Nematic Liquid Crystal Film Research Articles

Undamped Nonlinear Oscillations of the Molecular Director Induced in a Liquid Crystal Film by the Competition between the Orbital and Spin Angular Momentum Transfer from Light

Undamped nonlinear oscillations of the molecular director in a homeotropically aligned nematic liquid crystal film have been observed in the field of a normally incident laser beam with elliptical transverse cross-section and linear polarization along the direction orthogonal to the ellipse major axis. This phenomenon may be brought back to the competition between the longitudinal torques originating from the spin and orbital angular momenta the incident beam transfer to the sample.

Inhomogeneous Fréedericksz transition in nematic liquid crystals.

A theoretical and experimental analysis of a spatial instability developing in a homeotropically aligned nematic liquid crystal film is presented. The explanation for the existence of this instability is supplied through an amplitude equation. This model, which is valid in the vicinity of the Fréedericksz transition, assumes a strong difference between the nematic elastic constants. The first report of such an instability observed in the conditions accounted for by our model, was provided by Cladis and Torza [J. Appl. Phys. 6, 584 (1975)]. We repeated these experiments in order to confirm the validity of the model. Although carried out far from the Fréedericksz transition, these latter show a good qualitative agreement with the theoretical predictions. The nonlinear analysis allows to understand the dynamical behavior of an interface separating domains of stripes through the occurrence of a zigzag instability.

Beam coupling in nanotube-doped liquid-crystal films

Diffraction gratings written in films of nematic liquid crystals doped with multiwall carbon nanotubes were investigated by measurements of exponential beam-coupling coefficients. These phase gratings were induced by the interference modulation of two coherent optical beams, in conjunction with an externally applied dc field. Systematic and consistent results of the gain properties indicate that the bserved coherent-beam amplification depends strongly on the pump-to-probe intensity ratio.

A rheological theory for liquid crystal thin films

A macroscopic rheological theory for compressible isothermal nematic liquid crystal films is developed and used to characterize the interfacial elastic, viscous, and viscoelastic material properties. The derived expression for the film stress tensor includes elastic and viscous components. The asymmetric film viscous stress tensor takes into account the nematic ordering and is given in terms of the film rate of deformation and the surface Jaumann derivative. The material function that describes the anisotropic viscoelasticity is the dynamic film tension, which includes the film tension and dilational viscosities. Viscous dissipation due to film compressibility is described by the anisotropic dilational viscosity. Three characteristic film shear viscosities are defined according to whether the nematic orientation is along the velocity direction, the velocity gradient, or the unit normal. In addition the dependence of the rheological functions on curvature and film thickness has been identified. The rheological theory provides a theoretical framework to future studies of thin liquid crystal film stability and hydrodynamics, and liquid crystal foam rheology.

Patterns in hybrid nematic liquid-crystal films: topography and topology.

Spontaneous pattern formation in hybrid nematic liquid-crystal films on glycerol was studied with both polarized transmission and unpolarized monochromatic reflected light microscopy. These observations reveal that the patterns that are found in 1 to 10-microm-thick films are due to a combination of film topography, gradients in film thickness that act as an aligning field, and topology, connectivity requirements on the vector field. The patterns studied include cellular patterns, disclinations connected by 2pi-walls, and 2pi-walls that narrow to a point at the edge of the films.

Analysis of gratings induced in azo-dye doped liquid crystals

We investigate the formation of photoinduced gratings in a homeotropic film of nematic liquid crystal doped with methyl red. Using a grating translation technique, we determine the time evolution of the amplitudes and phase shifts of both index and absorption modulations. With low writing intensities, a fast grating caused by photoisomerization of the dye competes with a slower grating, originating from a reorientation of the director. This slower grating only appears if the light polarization has a nonzero projection on the director, i.e. at oblique incidence. The grating decays in the dark. We suggest that its formation may be explained by a light-induced dielectric torque, rather than a photorefractive effect. With high intensities, an even slower grating is formed at normal incidence. We stipulate that it originates from a change in the anchoring parameters caused by the adsorption of photoisomerized dyes on the surfaces. The resulting reorientation of the director is permanent.

High Contrast Light Modulator Based on PDNLC Monolayer

Interference effects in polymer dispersed nematic liquid crystals (PDNLC) films with monolayer arrangement of the LC droplets are investigated. Transmittance characteristics were measured both in dependence on the collection angle and incidence angle. The method of increasing of contrast of light modulators based on such films is discussed.

All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition–subtraction

We demonstrate two simple yet efficient all-optical image-processing techniques that use nonlinear photosensitive dye-doped nematic liquid-crystal films, namely, edge enhancement and image addition-subtraction operations. These films require no external bias and function at much lower optical powers and shorter response times than other conventional methods.

Holes and disclinations in hybrid nematic liquid crystal films.

Defect patterns in hybrid nematic liquid crystal films of pentylcyanobiphenyl (5CB) on a glycerol surface were studied with polarized transmission and monochromatic reflected light microscopy. We report that disclinations of apparent topological strength zero, stable pairs of +1 and -1 disclinations, and higher strength defects found in these films are stabilized by holes in the liquid crystal film. These holes are produced by a monolayer of surfactant on the glycerol surface. In addition, we describe the topology of the director field in the film during the coalescing of two such holes.

Influence of Polarizing Electric Field on Phase Transition Temperatures and Optical Transmission of PDLC Films

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. Using the thermally stimulated depolarization currents method and registering the optical signal simultaneously we studied the dependence of the phase transitions temperatures on the polarizing electric field. Thermal switch behavior and its dependence on polarizing electric field is obtained.

The surface-induced static director distribution in thin nematic liquid crystal films: A deuterium nuclear magnetic resonance spectroscopy study

We have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magnetic field have a top-hat shape different to the bowler-hat shape, which is well known as the director profile for the case with no magnetic field.

Capillary condensation of a nematic liquid crystal observed by force spectroscopy.

We have observed capillary condensation in nanometer-thin films of nematic liquid crystals using force spectroscopy. The liquid crystal was confined to a submicron gap between a flat substrate and a microsphere, attached to the cantilever of an atomic force microscope. A long-ranged and strongly attractive force due to capillary condensation of a nematic phase was observed at temperatures close to the bulk isotropic-nematic phase transition. The critical point, terminating the first-order confined isotropic-nematic transition line, was determined for the first time.

Fingering instabilities in driven thin nematic films

Motivated by experimental work (Cazabet et al., unpublished), we consider the possibility of fingering instabilities in thin films of nematic liquid crystals. We use lubrication theory on the flow equations for nematic liquid crystals to derive a simple model describing the evolution of the film height. As far as we are aware, this is the first time such a systematically derived, time-dependent thin film model for nematics has been presented. Simple “leading-order” solutions (depending on only one spatial coordinate) are found for two different flow driving mechanisms: (i) gravity perpendicular to the film and (ii) gravity parallel to the film (capillarity is also included in both cases). The effect of imposing two-dimensional perturbations to these solutions is studied. We find that for case (i) instability is possible, depending on whether or not there is complete wetting (i.e., whether or not the equilibrium contact angle of the droplet with the substrate is zero). For case (ii) we always have instability, as we would expect from the analogous result for Newtonian fluids [Phys. Fluids 8, 460 (1996); Europhys. Lett. 10, 25 (1989)].

Textures in Thin Free Suspended Nematic Liquid Crystal Film

In this paper we are concerning the orientational properties of free suspended nematic liquid crystal films. The experiments showed that orientation can be either homeotropic or planar. It is found that the orientation depends on the time during which the film was cooling from isotropic state to the liquid crystal one. For the slow cooling the liquid crystal had always home-ortopic orientation, for the fast cooling the resulting structure is planar. The analysis shows that the realized structure depends on a defect created in the film during cooling from isotropic state.

Analysis of the Dynamical Regimes Induced by a Laser Beam in Nematic Liquid Crystal Films

We report the analysis of the dynamical regimes induced in a homeotropically aligned nematic liquid crystal layer when a s- polarised laser beam acts at a small incident angle. For increasing light intensities these regimes are studied through an analysis performed on the variation of the polarisation state of a light probe beam. To measure with a good resolution the four Stokes parameters we use an experimental apparatus that include a pump-probe technique and a Four-Detector-Polarimeter (FDP). A preliminary description of the dynamical regimes, based on the analysis of the Fourier spectra, the autocorrelation functions and the attractors of the measured time series has been discussed. Moreover we calculated also the maximum Lyapunov exponent. For the last series of measurements this analysis suggest the presence of a transition towards chaos.

Acousto-optical effect induced by ultrasound pulses in a nematic liquid-crystal film

The reorientation of the director of a homeotropically aligned nematic liquid-crystal film subjected to ultrasound pulses is discussed. The relationship is investigated for the first time, to our knowledge, between the transmitted optical power and the high-frequency (approximately megahertz) pulsed ultrasound intensity. It is evident that there exists a power law governing the ultrasound-intensity-dependent optical transmission. However, the data indicate no existence of a characteristic threshold for the acousto-optical effect. Our results suggest that acoustic streaming, known to be responsible for acousto-optical effects in cw ultrasound experiments, should also play a similarly important role in the case involving a pulsed ultrasound field.

Supra Optical Nonlinearities (SON) of Methyl Red- and Azobenzene Liquid Crystal - Doped Nematic Liquid Crystals

In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.

Mechanical Theory for Nematic Thin Films

A mechanical model that describes the stability and structure of thin nematic liquid crystal films between two different isotropic fluids is presented. The model provides expressions for the structural disjoining pressure and the generalized Young−Laplace equation and is used to analyze the film as a single membrane and as two separate interfaces (microscopic model). The model is used to find expressions for the film tension (membrane model), the two film surface tensions (microscopic model), the contact angles of the film with the meniscus, and the film spreading coefficient. Asymmetric single-component nematic liquid crystal films are found to be stable because orientation gradients generate positive disjoining pressures. The disjoining pressure in nematic films scales with the inverse of the film thickness square, and this scaling produces an ordering of the surface tensions, such that the single-membrane film tension is greater than the two interface film tensions (microscopic model), which in turn ar...

Quasilinear electro-optical response in a polymer-dispersed nematic liquid crystal

Polymer-dispersed nematic liquid crystals are known to exhibit a quadratic electro-optic response. For practical applications, a linear dependence on the applied electric field is often required. In this letter, polymer-dispersed nematic liquid crystal films with a built-in dc electric field are shown to be good electro-optical devices with a linear response. The built-in electric field is obtained by energizing films with an external dc field. The electro-optical response dependence on the strength and frequency of the driving field is discussed.

Stimulated orientational and thermal scatterings and self-starting optical phase conjugation with nematic liquid crystals

A quantitative theory and experimental results on self-starting optical phase conjugation, using stimulated orientational and thermal scattering in nematic liquid crystal films, are presented. The coupled wave-material equations for the laser-induced refractive index changes, grating formation, and coherent wave mixing effects are developed. Analytical solutions are obtained for the case of negligible pump depletion, and numerical solutions for various input and generated signals, taking losses into account, are obtained. Experimentally, we demonstrate the feasibility of realizing these stimulated scattering and phase conjugation processes in thin (200 &mgr;m) nematic liquid crystal with a milliwatt-power cw laser. Theoretical estimates for various gain constants and threshold intensities, and their dependence on various physical parameters, are found to be in good agreement with experimental observations.