Nematic Liquid Crystal Film Research Articles

Enhanced linearly polarized lasing emission from nanoimprinted surface-emitting distributed feedback laser based on polymeric liquid crystals

The authors have demonstrated the enhancement of linearly polarized lasing emission intensity using a structure made by a simple fabrication process. The enhanced lasing is achieved using a nanoimprinted distributed feedback structure together with spin-coated polymeric liquid crystals. The backward linearly TE-polarized lasing emission is transformed to left-handed circularly polarized light (L-CPL) by employing a dye-doped polymeric nematic liquid crystal (PNLC) film as a (−1/4)λ[=(3/4)λ] plate. The L-CPL is effectively reflected by a L-polymeric cholesteric liquid crystal film as a reflector and transformed back to TE-polarized light by the PNLC film; as a result one-directional emission intensity is enhanced.

Characterization of the diffraction efficiency of polymer-liquid-crystal-polymer-slices gratings at all incidence angles

Recently, a novel holographic diffraction grating made of polymer slices alternated to homogeneous films of nematic liquid crystal (POLICRYPS) was realized. We study the optical performance of the POLICRYPS gratings by both numerical simulations and experiments. Characterization of the grating at normal and conical reading mount are performed. The diffraction efficiency depends strongly on the angles of incidence. Besides, the characterization of the diffraction efficiency at Bragg angle incidence is studied. A uniform high diffraction efficiency is achieved when the incident wave satisfies the Bragg condition.

High-resolution photorefractive gratings in nematic liquid crystals sandwiched with photoconductive polymer film

Photorefractive gratings with high grating resolution were observed in the 20 μm thick low-molar-mass nematic liquid crystal (NLC) cell with a separate photoconductive (PC) poly(N-vinylcarbazole) layer. An orientational grating with a grating spacing of 1.9 μm was produced. It is believed that a space–charge field with small fringe spacing forms in the PC layer and its evanescent component penetrates into the NLC layer. The penetrated evanescent field drives the NLC to reorient, and consequently the orientational grating forms. The model indicates that the modulated field exists in several hundred nanometers near the surface, and thus the orientational grating is not full of the NLC film, which is consistent with the observed phenomena of the multiple diffractions. Besides, asymmetric two-beam coupling of 11.2% was achieved for the grating with a grating spacing of 1.9 μm, and a net gain coefficient of larger than 62 cm−1 was obtained.

Fréedericksz transition in homeotropically aligned liquid crystals: a photopolarimetric characterization

AbstractThis work is aimed to the photopolarimetric characterization of the disorder evolution occurring in homeotropically aligned nematic liquid crystal films during the Fréedericksz transition. This order‐disorder transition is studied by monitoring the depolarization effects of the transmitted light. Depolarization mainly occurs because light undergoes random and local phase displacements. The measurements reveal unexpected depolarization effects at the transition, which we interpret in terms of director field unhomogeneity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of multiple circular walls on a thin film of nematic liquid crystal by laser scanning

We found that multiple circular walls (MCW) can be generated on a thin film of a nematic liquid crystal through a spiral scanning of a focused IR laser. The ratios between the radii of adjacent rings of MCW were almost constant. These constant ratios can be explained theoretically by minimization of the Frank elastic free energy of nematic medium. The director field on a MCW exhibits chiral symmetry-breaking although the elastic free energies of both chiral MCWs are degenerated, i.e., the director on a MCW can rotate clockwise or counterclockwise along the radial direction.

CAPACITIVE INTERDIGITAL SENSOR WITH INHOMOGENEOUS NEMATIC LIQUID CRYSTAL FILM

The performance of capacitive interdigital sensors involved with anisotropic and inhomogeneous nematic liquid crystal (LC) film is investigated. These sensors have potential applications in chemical and biological systems. The theory for modeling the permittivity tensor of the LC film as a function of the molecular orientation is presented. The LC film is handled as inhomogeneous material where molecules are assumed to have different orientations with respect to the frame axes. Under these conditions, fringing field capacitances as functions of the molecular deformations are calculated. Examples of modeled capacitive interdigital sensors in the present of different inhomogeneous distributions of LC films will be studied and discussed.

Soliton-like molecular deformations in a nematic liquid crystal film

We study the nature of molecular deformations in a nematic liquid crystal film with elastic energy under homeotropic boundary conditions. The deformation in terms of splay, twist and bend fields of the director axis is found to be governed by the completely integrable Davey–Stewartson-I (DS-I) equation in ( 2 + 1 ) dimensions. Using the line soliton and breather solutions of the DS-I equation, the director axis is constructed, the components of which exhibit damped spatial oscillations. However, the splay and bend fields of the director axis exhibit localized structures of deformation.

Molecular orientation in Langmuir–Blodgett films of polyfumarates carrying isopropylester and isopropylamide side chains

Molecular orientation in Langmuir–Blodgett (LB) films of poly(diisopropyl fumarate) (PDiPF) and poly(isopropyl N-isopropyl fumaramate) (PiPFAE), prepared by solution polymerization and emulsifier-free emulsion polymerization, respectively, was investigated. The π– A isotherms of both polymers indicated formation of stable Langmuir monolayers. The refractive index measurements as well as the polarized IR and UV spectroscopy of the LB films revealed that the backbones of the polymers were oriented parallel to the dipping direction of the substrate. The LB films were applied to a nematic liquid crystal alignment film, and the relationship between optical axes of the LB films and the alignment direction of the liquid crystal molecules was characterized. Utilization of PDiPF and PiPFAE as a matrix polymer for polymer dispersed liquid crystals was also examined, and a higher performance with a better contrast ratio was obtained for PiPFAE.

Orientational structure transformations caused by the electric-field-induced ionic modification of the interface in nematic droplets

The films of a polymer-dispersed nematic liquid crystal doped with an ionic surfactant were studied. The surface-anchoring modification effect caused by the local increase in the concentration of surface-active ions was observed at the polymer-liquid crystal interface under the action of an electric field. The modification of the boundary conditions leads to the transformation of the orientational structure of the nematic droplets and, as a result, to an appreciable change in their texture patterns and light-scattering efficiency at the interfaces. The monopolar director configurations (normal and curved) arising in the process of orientational structure transformations are considered and their typical textures are demonstrated. The possibility that the monopolar structure can be formed is theoretically substantiated by the computer simulation of the director field in a nematic droplet with the boundary conditions corresponding to the experiment.

Diffraction behavior of an azo-dye-doped nematic liquid crystal without applied electric field

Abstract We investigate the dependence of the two-beam coupling diffraction efficiency on the experimental parameters, and hence achieve the optimum experimental conditions for holographic recording in planar Methyl Red-doped nematic liquid crystal film without external electric field. Under the optimum experimental conditions, the diffraction efficiency of a holographic grating is improved to 21.4%, and holographic image with long-term stability is stored in the film. These results provide an experimental basis for holographic data storage in this liquid crystal film.

51.4: Broadband Quarter Wave Plate Using a Twisted Nematic Liquid Crystal Film and Two Uniaxial Films

AbstractWe designed a broadband quarter wave plate in the visible range using a twisted nematic liquid crystal film sandwiched between two compensation films. The quarter wave plate exhibits much wider bandwidth than the commercial product, which is composed of a half wave plate and a quarter wave plate.

Fullerene-containing liquid crystal spatiotemporal light modulators with surface-electromagnetic-wave-treated conducting layers

We have studied the switching characteristics of multilayer electrooptical structures comprising quartz substrates, transparent conducting layers, and an oriented nematic liquid crystal (NLC) film doped with photosensitive charge-transfer complexes based on electrooptically active organic monomer or polymer molecules and fullerenes. The transparent conducting layers treated with surface electromagnetic waves are used for the orientation of NLC molecules. The surface relief with variable permittivity, which is created by this treatment, not only ensures the alignment of NLC molecules, but also significantly influences the relaxation times of the electrooptical mesophase, thus making possible effective control over the switching times of the electrooptical light modulator within 1–1.5 ms.

Light depolarization effects during the Fréedericksz transition in nematic liquid crystals

This work is aimed to the photopolarimetric characterization of the disorder evolution occurring in homeotropically aligned nematic liquid crystal films during the electrically induced Fréedericksz transition. The molecular director dynamics and the transversal reorientation modes are investigated by the analysis of the depolarization of the light beam emerging from the sample. Our measurements reveal unexpected depolarization effects at the transition, which we interpret in terms of director field unhomogeneity and defects creation.

Chaotic Rotations Generated by Light in Nematic Liquid Crystals

Various nonlinear rotation regimes are observed in an optically excited nematic liquid crystal film under boundary conditions (for the light and the material) that are invariant by rotation. Th excitation light is circularly polarized, the intensity profile is circularly symmetric and the beam diameter at the sample location is a few times smaller than the cell thickness. A transition to chaos via quasiperiodicity is identified when the light intensity is taken as the control parameter. Transverse nonlocal effects are suggested to be the cause of the observed dynamics and a simple model consisting of a collection of coupled rotators is developed to provide a qualitative explanation. *Also at Laboratoire de Chimie-Physique-Matière et Rayonnement, Université Pierre et Marie Curie, Paris, France.

Bistable nematic liquid crystal device with flexoelectric switching

Motivated generally by potential applications in the liquid crystal display industry [8,35], and specifically by recent experimental, theoretical and numerical work [6,7,13,14,21,25,30,31], we consider a thin film of nematic liquid crystal (NLC), sandwiched between two parallel plates. Under certain simplifying assumptions, laid out in £2, we find that for monostable surfaces (i.e. only a single preferred director anchoring angle at each surface), two optically-distinct, steady, stable (equal energy) configurations of the director are achievable, that is, a bistable device. Moreover, it is found that the stability of both of these steady states may be destroyed by the application of a sufficiently large electric field, and that switching between the two states is possible, via the flexoelectric effect. Such a phenomenon could be used in NLC display devices, to reduce power consumption drastically. Previous theoretical demonstrations of such (switchable) bistable devices have either relied on having bistable bounding surfaces, that is, surfaces at which there are two preferred director orientations at the surface [7,14]; on having special (nonplanar) surface morphology within the cell that allows for two stable states (the zenithal bistable device (ZBD) [4,21], or, in the case of the Nemoptic BiNem technology [11,19], on flow effects and a very carefully applied electric field to effect the switching.

Electrooptic temperature sensor based on a Fabry-Pe/spl acute/rot resonator with a liquid crystal film

An electrooptic (EO) temperature sensor based on a Fabry-Pe/spl acute/rot (FP) resonator with a nematic liquid crystal (NLC) film is proposed. A novel compensation scheme of the EO effect allows for precise determination of temperature by monitoring the resonant wavelength of the NLC FP sensor. The proposed EO temperature sensor provides good stability, high resolution, self-calibration capability, and low driving voltage.

Light-induced chaotic rotations in nematic liquid crystals

Various nonlinear rotation regimes are observed in an optically excited nematic liquid-crystal film under boundary conditions (for the light and material) that are invariant by rotation. The excitation light is circularly polarized, the intensity profile is circularly symmetric, and the beam diameter at the sample location is a few times smaller than the cell thickness. A transition to chaos via quasiperiodicity is identified when the light intensity is taken as the control parameter. Transverse nonlocal effects are suggested to be the cause of the observed dynamics, and a simple model consisting of a collection of coupled rotators is developed to provide a qualitative explanation.

Computational modeling of texture formation and optical performance of liquid crystal films on patterned surfaces

An integrated microstructural‐optical model based on the tensorial Landau–de Gennes liquid crystal theory, the Matrix–Berreman optical model, and the finite‐difference time‐domain (FDTD) optical method is used to investigate texture formation and polarized light propagation in thin nematic liquid crystal (NLC) films for various anchoring boundary conditions mimicking surface conditions of an existing liquid crystal (LC)‐based biosensor device used to detect biological binding events. The integrated mathematical model of the optical device describes the signal generation process of the biosensor based on LC vision. The FDTD optical method predicts two important optical signatures of the transmitted polarized light: oscillations and nonsymmetric optical signals. However, the approximate Matrix–Berreman optical method cannot predict these important optical responses when strong lateral orientation gradients are present. The model predictions are found to be in good agreement with actual experimental results, and can be used to detect interfacial LC orientation due to bound biomolecules.

Two-wave mixing in a nematic liquid-crystal film sandwiched between photoconductingpolymeric layers

Degenerate two-wave-mixing effects were observed in hybrid cells of carbon-nanotubesuspension in a nematic liquid crystal sandwiched between photoconducting polymericlayers in the presence of an externally applied dc field. First-order self-diffractionefficiencies as high as 47% were achieved from Raman–Nath gratings recordedin the cells. The beam-coupling ratio increases with increasing the pump-beamintensity for a fixed dc voltage. Experimental results show that a lower polymericmolecular weight of photoconducting layers leads to a higher two-beam-coupling ratio.

A PHENOMENOLOGICAL APPROACH TO NORMAL FORM MODELING: A CASE STUDY IN LASER INDUCED NEMATODYNAMICS

An experimental setting for the polarimetric study of optically induced dynamical behavior in nematic liquid crystal films presented by G. Cipparrone, G. Russo, C. Versace, G. Strangi and V. Carbone allowed to identify most notably some behavior which was recognized as gluing bifurcations leading to chaos. This analysis of the data used a comparison with a model for the transition to chaos via gluing bifurcations in optically excited nematic liquid crystals previously proposed by G. Demeter and L. Kramer. The model of these last authors, relying on the original model for chaos by cascade of gluing bifurcations proposed by A. Arneodo, P. Coullet and C. Tresser about twenty years before, does not have the central symmetry which one would expect for minimal dimensional model for chaos in nematics in view of the time series near the gluing bifurcation. What we show here is that the simplest truncated normal forms for gluing with the appropriate symmetry and minimal dimension do exhibit time signals that are embarrassingly similar to the ones that could be found using the above-mentioned experimental settings. It so happens that the gluing bifurcation scenario itself is only visible in limited parameter ranges, and that a substantial aspect of the chaos that can be observed is due to other factors. First, out of the immediate neighborhood of the homoclinic curve, nonlinearity can produce expansion which easily produces chaos when combined with the recurrence induced by the homoclinic behavior. Also, pairs of symmetric homoclinic orbits create extreme sensitivity to noise, so that when the noiseless approach to attracting homoclinic pairs contains a rich behavior, minute noise can transform the complex damping into sustained chaos. As Leonid Shilnikov has taught us, combining global considerations and local spectral analysis near critical points is crucial to understand the phenomenology associated to homoclinic bifurcations in dissipative systems. We see here on an example how this helps construct a phenomenological approach to modeling experiments in nonlinear dissipative contexts.