Serguei Petrovich Palto

Using Fourier spectroscopy in the visible and near-IR ranges, the polarization optical spectra of radiation transmitted in waveguide mode through a thin layer of cholesteric liquid crystal (CLC), confined by transparent ITO electrodes, were studied. A series of spectral bands were experimentally detected, which are explained by the resonant absorption associated with the phase matching between modes in CLC and ITO layers. The experimental results are consistent with the numerical simulation data obtained within the experimental sample multilayer model using the finite-difference time-domain method for solving Maxwell’s equations.

The boundary conditions for the binary planar-homeotropic alignment of liquid crystals have been created using the photoalignment technique. Under these conditions, an orientational transition to the deformed lying helix state occurs in a cholesteric liquid crystal layer with the initial Grandjean texture under the action of an electric field. In the deformed lying helix state, the electric field-induced photonic stop bands exist for light propagating in the waveguide mode. In this study, the lasing effect has been investigated, which occurs at the short-wavelength edge of the tenth-order induced stop band overlapping with the amplified luminescence spectrum of a laser dye included in a cholesteric liquid crystal. It is shown that the electric field range in which the lasing effect occurs is consistent with the range of the increased diffraction efficiency of the deformed lying helix.

The spectral properties of chiral liquid crystal layers, where surface anchoring on one of the surfaces is binary (planar-homeotropic) modulated by photoinduced orientation, have been studied. Optical spectra for such layers have been measured not only along the normal to the layer, but also in the waveguide mode in a cholesteric liquid crystal layer. It has been shown that an electric field perpendicular to the layer induces an orientational transition of the original planar Grandjean structure with the helix axis perpendicular to the layer to a state with a deformed “lying” helix, when the axis lies in the plane of the layer. The effect of changing the direction of the helix axis is manifested in the emergence of photonic stop bands in the optical spectrum of light propagating in the waveguide mode.

Orientation transitions in a cholesteric liquid crystal (CLC) layer with negative dielectric anisotropy, under the influence of a non-uniform spatially periodic electric field created using a planar system of interdigitated electrodes, were studied experimentally and numerically. In the interelectrode space, transitions are observed from a planar Grandjean texture, with the helix axis perpendicular to the layer plane, to states with a lying helix, when the helix axis is parallel to the layer plane and perpendicular to the electrode stripes. It was found that the relaxation time of the induced state in the Grandjean zones, corresponding to two or more half-turns of the helix, significantly exceeded the relaxation time for the first Grandjean zone with one half-turn. An analysis of experimentally observed and numerically simulated textures shows that slow relaxation to the initial state in the second Grandjean zone, as well as in higher-order zones, is associated with the formation of local topologically equivalent states. In these states, the helix has a reduced integer number of helix half-turns throughout the layer thickness or unwound into the planar alignment state.

This paper reviews recent research performed at the liquid crystals laboratory of the A. V. Shubnikov Institute of Crystallography, Russian Academy of Sciences, focusing on photonic liquid crystalline structures induced by electric fields. Due to field-induced spatial modulation of the refractive index, such structures exhibit optical properties characteristic of photonic crystals. Two types of structures are discussed. The first type is induced in cholesteric liquid crystals with spontaneous formation of a helical director distribution. The orientation transition to a state with a lying helix – with the axis in the plane of the layer – is considered. The second type consists of homogeneous layers of non-chiral nematic liquid crystals, where the modulation of the refractive index arises due to the flexoelectric instability effect. In both cases, periodic boundary conditions of molecule orientation are crucial. Methods of forming boundary conditions and the photonic properties of structures are reviewed.

The latest studies of electrically induced photonic liquid crystal structures, performed at the Laboratory of Liquid Crystals of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, are reviewed. Due to the field-induced spatial modulation of the refractive index, these structure exhibit optical properties that are characteristic of photonic crystals. Two types of structures are discussed. The first is induced in cholesteric liquid crystals with spontaneous formation of a helical director distribution. The orientational transition to the state with a lying helix (i.e., axis lying in the layer plane) is considered. The second type includes homogeneous layers of nonchiral nematic liquid crystals, in which refractive index modulation is due to the effect of flexoelectric instability. In both cases the periodic boundary conditions for the molecular orientation are of fundamental importance. Both the methods for setting boundary conditions and the photonic properties of structures are considered.

Numerical studies of the waveguide properties of liquid crystal layers bounded by substrates with indium tin oxide (ITO) electrodes using the finite difference time-domain (FDTD) method are carried out. On the basis of the experimental transmittance spectra of ITO-coated glass substrates in the visible and near-infrared ranges, a Lorentz model describing the dielectric properties of the ITO electrodes is created. Then, by numerical modeling, optical systems including a homogeneously aligned LC layer between the thin alignment films and the ITO electrodes on the quartz substrates are studied. It is shown that, in the case of the use of traditional alignment films or their absence, the ITO electrodes lead to significant resonant losses in the waveguide mode for both TE- and TM-polarized light. The losses mechanism based on a phase-synchronized mode coupling occurring in relatively narrow spectral ranges is discussed. We also propose a method to control and exclude the losses using thin alignment films with a proper thickness and low refractive index.

Лев Михайлович Блинов и его жизнь в науке (1939–2023)