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.

Laser light emission in dye-doped chiral liquid crystals (CLCs) has been experimentally demonstrated for the homeotropic–planar (hybrid) orientation. A numerical simulation of this structure showed that the helicoid (helix) pitch period in CLC in the hybrid cell and the director distribution depend significantly on the anchoring energy at the homeotropic sample boundary. The anchoring energy plays the role of a factor facilitating the CLC helix unwinding. The lower anchoring energy, the smaller the pitch is and the closer to the natural CLC pitch it is. In this case, the length of the “screw"-type structure near the homeotropic cell boundary decreases. Thus, as the anchoring energy at the hybrid-cell homeotropic boundary decreases, the director distribution in the cell approaches the distribution in the planar (Grandjean) cell. This is confirmed by the laser light emission in the same spectral range as in the planar cell.

Статья посвящена крупному ученому и организатору науки, профессору Льву Михайловичу Блинову, посвятившему свою жизнь исследованию жидких кристаллов и организованных молекулярных структур. Лев Михайлович является основателем школы экспериментальных исследований органических пленок и жидких кристаллов в Советском Союзе, им воспитаны десятки молодых ученых, ставших кандидатами и докторами наук. Л.М. Блиновым написаны сотни научных статей, множество обзоров, монографий и учебников, читались лекции как в России, так и по всему миру. Кратко изложена биография Льва Михайловича, дан обзор его наиболее значимых достижений и отдельных публикаций, получивших широкий резонанс в научном сообществе. Приведены воспоминания учеников и коллег.

We propose a method for creating a periodic planar-homeotropic alignment (PHA) of liquid crystal (LC) on a film prepared by spin-coating a solution of a mixture of a photosensitive organic azo dye and a surfactant. Irradiation by polarised and non-polarised ultraviolet light provides areas with reliable planar and homeotropic LC alignment, respectively. Using a photomask, gratings with PHA are created, and optical properties of the LC cells based upon them are investigated. For the first time, the characteristic fine features of the field-induced transition of the initially standing chiral LC helix into a Deformed Lying Helix (DLH) configuration with the spiral axis lying in the LC layer plane are visualised and studied. The observed optical images illustrate the high-quality DLH pattern. Numerical simulations of the DLH reveal the peculiarities of the obtained structure and how it is formed.

We report on experimental investigations of the lasing effect in novel chiral liquid crystal (CLC) systems with a deformed lying helix (DLH). The lasing is studied for both odd- and even-order field-induced stop-bands, which are characteristic exclusively of the DLH state. The DLH state is achieved in special CLC cells with periodic boundary conditions, when the surface alignment is flipped between planar and vertical states. The alignment surfaces are prepared using focused ion-beam lithography. In an electric field, such CLC systems undergo an orientational transition, when the initial Grandjean-plane texture with the helix axis perpendicular to the CLC layer is transformed into the DLH state with the helix axis oriented in the plane of the layer. Due to field-induced strong deformation, the DLH system is characterized by a set of photonic stop-bands with a fine spectral structure; namely, on these fine-structured sub-bands, we have observed and studied the low-threshold lasing effect.

The field-induced orientational transition in layers of cholesteric liquid crystals with spatially periodic modulation of the surface anchoring is studied by numerical simulations and experimentally. The modulation of the surface anchoring is implemented using high-resolution focused ion-beam treatment of a polymer film providing planar alignment conditions. A specific feature of the orientational transition is that the thermodynamically stable initial planar structure with the helix axis along the normal to the layer transforms under an electric field into an equilibrium structure with a deformed lying helix (DLH) in the plane of the layer. For such a transition to occur the natural pitch of the cholesteric helix must be substantially less than the anchoring modulation period. The appearance of the DLH with a pitch corresponding to the anchoring modulation period results in strongly enhanced first-order diffraction efficiency. The orientational DLH transition is characterised by a rather narrow driving voltage range with a pronounced hysteresis. The waveguide lasing effect with characteristic of the deformed helix spectral modes is demonstrated in a range of the DLH transition. [GRAPHICS] .

Fluorescence and lasing in a structure with an electric-field-induced spatially periodic director modulation in the plane of a planar-oriented layer of a cholesteric liquid crystal (CLC) is experimentally investigated. The thickness of the CLC layer is chosen close to the natural pitch of the cholesteric helix, which corresponds to the second Grandjean zone. The electric-field-induced periodic field of the CLC director leads to spatial modulation of the refractive index and the appearance of optical properties typical of 1D photonic crystals, when light propagating in the plane of the CLC layer experiences Bragg reflection. In polarised light, the induced spatial modulation of the refractive index manifests itself in the form of banded domains oriented in the plane of the CLC layer perpendicular to the original (unperturbed electric field) direction of the director in the centre of the layer. For the electric field strengths that correspond to fundamentally different distributions of the field of a CLC director, the fluorescence spectra of the DCM laser dye are studied both for different geometries (including the waveguide regime) and for different levels of optical pumping. In the range of electrical voltages corresponding to the induction of a spatially periodic photonic structure, multimode lasing in the waveguide regime is detected. It is shown that the mode composition of lasing depends on the electric field, which affects the properties of the distributed feedback.

The amplification of spontaneous fluorescence in a planar layer of nematic liquid crystal doped with DCM dye in the mode of waveguide light propagation has been studied. The gain reaches the value α = 0.0014 μm –1 at a pump radiation intensity of 1.38 MW/cm 2 . Numerical simulation of the structure imitating the experimental cell showed qualitative agreement of the calculation results with the experimental data.

Waveguide lasing in a layer of nematic liquid crystal (NLC) doped with a dye has been experimentally observed. A chromium micrograting with a period of 1.76 μm has been deposited on the surface one of the cell substrates to provide a distributed feedback (DFB) and partially extract laser radiation to a quartz substrate. The DFB provides lasing in the ninth diffraction order for waveguide TM modes. Laser radiation is observed at the output of the substrate end face at an angle of 67.0° ± 1.5° with respect to the normal to the waveguide plane. An increase in voltage across the micrograting electrodes leads to red-shifted multimode lasing. A numerical simulation of a structure imitating the experimental cell has shown good agreement between the calculation results and experimental data.

Structures with a periodic in-plane liquid crystal director field modulation induced by an electric field are studied in cholesteric liquid crystals (CLCs). A phenomenon of the electric-field-induced instability in a planarly aligned cholesteric cell is used to create these undulated structures. The initial field-off state is planarly aligned with the cholesteric helix axis oriented perpendicular to the cell substrates. The interaction of the CLC with an electric field results in modulation of the refractive index, which is visualised as stripe domains oriented either along or perpendicular to the rubbing direction at cell alignment surfaces. The threshold electric field for the undulation appearance and a period of stripes are measured experimentally for three Grandjean zones (ratio d/p similar to 0.5, 1.0, and 1.5, where d is a cell thickness and p is the natural cholesteric pitch). For the zone with d/p similar to 1.0 using numerical simulations, we describe in detail the director distribution at an applied electric field. It is found that the in-plane undulated structure is characterised by a conical director rotation on moving along the alignment direction. The conical axis is tilted with respect to the alignment axis. The sign of the tilt angle depends on the handedness of CLC. [GRAPHICS] .

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 degrees) at an angle of about 67.0 degrees 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. [GRAPHICS] .

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.

We have studied the alignment of a nematic liquid crystal (LC) material on aluminum subwavelength nanogratings as a function of the period, p, and the slit width to period ratio, w/p. A method, based on Fourier analysis of the transmittance spectra of the LC grating system, has been applied. We show that the gratings provide stable planar alignment only for shorter periods and narrower slits (p < 400 nm, w/p < 2/3). As these parameters increase, the homogeneous surface alignment changes to domains with different tilt angles or to spatially modulated alignment. We have also obtained a 90° twisted LC director distribution, implying sufficiently strong azimuthal LC anchoring at the grating surface.

Waveguide lasing in a layer of a dye-doped nematic liquid crystal has been observed. The liquid-crystal layer was sandwiched between a quartz substrate and a glass cover plate on whose surface was deposited an interdigitated electrode system. This system had a period of 3.75 mu m and played a dual role, namely, it created a spatial periodicity of the waveguide medium refractive index (thus creating distributed feedback) and served as a diffraction grating coupling out a part of waveguide radiation into the glass cover plate. The distributed feedback ensured lasing in the 18th diffraction order for the TE modes and in the 19th order for the TM modes of the waveguide. The generated radiation was observed at the exit from the glass plate end face at the angles to the waveguide plane of 33.1 +/- 1.5 degrees for TM modes and 21.8 +/- 1.8 degrees for TE modes. The intensity and position of the TE emission line showed no regular dependence on the voltage on the electrodes. In the case of TM radiation, an increase in the voltage led to a short-wavelength shift of the laser line and to a decrease in its intensity.

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.

The paper presents the results of calculating the threshold pumping power necessary for laser generation in cholesteric liquid crystals doped with fluorescent dyes. Calculations have been performed with allowance for (i) specific spectral properties of the chiral cholesteric medium as a photonic structure and (ii) the spatially distributed character of feedback in the active medium. The dependences of the threshold pumping power on the sample thickness, dye concentration, and leading front widths of long pumping pulses have been studied.

We report on the results of calculating the conditions for light generation in cholesteric liquid crystals doped with fluorescent dyes using kinetic equations. Specific features of spectral properties of the chiral cholesteric medium as a photonic structure and spatially distributed type of the feedback in the active medium are taken into account. The expression is derived for the threshold pump radiation intensity as a function of the dye concentration and sample thickness. The importance of taking into account the distributed loss level in the active medium for calculating the optimal parameters of the medium and for matching the calculated values with the results of experiments is demonstrated.