transmitted radiation, are presented. It is shown that these characteristics can be considerably improved under quenching conditions for a film of polymer-dispersed ferroelectric liquid-crystal droplets.     

Random lasing in dye-doped polymer dispersed liquid crystal film

A dye-doped polymer-dispersed liquid crystal film was designed and fabricated,and random lasing action was studied.A mixture of laser dye,nematic liquid crystal,chiral dopant,and PVA was used to prepare the dye-doped polymer-dispersed liquid crystal film by means of microcapsules.Scanning electron microscopy analysis showed that most liquid crystal droplets in the polymer matrix ranged from 30 μm to 40 μm,the size of the liquid crystal droplets was small.Under frequency doubled 532 nm Nd:YAG laser-pumped optical excitation,a plurality of discrete and sharp random laser radiation peaks could be measured in the range of 575–590 nm.The line-width of the lasing peak was 0.2 nm and the threshold of the random lasing was 9 m J.Under heating,the emission peaks of random lasing disappeared.By detecting the emission light spot energy distribution,the mechanism of radiation was found to be random lasing.The random lasing radiation mechanism was then analyzed and discussed.Experimental results indicated that the size of the liquid crystal droplets is the decisive factor that influences the lasing mechanism.The surface anchor role can be ignored when the size of the liquid crystal droplets in the polymer matrix is small,which is beneficial to form multiple scattering.The transmission path of photons is similar to that in a ring cavity,providing feedback to obtain random lasing output.     

Polarization of light transmitted through a polymer film with nanosized droplets of liquid crystal

A method is developed for analyzing the state of polarization of a plane wave transmitted through a polymer-dispersed liquid-crystal (PDLC) film with nanosized liquid-crystal (LC) droplets. This method is based on the anisotropic-dipole approximation for describing scattering by a separate droplet and on the Foldy-Twersky approximation for describing propagation of light in a film. Equations are obtained that relate the ellipsometric parameters of coherent (direct) light transmitted through a PDLC film to the order parameters that characterize the morphological and structural properties of the film. Elliptic and circular polarizations and the rotation of the plane of polarization of a wave transmitted through a film are investigated under the normal illumination of the PDLC film by a linearly polarized plane wave. The order parameters of the PDLC film are determined as a function of a control field under the transition from a partially ordered structure of optical axes of LC droplets to a homeotropic structure.     

Coherent transmission and reflection of a two-dimensional planar photonic crystal

A method for modeling the radial distribution function for particles of a two-dimensional planar photonic crystal in the form of a monolayer of spatially ordered monodisperse spherical particles is proposed. The coherent transmission and reflection coefficients for layers under normal illumination are calculated in the quasi-crystalline approximation of the multiple wave scattering theory. The dependence of the coherent transmission and reflection of the layer on the degree of ordering of the spherical particles is investigated. The influence of the long-range order on the coherent transmission and reflection coefficients for layers with triangular, square, and hexagonal lattices is estimated. Monolayers of weakly absorbing dielectric and strongly absorbing metallic particles are considered.     

Direct computation of the twist elastic coefficient of a nematic liquid crystal via Monte Carlo simulations

A direct method for the computation of the twist Frank elasticity coefficient of a nematic liquid crystal is presented. The method, suitable for numerical calculations, is tested and applied in Monte Carlo simulations for a model system. The dependence of the elasticity coefficient on the temperature and density and its relation to the nematic order parameter are analyzed and discussed.     

Small-angle light scattering symmetry breaking in polymer-dispersed liquid crystal films with inhomogeneous electrically controlled interface anchoring

We have described the method of analyzing and reporting on the results of calculation of the small-angle structure of radiation scattered by a polymer-dispersed liquid crystal film with electrically controlled interfacial anchoring. The method is based on the interference approximation of the wave scattering theory and the hard disk model. Scattering from an individual liquid crystal droplet has been described using the anomalous diffraction approximation extended to the case of droplets with uniform and nonuniform interface anchoring at the droplet–polymer boundary. The director field structure in an individual droplet is determined from the solution of the problem of minimizing the volume density of the free energy. The electrooptical effect of symmetry breaking in the angular distribution of scattered radiation has been analyzed. This effect means that the intensities of radiation scattered within angles +θ _s and–θ _s relative to the direction of illumination in the scattering plane can be different. The effect is of the interference origin and is associated with asymmetry of the phase shift of the wavefront of an incident wave from individual parts of the droplet, which appears due to asymmetry of the director field structure in the droplet, caused by nonuniform anchoring of liquid crystal molecules with the polymer on its surface. This effect is analyzed in the case of normal illumination of the film depending on the interfacial anchoring at the liquid crystal–polymer interface, the orientation of the optical axes of droplets, their concentration, sizes, anisometry, and polydispersity.     

Preparation and characterization of polymer-dispersed liquid crystal (PDLC) with different cation exchange capacity (CEC) clays

This work describes the morphology and electro-optical properties of polymer-dispersed liquid crystals (PDLCs). These composites consist of the nematic liquid crystal E7, filled with different types of inorganic nanoparticles in Norland optical adhesive (NOA65) polymer matrices. Natural clays CL120 (trade name: CN-C34 with higher cation exchange capacity CEC) and CL42 (trade name: PK802 with lower CEC) are used as various inorganic nanofillers. Wide angle X-ray diffraction was exploited to examine the dispersion of the inorganic nanoparticles in PDLCs. The morphology of liquid crystal droplets in PDLCs is investigated by the field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) as well. Transmittance vs. applied voltage is measured to study the electro-optical properties and the response times of the PDLCs. It is suggested that doping with inorganic nanoparticles in PDLCs effectively reduces the driving voltage and improves the electro-optical characteristics.     

聚合物分散液晶增强散射的理想模型

介绍聚合物分散液晶技术新进展,应用光学原理对液晶微滴增强散射问题进行研究,提出液晶微滴最佳间距判据,液晶微滴间隔处聚合物材料光学厚度满足增反膜条件;提出液晶微滴最佳直径判据,液晶微滴直径满足双折射相消干涉条件;提出聚合物与液晶最佳配比的计算方法,应用晶体学原子堆积致密度知识给出最佳配比的计算结果.建立一种聚合物分散液晶增强散射的理想模型,进而从更基本的光学原理出发对异常散射理论模拟计算和实验上确定的液晶微滴最佳直径和聚合物与液晶最佳配比问题给出十分简单明确的理论解释.     

Coherent transmittance of a polymer dispersed liquid crystal film in a strong field: Effect of correlation and polydispersity of droplets

A method for computing the coherent transmittance of a film of polymer dispersed nematic liquid crystals aligned by an external field in the case of an oblique incidence of light is developed, and a theoretical analysis of this quantity is performed. The effects of close packing are considered in terms of the interference approximation of the theory of multiple wave scattering. The correlation arising in the spatial distribution of polydisperse droplets is taken into account by introducing partial distribution functions. It is shown that an increase in the concentration of liquid crystal droplets is accompanied by an increase in the coherent transmittance of the film.     

Depth-enhanced three-dimensional-two-dimensional convertible display based on modified integral imaging

A depth-enhanced three-dimensional-two-dimensional convertible display that uses a polymer-dispersed liquid crystal based on the principle of integral imaging is proposed. In the proposed method, a lens array is located behind a transmission-type display panel to form an array of point-light sources, and a polymer-dispersed liquid crystal is electrically controlled to pass or to scatter light coming from these point-light sources. Therefore, three-dimensional-two-dimensional conversion is accomplished electrically without any mechanical movement. Moreover, the nonimaging structure of the proposed method increases the expressible depth range considerably. We explain the method of operation and present experimental results.     

A computational approach to the optical Freedericksz transition

The optical Freedericksz transition in a homeotropic nematic liquid crystal cell is modeled using a solver which combines direct solution of Maxwell’s equations with a relaxation algorithm for the liquid crystal director. We find that even in the equal elastic constant case the continuous optical Freedericksz transition can be driven first order. For films in which the optical retardation of the extraordinary wave is sufficiently large, a whole set of discontinuous jumps in transmission coefficient can occur. These jumps correspond to the existence of optical resonances in the liquid crystal film. Our results agree in the short wavelength limit with paraxial approximation calculations, and provide a strong test of the FDTD method for anisotropic materials such as liquid crystals.     

Interference quenching of light transmitted through a monolayer film of polymer-dispersed nematic liquid crystal

A theoretical study of light transmission through a polymer with a monolayer ensemble of bipolar nematic droplets under applied electric film is performed. The possibility of interference quenching of monochromatic coherent light directly transmitted through the film is predicted theoretically and realized in the experiment.     

Phase transition in ellipsoidal droplets of nematic liquid crystals

We developed a simple method for the calculation of the director field distribution in the droplets of nematic liquid crystals (LCs) of any shape, allowing for the interaction of LCs with the droplet surface, as well as the influence of constant electric field. In contrast to different approaches, the approach that is developed in the present paper does not require any simplifying suppositions about the structure of the LC director field. The elastic-continuum theory is used, complemented with the possibility of consideration of point and linear defects. Calculations are performed using the Monte Carlo method on a simple grid. The triangulation technique is used to take the boundary conditions of droplets of a complex shape into account. The developed approach can be used for investigation of the properties of polymer-dispersed liquid crystals (PDLCs). The topological phase transitions in the nematic LC 4-cyano-4′-pentyl-biphenyl (5CB) in spherical and ellipsoidal droplets are investigated in this paper.     

Colossal light-induced refractive-index modulation for neutrons in holographic polymer-dispersed liquid crystals

We report strong diffraction of cold neutrons from an only 30 micro m thick holographic polymer-dispersed liquid crystal (H-PDLC) transmission grating. The light-induced refractive-index modulation for neutrons is about 10(-6), i.e., nearly 2 orders of magnitude larger than in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs a promising candidate for fabricating neutron-optical devices.