The alignment of liquid crystals (LC) can be arranged using specially prepared alignment layers. In the case of layer-by-layer deposition of a polymerizable LC, the upper surface of the preceding layer plays the role of the alignment layer. We succeeded in separating the film of the polymerizable LC RM257 aligned on the azo dye surface from the substrate and using its opposite side to align the next LC layer. The "Brilliant yellow" azo dye as photoalignment layer was irradiated by linearly polarized light with subsequent coating of RM257 layer, followed by its UV-curing. Then, a drop of UV-polymerizable glue was applied to the surface of the polymer film and the second substrate was attached. After fixing the UV glue, the resulting sandwich was mechanically separated. In the process of substrate separation, the azo dye remains on the mother substrate, and the oriented polymerized film of RM257 is transferred to the second substrate. The resulting alignment surface provides high-quality orientation of LC materials-both uniform and with a complex spatially periodic distribution of the LC director typical for diffraction optical elements. This approach eliminates transmission losses associated with the azo dye absorption, significantly improves the adhesion of the film, and allows for transferring the aligned film onto various substrates.

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.

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.

The “Patterned-Liquid-Crystal for Novel Displays” is a Special Issue focused on new insights and explorations in the field of liquid crystals arranged in a periodic patterned way [...]

Liquid crystals self-assemble on nanopatterned alignment layers into purely soft matter metasurfaces sensitive to external stimuli and imparting tailored spatial modulations to transmitted light wavefronts. Upon fine optimization, they are capable of efficient light deflection by virtue of anomalous refraction into a dominating transmission diffraction order. To expand the spectral range and acquire additional functionality, we put forward the double-sided metasurface design based on the liquid crystal alignment by a pair of complementing patterned substrates. We numerically optimize, fabricate, and experimentally characterize metasurfaces refracting red light with an efficiency of up to 70% and sustaining the efficiency above 50% in a broad range of visible wavelengths exceeding 500 nm. We verify that the refraction is reversibly switched in less than 10 ms by voltages of a few volts. We also report on a remarkable mechanical reconfigurability, as micrometer-scale relative substrate shift flips the refraction direction.

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 propose, optimize, fabricate and test beam-steering elements based on double-sided liquid-crystal (LC) metasurfaces allowing for diffractive and spectral multiplexing, and thus covering three different beam deflection directions each. While straightforward parallel design requires one diffractive beam-steering element per a direction determined by Bragg’s law, double-sided LC-metasurfaces are remarkably flexible and allow optimization for three operation modes at different applied voltages: zero-voltage mode with dominant +1 order red light and +2 order blue light diffraction; intermediate mode at 1.4–1.6 V voltage with dominant +1 order blue light diffraction; and transparent mode at 5 V voltage. We comprehensively study three such elements with 6, 8 and 10 micrometer periods and verify their capability of deflecting blue and red light beams with 40–70% efficiencies in nine target directions with 10 ms characteristic switching times.

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.

Dember photovoltaic effect (Dember effect) was used to study permeability of a thin layer of lyotropic liquid crystal (dipalmitoylphosphatidylcholine) at various temperatures. By photo dissociation of a photosensitive ionic dye dissolved in a lyotropic liquid crystal, an inhomogeneous distribution of charge carriers was induced in a thin liquid crystal layer. The diffusion relaxation of these charge distributions depends on the structure of lyotropic liquid crystal and show up itself in the temperature dependence of pulsed photoelectric response. The data obtained are in a good agreement with the results of dielectric measurements. In perspective, above method can be applied to examine permeability of natural and synthetic lipid vesicles (cells, exosomes, liposomes and other vesicles) relevant in medicine and to characterize the structural phase transitions in lipid membranes when modified by molecules of interest (e.g. cholesterol in liposomes to increase rigidity and stability).

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] .

The conditions for the emergence of a fast electro-optical mode in a liquid crystal cell with a short period ( p = 1.8 μm) of interdigitated electrodes have been investigated. The electro-optical switching in two liquid crystal mixtures differing in viscosity and dielectric and optical anisotropies has been studied through numerical simulations and experimentally. The fast mode switching times are shown to be in the submillisecond range and to be related to the switching of a near-electrode liquid crystal layer comparable in thickness to the period of the electrode structures. The inclusion of the entire liquid crystal volume in the switching process is characterized by a slow mode with considerably longer times.