Nematic Liquid Crystal Film Research Articles

Bifurcation analysis of optically induced dynamics in nematic liquid crystals: elliptical polarization at normal incidence

We present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by an elliptically polarized beam at normal incidence with the intensity and the polarization state of light as the control parameters. The asymmetry arising from the elliptical polarization of the excitation lightwave is shown to affect dramatically the dynamics, and various new dynamical behaviors are reported: (i) quasi-periodic rotations for almost circular polarization; (ii) a discontinuous transition, identified as a homoclinic bifurcation, to a largely reoriented state over a large range of ellipticity values; (iii) oscillations associated with large reorientation; and (iv) optical multistability between several distinct dynamical regimes.

Bifurcation analysis of optically induced dynamics in nematic liquid crystals: circular polarization at normal incidence

We present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by an elliptically polarized beam at normal incidence with the intensity and the polarization state of light as the control parameters. The asymmetry arising from the elliptical polarization of the excitation lightwave is shown to affect dramatically the dynamics, and various new dynamical behaviors are reported: (i) quasi-periodic rotations for almost circular polarization; (ii) a discontinuous transition, identified as a homoclinic bifurcation, to a largely reoriented state over a large range of ellipticity values; (iii) oscillations associated with large reorientation; and (iv) optical multistability between several distinct dynamical regimes.

Computational modeling of the propagation of light through liquid crystals containing twist disclinations based on the finite-difference time-domain method

The finite-difference time-domain (FDTD) method is used to compute propagation of light through textured uniaxial nematic-liquid crystal (NLC) films containing various types of twist disclination (defect) lines. Computational modeling by the FDTD method provides an accurate prediction of the optical response in multidimensional and multiscale heterogeneities in NLC films in which significant spatial optic axis gradients are present. The computations based on the FDTD method are compared with those of the classic Berreman matrix-type method. As expected, significant deviations between predictions from the two methods are observed near the twist disclination line defects because lateral optic axis gradients are ignored in the matrix Berreman method. It is shown that the failure of Berreman's method to take into account lateral optic axis gradient effects leads to significant deviations in optical output. In addition, it is shown that the FDTD method is able to distinguish clearly different types of twist disclination lines. The FDTD optical simulation method can be used for understanding fundamental relationships between optical response and complex NLC defect textures in new liquid-crystal applications including liquid-crystal-based biosensors and rheo-optical characterization of flowing liquid crystals.

Comment on “Numerical investigation of optically induced director oscillations in nematic liquid crystals”

A nonlinear optical system based on nematic liquid crystals film was numerically investigated by Demeter and Kramer in [Phys. Rev. E 64, 020701(R) (2001)]. They show that the uncommon route to chaos via a cascade of homoclinic gluing bifurcations predicted by a perturbative approach surprinsingly does not exist when the problem is solved exactly. On the other hand, the system still exhibits a secondary instability but its threshold is found to be much higher than previously reported. We clarify the reason for the failure of the perturbative approach and its limits of applicability. Moreover, the significant increase of the secondary instability threshold can be understood from a nonrealistic amplification of light intensity introduced by the perturbative model.

Multiguide directional coupler using switchable liquid-crystalline optical channels

A multiguide directional coupler with a gratinglike indium-tin-oxide electrode on a planar nematic liquid-crystal film is studied. The linearly polarized beam can be coupled into adjacent channels with the periodic modulation of the refractive index by applying a periodic electric field. The intensity distribution of coupling in the transverse direction varies with the distance of beam propagation. The coupling effects, which depend on the polarization of the incident beam and the temperature of the liquid crystals, are also discussed.

Quenched disorder in a liquid-crystal biosensor: Adsorbed nanoparticles at confining walls

We analyze the response of a nematic liquid-crystal film, confined between parallel walls, to the presence of nanoscopic particles adsorbed at the walls. This is done for a variety of patterns of adsorption (random and periodic) and operational conditions of the system that can be controlled in experimental liquid-crystal-based devices. We compute simulated optical textures and the total optical output of the sensor between crossed polars, as well as the correlation function for the liquid-crystal tensor order parameter; we use these observables to discuss the gradual destruction of the original uniform orientation. For large concentrations of particles adsorbed in random patterns, the liquid crystal at the center of the sensor adopts a multidomain state, characterized by a small correlation length of the tensor order parameter, and also by a loss of optical anisotropy under observation through crossed polars. In contrast, for particles adsorbed in periodic patterns, the nematic at the center of the cell can remain in a monodomain orientation state, provided the patterns in opposite walls are synchronized.

Optical Phase Conjugation and Wavefront Correction by Thin Nematic Liquid Crystal Cells

ABSTRACT We report a study of optical phase conjugation in thin nematic cells by using low intensity of the incident beams and without any external applied field. The reported effect exploits the colossal optical nonlinearity recently observed in thin nematic liquid crystals films doped with a small amount of methyl red. The high conjugated reflectivity values obtained with the analysed samples, allows successful correction of severely aberrated wavefronts of very weak light beams.

Dynamics of cross-polarization-stimulated orientation scattering in a nematic liquid-crystal film

We have studied the dynamic evolution of stimulated cross-polarization scattering in a nematic liquid-crystal film. The effect is initiated by two-beam coupling between a cw incident polarized laser and its orthogonally polarized multifrequency noise components scattered by fluctuations in the director axis. Complex time- and intensity-dependent oscillations and energy exchanges among the pump and stimulated waves were observed. Imparting an ac modulation to the film further amplified the characteristic oscillations while it suppressed the background noise. The conditions for stimulated laser generation and the observed frequency spectra of the intensity oscillations are in good agreement with a quantitative theory that accounts for the temporal-spatial dependence of the wave mixing process.

Interference and ion effects in the electro‐optical response of PDNLC films

Abstract— We have considered the interference in polymer‐dispersed nematic liquid‐crystal (PDNLC) films caused by the superposition of light beams that pass through and between the LC droplets. The relative phase retardation of the beams depends on the applied voltage, and, as a consequence, interference oscillations can occur at transmittance‐voltage curves. Typical forms of the curves for composite films with various structures are presented. Interference oscillations were revealed not only in the static electro‐optical characteristics but in the dynamic ones also. For PDLC films based on commercially available components produced without thorough purification, the combination of interference and ionic effects is observed in the dynamics of the optical response. The relaxation of transmittance due to the depolarization ionic field is presented, depending on droplet size, temperature, pulse duration, and the concentration of the ionic admixture.

Molecular field theory for nematic liquid crystal film with finite layers

The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl-Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N-I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl-Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase.

Nonlinear optical property of azo-dye doped liquid crystals determined by biphotonic Z-scan technique

This work investigates the optical Kerr property of azo-dye doped nematic liquid crystal films using the biphotonic Z-scan technique. The results indicate that the nonlinear effect measured using the Z-scan technique with a red light can be modulated or switched with the simultaneous application of a green light, because of photoisomerization and thermal effects, as determined by dynamic measurements. The former dominates in the early stage when the green light is applied, while the latter dominates in the later stage.

Mechanistic Study of the Anchoring Behavior of Liquid Crystals Supported on Metal Salts and Their Orientational Responses to Dimethyl Methylphosphonate

This paper reports an investigation of the orientational behavior of films of nematic liquid crystal (LC) 4‘-pentyl-4-biphenyl-carbonitrile (5CB) supported on a variety of metal perchlorate salts and their orientational response to dimethyl methylphosphonate (DMMP). It is found that 5CB supported on perchlorate salts comprised of metal ions with high electron affinities (>15.64 eV), such as Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+, and Fe3+, assumes a homeotropic orientation whereas 5CB supported on salts of metal ions with low electron affinities, including Mn2+, Mg2+, Ag+, Cs+, and Na+, assumes a planar or tilted orientation. The role of the nitrile group of 5CB in determining the orientational behavior of the LC via its coordination with the metal ions is supported by polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS). A new nitrile stretching peak is measured with Cu2+, Ni2+, and Cd2+ (which cause homeotropic anchoring) but not with Ag+, Na+, and Mg2+ (which cause tilted or planar orientations of 5CB). Upon exposure to 100 ppb of DMMP, the 5CB supported on the perchlorate salts of Al3+, Eu3+, and Cu2+ undergoes a homeotropic-to-planar anchoring transition while 5CB supported on the other metal ions shows no measurable orientational response to DMMP. These results, when combined, support the view that the orientational response of 5CB to DMMP on metal perchlorate salts results from a competitive binding of 5CB and DMMP to the cations of the metal salts.

Evolution of a thin film of nematic liquid crystal with anisotropic surface energy

We use lubrication theory on the flow equations for nematic liquid crystals to derive a simple model describing the evolution of the film height under gravity, in the case of finite surface “anchoring energy” at the free surface and at the rigid substrate. This means that the molecules of the nematic have a preferred alignment at interfaces, modelled by a single-well potential surface energy (first introduced by Rapini & Papoular [9]). This paper generalises the earlier work of Ben Amar & Cummings, in which the orientation of the nematic liquid crystal molecules is effectively specified at both surfaces (strong anchoring; isotropic surface tension). Additional terms, analogous in some sense to Marangoni terms, are introduced into the PDE governing the film height evolution. The stability of the derived model is considered, and stability criteria are presented and discussed. The existence of static, drop-like solutions to the model is also briefly considered.

Thermodynamic Model of Surfactant Adsorption on Soft Liquid Crystal Interfaces

The Gibbs adsorption isotherm for planar liquid crystal/fluid interfaces is derived using the anisotropic Gibbs-Duhem equation. The Gibbs adsorption isotherm for planar interfaces is used to analyze the adsorption-driven orientation transition in aqueous solutions of anionic surfactants in contact with rodlike uniaxial nematic liquid crystal films. In qualitative agreement with experiments, the model predicts that, as the surfactant concentration increases, the tangential (planar) average molecular orientation of the liquid crystal with respect to the interface undergoes a transition to a normal (homeotropic) orientation. The anchoring coefficient or strength of anisotropic component of the interfacial tension is shown to depend on the surfactant's concentration. Analyzing the response to addition of a co-cation, the model reveals that, as the fractional coverage of the surfactant's chains increases, the interpenetration of liquid crystal molecules between the adsorbed surfactant tails promotes the orientation transition; at even higher surfactant chain concentrations, interpenetration is hindered because of lack of available space and a random surface orientation emerges. Thus, for aqueous surfactant solutions in contact with nematic liquid crystals, increasing the surfactant concentration leads to the following interfacial liquid crystal orientation transition cascade, planar orientation --> homeotropic orientation --> random orientation, which can lead to new sensor capabilities and surface structuring processes.

The geometrical optics approach for multidimensional liquid crystal cells

We present an approach based on the geometrical optics approximation (GOA) for analysis of liquid crystal cells whose director varies in more than one spatial dimension (multidimensional liquid crystal cells). The GOA is applied to calculate light transmittance and far field diffraction patterns for two- and three-dimensional nematic liquid crystal films. Important features of the GOA, such as a method of eliminating non-convergence problems that can occur during the iterative numerical solution of the equations for the amplitudes of the electromagnetic field, are described. We compare the results obtained from the GOA with those produced by the quasi-one-dimensional Jones calculus and the beam propagation method, where the latter is applicable. It was found that the refraction (or ray bending) effects, produced by the GOA, are more important than effects of diffraction and light scattering, which means that the GOA (unlike the Jones calculus) is accurate for the considered type of liquid crystal cells, whose director varies on the micron scale. The GOA is about as fast as the Jones method and is applicable for calculating optical properties of liquid crystal cells with any number of dimensions of director variations.

Quasistatic electric-field-modulated optical pattern transition in a thin nematic liquid-crystal film with a single feedback mirror

We present here the pattern formation in the transverse profile of a continuous-wave laser beam passing through a parallel planar-aligned nematic liquid crystal (NLC) film biased by a quasistatic electric field and then reflected back to the sample cell by a single planar mirror. The effect of the biasing voltage is studied through the ability to change the nonlinearity by modulating the orientation of the NLC molecules electrically. By suitably modulating the quasistatic electric field the optical pattern transition from the hexagon to the roll is achieved. The pattern transition can be explained by the linear stability analysis for the nonlinear liquid crystal film with optical feedback.

Influence of Frank Elastic Constant Anisotropy on Optical Pattern Formation Using Nematic Liquid Crystal Films

We investigate the influence of the Frank elastic constant anisotropy on optical pattern formation using nematic liquid crystal (NLC) films in one-feedback-mirror systems. The governing diffusion-like equation for the parallel planar-aligned and the vertical-aligned NLC films with an externally biasing quasi-static electric field under the hard boundary assumption is derived. The intrinsic Frank elastic constant anisotropy induces the anisotropy of the diffusion length, which results in the anisotropic distribution of the threshold intensity for optical pattern formation. The effects of the elastic constant anisotropy on optical pattern formation are studied for the first time and the obtained numerical results can reasonably explain the optical patterns that are formed.

Voltage-controlled multiguide directional coupler formed in a planar nematic liquid crystal film

A voltage-controllable multiguide directional coupler is designed with a nematic liquid crystal cell, comprising a grating-like indium-tin-oxide electrode. An incident polarized laser beam can couple into adjacent channels by periodic distribution of the refractive index. This capability is attributed to the reorientation of the periodic liquid crystal caused by the application of a voltage. The coupling variation of the optical power in the transverse direction parallel to the waveguide surface is investigated. This work presents a promising coupling function for potential applications of optical communication devices.

Optical Compensation Method for In-plane Switching Twisted Nematic Mode

We have proposed an in-plane switching twisted nematic (IT) mode. The characteristics of the IT mode are the wide viewing angle, the narrow cell gap error tolerance, and the relative fast response time. However, an IT mode needs a high electric field to obtain a high contrast ratio. To improve this situation, we propose a compensation method using a twisted nematic liquid crystal (LC) film or a twisted discotic LC film. Specifically, the latter can achieve the wide viewing angle, the high contrast ratio and the decrease in the driving voltage. In this paper, a compensation method using a film is suggested and evaluated by numerical calculation.

Molecular reorientation based diffraction in dihydropyridin doped nematic liquid crystal film

The change of refractive index, originated by photo induced molecular reorientation, brings about a significant diffraction capability in a liquid crystal system doped by 1,4-dihydropyridin. Results of a grating diffraction experiment are reported for the constructed sample. The accessible diffraction efficiency is roughly of the order of 10% under the optimum conditions and analyzed results propose this novel system to be appropriate and promising for the excitation wavelength of an He–Cd laser.