Selective reflection of light from a layer of cholesteric liquid crystal with induced inhomogeneity of helix pitch

A possibility to induce a planar defect in a plane-layer structure of a cholesteric liquid crystal via an external electric field is shown experimentally. The results of studies of behavior of the selective reflection (transmission) coefficient of light for a layer of the cholesteric liquid crystal, where planar defects are induced, are presented     

Chiral molecules split light: Reflection and refraction in a chiral liquid

A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.     

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.     

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.     

Transversal structure of liquid crystal in a light wave field

The change in the orientation of the director of a nematic liquid crystal in a light wave field under a uniform illumination is analyzed. A system of equations for the distribution of the director angle of inclination is derived for the case of normal incidence. This system accounts for the finite energy of interaction between the nematic and substrate. The dependence of the director distribution profile on the incident light intensity and cohesion energy is studied at fields considerably above the threshold.     

Untwisting of the helical structure in a plane layer of chiral liquid crystal

The untwisting of the helical structure of a chiral liquid crystal (CLC) in a thin plane layer exposed to an external action (temperature or field) and its dependence on the molecular adhesive forces at the layer boundaries are studied theoretically. It is shown that the critical electric (magnetic) field for complete untwisting in a thin layer may be appreciably lower than in the corresponding bulk CLC sample, and, contrary to the latter, the untwisting proceeds jumpwise. The expressions relating the jump temperature (field), i.e., the magnitude of untwisting action, to the CLC material parameters, layer thickness, and surface adhesive potential are given. The jump temperature (field) hysteresis is studied. In particular, it is shown that, for certain parameters, the untwisted helix remains untwisted after the removal of external action. The revealed qualitative regularities of untwisting are illustrated by numerical computations with the use of particular parameters of a CLC layer.     

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.     

Electric convection in a nematic liquid crystal in a varying electric field

The emergence of electric convection in a nematic liquid crystal under the action of a varying electric field is analyzed in the 1D approximation. A new mode of external field variation with subharmonic oscillations is proposed. The behavior of several classes of synchronous perturbations and subharmonic oscillations is analyzed; the evolution of the fields of the director and of the volume charge is studied. Parametric instability regions are specified and the critical frequencies of transitions between response modes are determined. Stability maps for a nematic liquid crystal are plotted on the frequency-amplitude plane.     

Light scattering in cholesteric liquid crystals with a large pitch

Optical properties of cholesteric liquid crystals with a pitch larger than the wavelength of light are considered. Normal waves of the medium and the Green function of the electromagnetic field are analyzed. A general algorithm based on the application of the Kirchhoff method is proposed for calculating the scattered light intensity in media with a one-dimensional periodic structure. The WKB vector method is used for calculating the spatial correlation function of thermal fluctuations of the director. It is found that the transformation of two fluctuation modes takes place is some regions. The angular and polarization dependences of the intensity of light scattered from fluctuations of the director are calculated. It is found, in particular, that the intensity of scattering is a nonmonotonic function of the size of the system.     

Deformation of a pretilted nematic liquid crystal layer in an electric field

The deformation of the optical axis of a nematic liquid crystal layer under the influence of an electric field is caused by the dielectric torque. This momentum counteracts with an elastic torque generated by interfacial forces between the surface of the electrode and the liquid crystal. By means of a variational analysis, the deformation profiles within the liquid crystal layer are calculated, assuming large interfacial energies and various angles of pretilt of the liquid crystal directors with respect to the electrodes. Both the extreme case of a homeotropic (vertical) alignment on one electrode and a homogeneous (parallel) alignment on the opposite electrode, as well as the general case of arbitrary and different alignments on both electrodes, lead to heavily pretilted liquid crystal layers, resulting in definite deformation profiles without disclinations. Liquid crystal cells prepared in this way neither show threshold voltages nor delay times when electrical fields are applied. Measured and calculated characteristics of such liquid crystal cells are presented, they show good agreement.     

Electric field quenching of thermal fluctuations of orientation in a nematic liquid crystal

Using Rayleigh scattering and light beating spectroscopy, we observe the thermally excited angular fluctuations of the di-rector in the nematic phase of MBBA (p-methoxybenzylidene-p-n-butyl-aniline) in presence of a stabilizing a.c. electric field. Both the intensity and damping time of these fluctuations are found to decrease when increasing the field intensity, allowing the measurement of the bend Frank elastic constant.     

Electric field tunable photonic Hall effect with liquid crystals

The Mie theory for electromagnetic scattering is extended to the case of coated metal sphere with liquid crystals. A new set of vector basis functions for the electric displacement vector inside the liquid crystal layer has been constructed. The expansion coefficients of transmitted and scattered fields are obtained analytically by applying the continuous boundary conditions. The dependence of the scattering property on the geometrical parameters has been investigated in detail. The appearance of photonic Hall effect for such a Mie scatterer is confirmed. It is interesting that such a photonic Hall effect not only depends on the ratio of the inner to outer radius of coated sphere, it is also tunable by the application of an external voltage.     

Orientational self-pumped phase conjugation of a light beam in a layer of a nematic liquid crystal with nonreciprocal feedback

Self-pumped phase conjugation of a Nd: YAG laser beam by forward stimulated orientational scattering in a nematic liquid crystal layer with a nonreciprocal feedback loop is investigated. It is found that the threshold of phase-conjugate wave generation is the lowest when the phase shift difference of pumping and scattering waves in the loop is /2.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 3–4, pp. 304–311, March–April, 1995.     

液体表面光场环状暗斑效应及折射率测量

实验发现透过液体介质的光场出现了一环状暗斑效应。进一步研究表明,该效应是由于光在液体介质表面发生全反射引起的,环状暗斑半径的大小与液体折射率和液体厚度有关,当液体折射率一定时,液体厚度越厚,暗斑半径越大;光液体厚度一定时,折射率越大,暗斑半径越小。进而建立了环状暗斑半径与液体折射率之间的关系,提出了一种测量液体折射率的新方法。     

Pixel size and pitch measurements of liquid crystal spatial light modulator by optical diffraction

We present a simple technique for the determination of pixel size and pitch of liquid crystal (LC) based spatial light modulator (SLM). The proposed method is based on optical diffraction from pixelated LC panel that has been modeled as a two-dimensional array of rectangular apertures. A novel yet simple, two-plane measurement technique is implemented to circumvent the difficulty in absolute distance measurement. Experimental results are presented for electrically addressed twisted nematic LC-SLM removed from the display projector.