A. R. Geivandov

Switching time dispersion and retention of bistable states in Langmuir-Blodgett ferroelectric films

Polarization switching and retention of each of the two polarized states in Langmuir—Blodgett (LB) ferroelectric films are studied using nonlinear dielectric spectroscopy. It is found that polarized states can be preserved for a long time, but the polarization dynamics in 10–40-nm-thick LB films is characterized by a considerable switching-time dispersion. In addition, ferroelectric LB films exhibit clearly manifested asymmetry of switching to states with opposite directions of polarization. To explain the experimental results, a polarization-switching model is proposed that takes into account the energy of interaction of a ferroelectric polymer with boundary surfaces. The effect of inhomogeneity of the LB film structure on the ferroelectric switching dynamics is also discussed.

Lyotropic liquid crystal guest–host material and anisotropic thin films for optical applications

We propose a new approach for the production of thin film optical functional materials. The method is based on molecular design whereby two different types of lyotropic liquid crystals (LC), lyotropic LC based on columnar supramolecules and water-soluble rod-like polymer molecules are mixed. The resulting lyotropic guest–host system allows production of optical retardation films with tunable optical anisotropy controlled by composition of the guest–host system. Coatable retarders can be used in modern liquid crystal displays and TVs for optical compensation and enhancement of the LCDs performance.

Fast electric field switched 2D-photonic liquid crystals

We demonstrate field-induced 2D-photonic liquid crystals (LC). The 2D spatially periodic modulation of the LC director field is achieved using a geometry with two crossed interdigitated systems of electrodes located at opposite sides of the LC layer. With a special method of dual-field driving, a very fast switching between different spatially periodic LC director distributions is achieved. The director field distribution and potential use of these photonic crystals for fast switched multidirectional lasing is discussed.

Liquid crystal on subwavelength metal gratings

Optical and electrooptical properties of a system consisting of subwavelength metal gratings and nematic liquid crystal layer are studied. Aluminium gratings that also act as interdigitated electrodes are produced by focused ion beam lithography. It is found that a liquid crystal layer strongly influences both the resonance and light polarization properties characteristic of the gratings. Enhanced transmittance is observed not only for the TM-polarized light in the near infrared spectral range but also for the TE-polarized light in the visible range. Although the electrodes are separated by nanosized slits, and the electric field is strongly localized near the surface, a pronounced electrooptical effect is registered. The effect is explained in terms of local reorientation of liquid crystal molecules at the grating surface and propagation of the orientational deformation from the surface into the bulk of the liquid crystal layer.

Mathematical model of compensation for dispersion of light polarization states in electrooptical systems based on chiral liquid crystals

Spectral dispersion of the polarization states occurs after passage of linearly polarized light through a thin layer of a chiral nematic crystal along the helical axis. It is clearly pronounced for crystals with a high optical anisotropy and a helical pitch somewhat larger than the light wavelength. The results of numerical analysis of the spectral dependence of polarization states at the output of the liquid crystal layer and the method for compensating for the dispersion are presented for the first time. It is shown that polarization dispersion can be eliminated using phase plates of two types, one of which should have a high anomalous birefringence dispersion. The possibility of developing fast light modulators operating in a wide spectral range is discussed.

Plasmon electro-optic effect in a subwavelength metallic nanograting with a nematic liquid crystal

The electro-optic effect in hybrid structures based on subwavelength metallic nanogratings in contact with a layer of a nematic liquid crystal has been experimentally studied. Metallic gratings are fabricated in the form of interdigitated electrodes, which makes it possible to use them not only as optical elements but also for the production of an electric field in a thin surface region of the layer of the liquid crystal. It has been shown that, owing to the electric-field-induced reorientation of molecules of the liquid crystal near the surface of the grating, it is possible to significantly control the spectral features of the transmission of light, which are caused by the excitation of surface plasmons. The electro-optic effect is superfast for liquid crystal devices because a change in the optical properties of the system requires the reorientation of molecules only in a very thin surface layer of the liquid crystal.

Antiferroelectric and ferroelectric switching of a liquid crystal in Langmuir-Blodgett films under strong confinement conditions

An antiferroelectric liquid-crystal (LC) material formed of banana-shaped molecules in the sandwich metal-thin Langmuir-Blodgett film-metal geometry, which is typical of solid-state technology, is investigated. Upon heating the thin-film elements, the material passes to the high-temperature (68–127°C) LC B2 phase, which, despite severe limitation on the film thickness (∼100 nm or less), exhibits antiferroelectric switching, which was previously observed only in bulk samples (10 μm thick) of the LC studied. At film thicknesses smaller than ∼40 nm, ferroelectric switching is observed, which is caused by the stabilization of the ferroelectric LC phase by the boundary surfaces. The largest values of the switched polarization of films (∼400 nC/cm2) are comparable with the polarization of bulk samples. The coercive field increases with decreasing film thickness and reaches ∼106 V/cm for the thinnest films. This value corresponds to the intrinsic coercive field of the ferroelectric under conditions of a strong surface effect, which suppresses domain processes.

Fourier Spectroscopy as a Method of Investigation of Photoelectric Properties of Organic Systems

A new method of investigation of photoelectric properties of layered thin-film structures based on broadband Fourier spectroscopy exhibiting a harmonically modulated optical delay is proposed. In contrast to traditional approaches to study photoelectric properties, which are based on application of dispersive spectral devices, the proposed method allows not only simultaneously covering the ultraviolet, visible, and infrared spectral ranges, while demonstrating a wide dynamic range and high spectral resolution, but also easily varying low-frequency modulation of the action of light. The capabilities of the method are demonstrated using a polycrystalline organic heterostructure as an example. Its spectral sensitivity, speed, and specific detectivity are measured. A model and an equivalent electric circuit are proposed for explanation of the results of the measurements.

Lasing in micro-grating liquid crystal systems

ABSTRACT A waveguide lasing effect has been observed and investigated in a dye-doped layer of a nematic liquid crystal (NLC) between two quartz plates. One of the plates has an electrode micro-grating, which allows (i) creating the feedback, (ii) guiding a part of the lasing emission into the quartz substrate and (iii) modulating the NLC refractive index by an electric field. At 0 V, a single Transverse Magnetic mode (TM)-polarised mode lasing has been observed. The emission goes out from the edge of the quartz plate in a narrow angular sector (±1.5°) at an angle of about 67.0° with respect to the NLC layer normal. At voltage applied, a number of additional lasing modes propagating at the same angle, but located at different wavelengths, appear. The experimental results are interpreted on account of numerical simulations based on the finite difference time domain method. Graphical Abstract

Micro- and nanostructures for the spatially periodic orientation of liquid crystals obtained by focused ion beam milling

Micro- and nanostructured surfaces creating spatially periodic boundary conditions of the alignment of nematic liquid crystals in two mutually orthogonal directions perpendicular and parallel to the surface are obtained by focused ion beam milling. It is shown that ion milling provides an easy axis along the normal and sufficiently strong anchoring energy. The value of this energy can noticeably exceed the energy of the planar anchoring of liquid crystals with typical orienting surfaces on the basis of polymer films. Using the numerical simulation, the anchoring energy values necessary for an implementation of a deep modulation of the director field with a spatial period of hundreds of nanometers are determined, which is important for creation of photonic liquid-crystal systems.