Nematic Liquid Crystal Film Research Articles

Mechanically Induced Electric Polarization in Polymer Dispersed Nematic Liquid Crystal Films

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. In order to induce anisotropy and consequently electrical polarization, the films have been hot stretched and pressed. We studied the dependence of the polarization upon temperature, frequency and amplitude of the applied electric field. High values of the order of μC/cm2, have been obtained. The frequency dependence of the polarization indicates relaxation times of the order of milliseconds.

Extremely nonlinear photosensitive nematic liquid crystal film

Optically induced nematic liquid crystal (NLC) axis reorientations result in extraordinarily large nonlinear refractive index changes. We discuss the origins of these nonlinearities, and recently, obtained results of experimental studies of photovoltage generation, dynamic grating diffraction, and image conversion using dye-doped NLC films in all-optical configurations are presented. These processes are characterized by unprecedented low threshold laser powers. For example, we have demonstrated incoherent–coherent image conversion with laser intensity on the order of 70 microWatts/cm 2.

Laser- and Electric-Field-Induced Local Refraction Index Change in a Nematic Liquid Crystal Film

We study the Ar+ laser- and electric-field-induced local refraction index change in a planar-parallel-aligned nematic liquid crystal (NLC) film. The laser that is normally incident on the film is strong enough to induce a spatial nonuniformity of the molecular reorientation. The experimental results show that the change of the refraction index of the NLC film is always less than that calculated by the theory in current usage. This is because of the finite beam size effect which is different from the infinite plane wave assumption in the current theory. In this paper, we report some interesting phenomena which result from the finite beam size effect especially near the occurrence of the Fredericksz transition. The experimental results are compared to the theoretical calculations.

All-optical image processing with a supranonlinear dye-doped liquid-crystal film

We demonstrate a dynamic all-optical image-intensity-inversion technique that uses self- and mutual-phase-modulation effects with a highly nonlinear nematic liquid-crystal film placed in an intermediate focal plane. This process requires submilliwatt optical power, responds in a few milliseconds, and can be realized over a very broad spectral range.

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.

Mechanical theory of structural disjoining pressure in liquid crystal films

A mechanical theory of structural disjoining pressure in nematic liquid crystal films is developed based on Laplace's interfacial stress balance equation. Identification of the interfacial stresses in nematic liquid crystal interfaces leads to two contributions to the structural disjoining pressure. It is shown that tensor order parameter gradients across the film give rise to bulk Ericksen stresses, whose normal component results in a disjoining pressure that tends to stabilize the film. In addition, tangential gradients in the tensor order parameter give rise to gradients in interfacial bending stresses whose normal component results in a film pressure that may be disjoining or conjoining. Phenomenological expressions for the two structural disjoining pressures are obtained using the classical equations of liquid crystal bulk and surface elasticity.

Polarimetric study of the optically induced dynamical behaviour in nematic liquid crystal films

We report the photopolarimetric study of the molecular director dynamics observed in a homeotropically aligned nematic liquid crystal layer under the action of a small angle incident s-polarised laser beam. For increasing light intensities several interesting dynamical regimes have been observed. In order to accomplish this aim, we propose a new experimental apparatus which, using a pump-probe technique and a new polarimeter (four-detector polarimeter), allows us to measure with a good resolution the variation in the polarisation state of the laser probe during the molecular reorientation induced by a laser beam in the sample. A preliminary description of the dynamical regimes, based on the analysis of the Fourier spectra and the autocorrelation functions of the measured time series, have been reported.

Hybridly Aligned Liquid Crystal Films. A Monte Carlo Study of Molecular Organization and Thermodynamics

A Monte Cario simulation of a nematic Lebwohl-Lasher hybrid liquid crystal film, confined between two surfaces with antagonistic anchoring alignment, is presented. We have calculated the ordering and the molecular organization inside the film for different lateral sizes. The influence of these boundary conditions on the nematic isotropic phase transition is also investigated.

Multistable alignment in free suspended nematic liquid crystal films.

Two different types of orientation were observed in free suspended nematic liquid crystal films. The alignment was either homeotropic or planar, depending on the time during which the film was cooling from the isotropic state to the liquid crystal one. It was shown that nucleation of defects makes planar orientation preferable.

Dynamical Regimes and Motion Intermittence Induced by a Laser Beam in a Nematic Liquid Crystal Film

When an s-polarised laser beam shines on a homeotropically aligned film of nematic liquid crystal at small incidence angle, undamped oscillations of the molecular director may be produced. At high beam intensities, the oscillations break up into deterministic chaos. Although this effect has been known for a long time and the route to chaos has been qualitatively analysed, no detailed study of the director motion has been carried out. We designed an experimental apparatus to monitor the dynamics of the director, as described by its two polar angles. We observed different dynamical regimes, depending on laser intensity: steady states, oscillations, rotations, and, at the highest laser intensities, deterministic chaos. Moreover, the transitions between the oscillation and rotation regimes are characterized by intermittency.

Experimental Results on a New Integrated Beam Deflector/Switch Based on Liquid Crystals

Liquid crystals have effective electro-optic coefficients that are orders of magnitude larger than other materials used in integrated optics. A number of integrated optical devices based on these materials have been recently proposed. We have designed and realized an integrated device in a three-stage planar waveguide, having as middle stage a nematic liquid crystal film. We studied the device performance in different geometries using TE polarized light. By a proper choice of the material parameters we measured time responses in the microsecond range. Our experimental results confirm the possibility of employing such a device working as an optical switch and/or beam deflector.

Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection

Optically induced nematic liquid crystal axis reorientation results in extraordinarily large nonlinear refractive index changes that could find practical applications in conjunction with cw or long-pulse lasers. We discuss the origins of these nonlinearities, and present the results of recent experimental studies of image conversion, optical limiting and sensor protection using aligned dye-doped nematic liquid crystal films in all-optical configurations. These processes are characterized by unprecedented low threshold laser powers, thus presenting nonlinear photosensitive nematic liquid crystals as promising next generation image processing and optical switching/limiting material.

Photopolarimetric characterization of the transition between two turbulent states in a nematic liquid crystal film.

This work was aimed at the photopolarimetric characterization of the transition between two dynamic scattering modes that take place in a planarly aligned nematic liquid crystal sample, under the effect of an external low-frequency electric field. The time evolution of the degree of polarization and the behavior of the radiation entropy of the transmitted light allow us to interpret the transition between two turbulent states, or dynamic scattering modes, as a decay from a two-dimensional (2D) to a (3D) turbulence.

Inelastic scattering of atoms and molecules from liquid crystal surfaces

Collisions of an atomic beam of He atoms with the surface of an aligned nematic liquid crystal film (4′-pentyl-4-cyano biphenyl, 5CB) initiate a transient change in the alignment of the surface region of the film. The effect is greater when the He atoms approach perpendicular to the alignment director of the sample than when the approach is parallel to the director. The relaxation of the system back to its initial alignment has a temperature dependence which suggests that rotational diffusion is the relaxation mechanism. Laser induced fluorescence spectra of I2 molecules scattered from the isotropic phase and from the nematic phase of 5CB indicate that molecules scattered from aligned 5CB are more internally excited than those scattered from an isotropic 5CB sample. In total, these results indicate that the dynamics of collisions at the gas–liquid crystal interface are strongly dependent upon the molecular order in the liquid crystal.

Fluid Motion Effect in Self-Phase Modulation of an Intense Laser Beam Interacting with a Nematic Liquid Crystal Film

We study the dynamics of the self-phase modulation of strong optical fields in a nematics substance. In particularly, the backflow effect is discussed.

Dielectric relaxation in polymer dispersed nematic liquid crystal films

Dielectric absorption studies for polymer dispersed liquid crystal (PDLC) samples of different nematic (4-hexyl-4'-cyanobiphenyl, 6CB) contents (20, 30, and 40 wt%) have been carried out in the frequency range from 10kHz to 10MHz. A method for evaluation of the fraction of the molecules which are not affected by the polymer surface anchoring forces in the nematic droplets is proposed.

Coupling of the remanent polarisation in thin film oxide ferroelectrics with nematic liquid crystals

The technique of domain visualisation, revealed by a film of nematic liquid crystal (NLC) on a ferroelectric crystal substrate, is demonstrated for the first time to be applicable to thin films of lead zirconate-lead titanate (PZT) ferroelectric ceramics. PZT film thicknesses <1 µm were deposited on indium tin oxide coated glass substrates. Commercially available NLC materials with positive dielectric anisotropy were employed. The electro-optic properties of devices incorporating the ferroelectric ceramic and NLC are reported. The poled PZT grains induce a local Freedericksz transition in the NLC and it is proposed that the ferroelectric-NLC interaction is primarily due to the depolarisation field due to the ferroelectric remanent polarisation.

Dye-doped photorefractive liquid crystals for dynamic and storage holographic grating formation and spatial light modulation

The basic mechanisms of photo-induced space charge field formation, director axis re-orientation, and refractive index changes in fullerene C60- and dye-doped nematic liquid crystals films are presented. In particular, in aligned methyl-red-doped nematic liquid crystal film, we observe a nonlinear index change coefficient as high as 10 cm/sup 2//W, associated with purely optically induced liquid crystal director axis re-orientation. Experimental observations of dynamic and high-resolution storage holographic grating formation, two beam coupling with gain of nearly 3000 cm/sup -1/, optical limiting action at nanowatt cw laser power, and incoherent-coherent image conversion at /spl mu/W/cm/sup 2/ light intensity level are discussed.

A Fourier-transform-based interferometric technique for measuring the elastic anisotropy of a nematic liquid crystal

We propose a method based on an interferometric technique to measure the molecular distortion induced by an incident light beam on a nematic liquid crystal film. An analysis of the fringe pattern generated by means of a Fourier transform method allows comparison of the measurements with the theoretical molecular distribution. From this analysis an estimation of the elastic anisotropy of the liquid crystal is derived which is in good agreement with previous results.

Photo-Induced Space Charge Fields, Photo-Voltaic, Photorefractivity, and Optical Wave Mixing in Nematic Liquid Crystals

We present a detailed discussion of the fundamental mechanisms for recently observed nonlinear electro-optical wave mixing effects in nematic liquid crystal films. Experimental results on photoconductivity, photorefractive-like two beam coupling, beam amplification, self-diffraction, and storage holographic grating formation in dye- and Fullerene C60 and C61 doped nematic films are presented along with a review of two- and four- wave mixing theories. Nematic liquid crystal films are shown to be potentially useful for adaptive and optical storage applications.