Anisotropy of the turbidity of a nematic liquid crystal

The absorption coefficient of a nematic liquid present an important anisotropy. The measurement of this coefficient in three simple geometries allows the determination of the three Frank elastic constants on the same sample. We give the results obtained for MBBA at room temperature.     

Interaction of light with a dye-doped nematic liquid crystal

The interaction of linearly and circularly polarized beams with a nematic liquid crystal doped with light-absorbing dyes has been studied by light-diffraction and microprojection methods. It has been found that there is a threshold for the emergence of light-induced anisotropic structures, which, depending on the type of dye, can be axisymmetric or extended in a direction determined by the light field. Possible mechanisms leading to the formation of the anisotropic structures are discussed. Zh. éksp. Teor. Fiz. 111, 2059–2073 (June 1997)     

Anisotropy of diffusion in nematic alkyloxybenzoic acids

The existence of monomer molecules in the nematic phase of substituted benzoic acids permits the measurement of the anisotropy of diffusion coefficients by a single NMR-method.     

Scattering and amplification of light in a layer of a nematic liquid crystal

The role of light scattering in liquid crystal lasers can be of two kinds. It can induce undesirable losses increasing the lasing threshold or it can promote the appearance of feedback necessary for lasing. The light amplification spectra of the liquid crystal in the isotropic and light-scattering nematic phases have been measured. For comparison, at a close pump power, the amplification spectra have been measured on a TiO₂ nanoparticle suspension in an isotropic solvent, where the mean free path of photons is the same as that in the liquid crystal. The amplifications in two systems are significantly different, because the shapes of their scattering patterns are strongly different.     

Nanostructures in a nematic liquid crystal

The effect of nanoobjects, molecular objects, and α particles introduced into a liquid-crystal matrix that is placed in a dc electric field is studied theoretically and experimentally. The introduced particles form a charged defect, the radius of which is much larger than the particle sizes, in the orientational structure of the liquid crystal. This defect is easily observed under an optical microscope, and its image agrees with calculated data. It is suggested that this effect be used for detection and diagnostics of small objects and for design of ionizing radiation detectors. The phenomena revealed in this work can help in producing new charged-defect-saturated liquid-crystal structures formed by introducing nanoparticles in a medium or by irradiating a medium by ionizing radiation. A nonmechanical principle of transport of nanoparticles and their regular incorporation into the lattice is implemented. The essence of this principle is control of molecular orientations in a liquid crystal by quasi-stationary electric fields.     

Opto-acoustic modulator with a nematic liquid crystal

A liquid-crystal optical modulator working with acoustic signals is considered. It is shown that it can be operated also with white light up to frequencies of at least 1 MHz.     

Phase measurements of a twisted nematic liquid crystal spatial light modulator with a common-path interferometer

We present a novel and high accuracy (±1°) technique for measuring the difference in phase delay between the positive and negative eigenvectors of a twisted nematic liquid crystal spatial light modulator (LCSLM) as a function of applied voltage. We show a new architecture for a common-path interferometer where we simultaneously generate both the positive and negative eigenvectors of the LCSLM. We use an electro-optic light modulator to provide a time varying phase shift between the two eigenvectors and measure the phase delay difference between the two eigenvectors. We compare the phase delay values with those obtained using a Mach–Zehnder interferometer and find good agreement.     

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.     

Thermal orientation effect in a nematic liquid crystal

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.     

Onset of parametric electroconvection in a nematic liquid crystal

The behavior of a nematic liquid crystal (NLC) in an external ac electric field is studied in terms of the weak electrolyte model. Since electroconvection of certain types of NLCs can occur in a constant field in an oscillatory mode, a quasi-periodic response of a nematic to the external action is possible in an ac field, as well as a sharp reduction of the electroconvective stability thresholds at resonance frequencies. Stability maps for NLCs are plotted on the plane of amplitude-frequency parameters of the external field. Phase portraits are constructed for subharmonic and synchronous perturbations. The optical response of a NLC in the ac electric field is studied; the time dependences of the relative intensity of light, which passed through the nematic liquid crystal cell, are obtained.     

Geometrical optics of a twist-oriented nematic liquid crystal

We have systematically analyzed the spatial dynamics of the polarization state of light in smoothly inhomogeneous anisotropic media. As a medium of this kind, we have considered a nematic liquid crystal with twist orientation. A system of coupled equations has been solved with respect to the Cartesian components of the electric-field component using a rotating coordinate system. We have shown that, upon propagation of light in this medium, the state of the polarization experiences oscillations the spatial frequency of which depends on the thickness of the specimen. Our analysis also has shown that, at an appropriate choice of the thickness, this twist cell behaves as a broadband half-wave plate.     

A biaxial nematic liquid crystal

We report a biaxial nematic liquid crystal in a simple thermotropic system.     

Influence of barrier effects at interfaces on dynamic scattering of light in a nematic liquid crystal

The intensity of light dynamic scattering by a nematic liquid crystal with negative dielectric anisotropy (ZhK-440) as a function of the constant electric field strength has been studied under different interfacial conditions. It has been shown that potential barriers that arise at the interfaces not only influence the scattering intensity, but may also radically change the form of the dependence; i.e., the curves of optical transmission in the direction of the incident beam may have a minimum under certain field strengths. At higher strengths, the cells become totally transparent again. This anomalous behavior of the transmission curve is associated with the fact that the conductivity of the cells drops below a critical value with growing field strength, as the resistance of the space charge region at the inversely biased junction of the extraction electrode rises. In addition, it has been shown that the high resistance of the cells at low voltages may be attributed not only to a low concentration of impurities in the liquid crystal, but also to a low emissivity of the injecting electrode and a weak electron affinity of the extraction electrode.     

Acousto-optical and acousto-dielectric effects in a nematic liquid crystal

This paper deals with the optical transmission and the dielectric effects in a nematic liquid crystal film (MBBA) of homeotropic state subjected to ultrasound. As in the case of the application of shear vibration, it is found that not only the transmitted light intensity through the film but also the dielectric constant and the tanδ of the film change with ultrasound intensity. There exists a threshold sound pressure above which the effects are appreciable. The threshold sound pressure is almost inversely proportional to the film thickness and the ultrasound frequency.