A. A. Karetnikov

Polarimetric method for the determination of the optical axis tilt angle at boundaries of a liquid-crystal layer

A polarimetric method is proposed to determine the tilt angle of the local optical axis of a liquid crystal at layer boundaries. The method is based on the measurement of the azimuthal polarization angles of light waves passed through a liquid-crystal cell or reflected from it. This method requires neither the knowledge of the refractive indices of a liquid crystal nor the homogeneity of the layer. The method can be applied in the whole range of tilt angles. Experimental data on the measurement of the tilt angles in cells assembled by the scheme of a prism liquid-crystal polarizer are presented.

Local Dynamics of Director Reorientation under Electric Field in Helical LC Structure

The influence of an electric field on the trajectory of an extraordinary light ray in a layer of a chiral liquid crystal (LC) with a 180° turn of the director is studied. In the absence of the electric field and at a large angle of incidence the ray reflects inside the layer and return back through the surface which it entered. The applied electric field distorts the initial configuration of the director. It results in a change of the ray trajectory so that the light is propagated through the LC cell. The study of the temporal characteristics of the effect at various angles of incidence of light on the layer makes it possible to examine the local reorientation of the director inside the cell.

Electric field effect on refraction of light in a layer of chiral liquid crystal with large pitch

The effect of an alternating electric field on the trajectory of an extraordinary light wave in a layer of a chiral liquid crystal with a 180° turn of the director is studied. In this structure, in the absence of the field and at a large angle of incidence of the light wave on the liquid crystal layer, the light undergoes refraction inside the layer. It is shown that the deformation of the director that arises under the action of the electric field changes the character of refraction of the extraordinary wave and the layer begins to transmit the light. The threshold voltage of this effect is determined. The dynamics of the effect is studied. At large voltages, in addition to the extraordinary wave, an ordinary light wave is observed in the light passed through the cell. The ordinary wave intensity is modulated by the initial frequency of the control signal, whereas the extraordinary wave intensity is modulated by the double initial frequency.

Propagation of light through a forbidden zone in chiral media

The particular features of the propagation of light in uniaxial chiral liquid crystals with a large pitch of the spiral are considered. There exist forbidden zones in these systems for fairly large angles of incidence of an extraordinary ray. On the one hand, this results in an efficient reflection of the wave from the zone boundary, and, on the other hand, this causes the wave to decay inside the zone. A case of narrow forbidden zones is studied, and it is shown that optical effects that arise upon propagation of rays near turning points are equivalent to the tunnel and over-barrier reflection effects. The angular dependences of the intensities of rays that were refracted in a forbidden zone and transmitted through it are calculated. The percolation effect is experimentally studied in a mixture of a nematic liquid crystal with a chiral addition. The intensity of a transmitted extraordinary ray is studied as a function of the angle of incidence, which determines the width of the forbidden zone. Both the over-barrier reflection and the percolation effects are observed. The calculation results are shown to agree with experiment.

Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Fréedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.

Determination of the director pretilt angle at liquid-crystal layer boundaries by polarimetric and crystal rotation methods in the same cell

The pretilt angle of the director at the liquid crystal–glass interface is determined with the polarimetric and crystal rotation methods. The polarimetric method has been modified with the aim of increasing the measurement accuracy. The results obtained with the modified polarimetric method are verified with the use of reference plane-parallel iceland spar plates with known pretilt angles of the optical axes. It is shown that the pretilt angles found with the crystal rotation and modified polarimetric methods coincide within the measurement accuracy.

Peculiarities of light propagation in chiral liquid crystal cells in an external electric field

We investigate the propagation of the extraordinary ray in a cell with a chiral liquid crystal at oblique incidence. For a 180° twist cell, we study the dependence of the minimum incidence angle at which the light does not yet pass through the cell on the applied voltage. The orientational structure of a liquid crystal in an external electric field has been calculated by directly minimizing the free energy. This has allowed the ray trajectories and the limiting refraction angles to be determined. The results of our calculations are consistent with the experiment. We show that allowance for the electric field nonuniformity in a chiral system is important for agreement between theory and experiment.

The influence of an external electric field on the propagation of light waves in cholesteric liquid crystal cells

The specific features of light transmission in a cholesteric liquid crystal (LC) cell with a director rotated by 90° have been investigated. In this structure, where a light wave is incident at a large angle with respect to the LC surface, the light is reflected (refracted) in the LC layer near the opposite boundary. It is shown that the application of an electric field changes the character of extraordinary wave refraction, as a result of which light starts passing through a cell. The transmission threshold voltage is determined, and its dependence on the angle of incidence of light is obtained. The dependence of the transmitted-light intensity on the voltage across the cell is obtained as well. The same dependences are also derived by numerical calculations with allowance for the turning points and extinction.

Fast electro-optical response of a cell with a homeoplanar layer of a nematic liquid crystal

The refraction of light, i.e., the turn of an extraordinary ray in the liquid crystal layer similar to total internal reflection at an interface between two media, has been studied in a cell with the homeoplanar orientation of the director. The rise, τon, and decay, τoff, times of optical responses have been obtained for various angles of incidence of light on a liquid crystal layer subjected to an electric field. The times τon and τoff of optical responses for the angles of incidence much larger than the angle of total internal reflection are 1–2 ms, which is three orders of magnitude smaller than the relaxation time of an optical response in the case of normal incidence of the ray.

Extraordinary ray refraction in a large pitch helical medium

The phenomenon of extraordinary ray refraction in a helical liquid crystal with large (compared to the light wavelength) pitch has been studied by theoretical and experimental methods. At a sufficiently large angle of incidence relative to the pitch axis, the extraordinary ray exhibits reflection (reversal) from a certain layer of the medium. The ordinary ray, for which the system is optically isotropic, exhibits no such reflection. The experimental dependences of the transmitted and reflected (reversed) rays are described using the geometrical optics approximation taking into account the optical losses for scattering inside the liquid crystal.