E. V. Aksenova

Coherent backscattering of light in nematic liquid crystals

Multiple light scattering by director fluctuations in nematic liquid crystals is considered. A uniform director orientation is assumed to be specified by an applied magnetic field. The coherent backscattering effect, which consists in the presence of a sharp light backscattering peak, is studied. The Bethe-Salpeter equation is used to calculate the multiple scattering intensity taking into account the contributions of ladder and cyclic diagrams. An analytical expression for the angular and polarization dependences of the coherent backscattering intensity is obtained in terms of the diffusion approximation. The calculation and experimental results are compared. The developed theory is shown to qualitatively describe the elliptical shape of the backscattering cone, to explain the absence of a coherent contribution for crossed polarizations, and to calculate the relative peak height.

Light propagation in chiral media with large pitch

Light propagation in uniaxial chiral media with large pitch is studied. In these systems there are forbidden zones for extraordinary beams, which lead to effective reflection on zone boundaries and to wave damping inside the forbidden zone. We analyze the vicinities of the turning points and the transition of an extraordinary wave through the forbidden zone. Narrow forbidden zones with merging turning points are studied in detail. The transition through the forbidden zone is studied experimentally in nematic liquid crystal doped with a chiral addition. There is a good agreement between experimental results and theoretical calculations.

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 and scattering of light in stratified media

The propagation and scattering of light in stratified media are considered. Based on the Maxwell equations, the present approaches to the solution of these problems are formulated in a unified way. The particular features of the wave propagation in stratified media are discussed. Scalar and vector fields are considered. Media with small-and large-scale regular inhomogeneities are examined. The construction of the Green’s function of the wave equation in a spatially homogeneous medium is discussed. Stratified isotropic and anisotropic media are analyzed. The scattering of light in a stratified medium is studied with emphasis on the Kirchhoff method, as this makes it possible to obtain calculation formulas in a form convenient for comparing the theory with the experiment. The propagation of waves in photonic crystals and the formation of forbidden zones in such objects are briefly considered.

Light propagation in a helical medium with a large-scale periodicity

The propagation of light in helical media whose pitch greatly exceeds the radiation wavelength has been investigated. It is shown that such a system includes forbidden zones corresponding to the effect of ray turning. The propagation of an extraordinary ray corresponding to a very narrow forbidden zone has been analyzed in detail. The radiation wave field has been obtained both far from a turning point and in the close vicinity of such a point. It is shown that, in the vicinity of a turning point, mode transformation takes place and the wave acquires an elliptical polarization. The parameters of the ellipticity and the relations between the amplitudes of the waves passed through the forbidden zone and “reflected” from it are calculated.

Waveguide propagation of light in cholesterics with large pitch

Abstract The Greens function and waveguide propagation of electromagnetic field in cholesteric liquid crystals with a pitch being large compared to the wavelength is considered. This function is constructed using the solution of the Maxwell equations. The behavior in the far zone is analysed in detail. The periodic system is distinguished from an anisotropic medium by a discontinuity of the wave vector surface and a break of the beam vector surface. Trajectories of beams in such a medium are not plane. The forbidden zone corresponds to capture of beams and formation of a wave channel. Within this wave channel the Greens function asymptotic differs from 1/r behavior.

Simulation of radiation transfer and coherent backscattering in nematic liquid crystals.

We consider the multiple scattering of light by fluctuations of the director in a nematic liquid crystal. Using methods of numerical simulation the peak of the coherent backscattering and the coefficients of anisotropic diffusion are calculated. The calculations were carried out without simplifying assumptions on the properties of the liquid crystal. The process of multiple scattering was simulated as a random walk of photons in the medium. We investigated in detail the transition to the diffusion regime. The dependence of the diffusion coefficients on the applied magnetic field and the wavelength of light were studied. The results of simulation showed a nonmonotonic dependence of the diffusion coefficients on the external magnetic field. For calculation of the peak of the coherent backscattering we used the semianalytical approach as long as in nematic liquid crystals this peak is extremely narrow. The parameters of the backscattering peak and of diffusion coefficients which were found in numerical simulations were compared with the experimental data and the results of analytical calculation.

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.

Calculation of correlation function of the director fluctuations in cholesteric liquid crystals by WKB method

The spatial correlation functions of the thermal fluctuations in systems with smoothly varying structure are calculated by means of the WKB method. As a particular physical problem we consider the behavior of director fluctuations in cholesteric liquid crystals possessing one-dimensional spatial periodicity. The problem leads to the solution of set of two second order differential equations with periodic coefficients. It is shown that in this physical system there exist regions where the WKB approximation is not valid. The analysis of these regions is similar to that of the turning points in quantum mechanics. Contrary to standard approach in our problem the turning point has fourth-order singularity and only decaying solutions have physical sense. We find WKB solutions for normal modes of director fluctuations in cholesteric liquid crystals far from the turning point as well as in its vicinity. We obtain that two fluctuating modes interact in the vicinity of the turning point, but any of these modes does...