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.

In a layer of a chiral liquid crystal with negative dielectric anisotropy and the thickness d corresponding to the second Grandjean zone ( 3p_0/4 < d < 5p_0/4 , where p_0 is the natural helical pitch) under the action of a planar electric field, an orientational transition occurs from the ground state twisted by the angle 2π to a topologically equivalent untwisted state (0 state). Although the 0 state is metastable and long-lived, it can rapidly be transformed back to the 2π state by an electric field pulse of a comparatively small amplitude. The direct transition to the 0 state is induced by the interaction of the electric field with negative dielectric anisotropy, while the rapid reverse transition can be attributed to the flexoelectric interaction.

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.

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.

The optical spectra of the cholesteric liquid crystal (CLC) layers under conditions of hybrid anchoring show a short-wave shift under a pulsed electric field. This behavior is anomalous because it is associated with a decrease in the pitch of the cholesteric spiral, which is atypical at conditions when the electric field is perpendicular to the axis of the CLC spiral. An analytical model of the phenomenon is discussed, according to which the spiral pitch under hybrid boundary conditions can be greater than the natural pitch in an unlimited volume of CLC. An in-plane electric field, being localized near the homeotropic-alignment surface, can be treated as effectively influencing the azimuthal anchoring and leading to a variety of metastable states with both increased and decreased pitch. These metastable states with local minima of free energy prevent the spiral from unwinding, and corresponding bands of selective reflection can even be shifted to the short-wave region of the spectrum. The observed effect is also studied numerically. It is shown by numerical simulations that the localized electric field from short-pitch electrodes can also modify zenithal anchoring, which should allow for defect-free controlling of the spiral pitch and spectral stop-band location.

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.

Spectral singularities of the ampere–watt sensitivity of photoelectric structures consisting of a transparent indium–tin oxide electrode, a photosensitive organic layer, and an aluminum electrode have been studied. The structures have been formed on a quartz glass substrate. The photosensitive layer has been vacuum-evaporated either from zinc phthalocyanine ZnPc (exhibiting donor properties) and C 70 fullerene (acceptor) organic precursors or from a ZnPc:Cr 70 donor–acceptor blend. Using computer simulation, the structure of absorption bands has been determined in a wide spectral range for all three above systems. This has made it possible to calculate the absorbed and reflected fractions of radiation incident on the sample and explain the singular spectral behavior of the ampere–watt sensitivity of the ZnPc:C 70 blend. It has been shown that the photosensitivity of the blend reaches a maximum near the overlap of the absorption bands of donor and acceptor molecules.

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.

In a planarly aligned layer of a nematic liquid crystal (LC) the applied electric field can induce a periodic stripe pattern, which is characteristic of the well-known flexoelectric instability effect. We have studied the flexoelectric instability in LC layers with periodic planar-homeotropic alignment (PHA) at one of the layer surfaces and compared the results to the case of the homogeneously aligned layer. It is found that the PHA is more favourable for the flexoelectric spatial modulation. The flexoelectric grating is formed significantly faster in the case of short-period PHA as compared to the cell with homogeneous alignment, which is due to both PHA periodicity and flexoelectric polarisation that in the case of PHA is non-zero even in the field-off state. The impressive finding is that the flexoelectric gratings are spatially synchronised with PHA and free of fork-like defects. The Fourier analysis of the patterns shows that the wavenumber of the flexoelectric gratings is multiple of the PHA grating wavenumber, which points to the spatial resonance of the flexoelectric instability with the PHA harmonics. The improved dynamics with the synchronisation opens perspectives for new electrooptical and photonic applications of the flexoelectric gratings.

Liquid crystal (LC) metasurfaces self-assemble on polymer alignment layers patterned by focused ion beam inducing micrometre and submicrometre scale modulations of LC director imparting specific wavefront profiles to the transmitted light required for versatile optical functionalities. We report on the design, optimisation, fabrication and characterisation of LC metasurfaces performing as Dammann gratings distributing light uniformly in many diffraction channels. Comparing them with the previously reported diffracting and beam-steering LC metasurfaces, we demonstrate how the same approach can be applied to create photonic devices with qualitatively different diffraction properties switchable between diffracting and transparent states by low voltage within several milliseconds. Analysing the achievements, we elucidate the future potential of the LC metasurface concept and discuss particular application prospects.

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.

Controlling the phase of light with a high efficiency and precision is essential for applications in imaging, tunable devices, and optical systems. Spatial light modulators (SLMs) based on liquid crystals (LCs) have been regarded as one of the best choices for the generation of phase profiles for the steering of light. The upper glass substrate has an unpatterned electrode for a common electrode, while the lower glass substrate has one-dimensional micro-patterned electrodes for controlling the single pixel level by the applied voltages. By applying different voltages to each electrode to create a sawtooth-shaped phase profile, the collimated input beam is deflected to the desired angle. To maximize the diffraction efficiency (DE) values, an advanced simulation method has been developed to find the optimized phase profile through the analysis of LC director distributions. The resulting diffraction patterns are investigated both computationally and experimentally, with a good agreement between the results obtained. Finally, the beam deflector (BD) system with an advanced driving algorithm has a high 1st order DE, about 60%, 37%, and 7.5% at 1°, 2.5°, and a maximum steering angle of 7.5°, respectively. The LC director distributions in relation to various diffraction angles are simulated and an experimental success in realizing enhanced DE for the beam steering device is presented.

We study versatile soft-matter metasurfaces based on self-assembling of nematic liquid crystal on polymer alignment layers processed with a focused ion beam. Digital control of the beam path allows imprinting patterns that induce different complex distributions of the refractive index within several micrometer thick liquid crystal layers. We optimize them to implement various optical functionalities, such as broadband anomalous refraction, wide-aperture focusing, and beam splitting in tens of channels.

We report the formation of high optical power microlenses in the near-surface region of the liquid crystal layer. Such microlenses, possessing a very small focal length f at a rather large aperture A (f/A∼2), are able to focus the light into spots of a characteristic size comparable with the wavelength. Using numerical modeling, a specific patterning profile of a liquid crystal (LC) alignment surface by an ion beam is proposed to provide the aligning properties necessary for the formation of an array of microlenses with a focal length comparable to the LC cell thickness. The proposed microlens arrays are produced, and their optical properties are discussed.

The photoelectric properties of thin films based on an organic composition of fullerene (C 60 ), zinc phthalocyanine (ZnPc), and a ferroelectric copolymer of vinylidene fluoride with trifluoroethylene P(VDF–TrFE) are studied. Along with well-pronounced photoelectric properties (which are characteristic of fullerene–phthalocyanine mixtures), this material is shown to have ferroelectric properties as well. Ferroelectric switching of polarization is found to cause bistable switching of the photoelectric effect with a significant increase in the ampere–watt sensitivity in one of the polarized states. This effect is explained by a change in the built-in local electric field, which depends on the direction of the ferroelectric polarization vector and acts inside the cavities of the ZnPc/C 60 molecular heterojunctions.

For LCs with a non-zero flexoelectric coefficient difference (e1–e3) and low dielectric anisotropy, electric fields exceeding certain threshold values result in transitions from the homogeneous planarly aligned state to the spatially periodic one. Field-induced grating is characterized by rotation of the LC director about the alignment axis with the wavevector of the grating oriented perpendicular to the initial alignment direction. The rotation sign is defined by both the electric field vector and the sign of the (e1–e3) difference. The wavenumber characterizing the field-induced periodicity is increased linearly with the applied voltage starting from a threshold value of about π/d, where d is the thickness of the layer. Two sets of properties of the field-induced gratings are studied in this paper using numerical simulations: (i) the dynamics of the grating appearance and relaxation; (ii) the transmittance and reflectance spectra, showing photonic stop bands in the waveguide mode. It is shown that under ideal conditions, the characteristic time of formation for a spatially limited grating is determined by the amplitude of the electric voltage and the size of the grating itself in the direction of the wave vector. For large gratings, this time can be drastically reduced via spatial modulation of the LC anchoring on one of the alignment surfaces. In the last case, the time is defined not by the grating size, but the period of the spatial modulation of the anchoring. The spectral structure of the field-induced stop bands and their use in LC photonics are also discussed.

Liquid crystals subjected to modulated surface alignment assemble into metasurface-type structures capable of various flat-optical functionalities, including light diffraction and focusing, deflection and splitting. Remaining in a fluid phase, they are susceptible to external stimuli, and, in particular, can be efficiently controled by low voltages. We overview the existing approaches to the design and fabrication of liquid-crystal metasurfaces, highlight their realized optical functions and discuss the applied potential in emerging photonic devices.