Photoalignment Technique Research Articles

Alignment assessment of anisotropic liquid crystals through an automated image processing algorithm

Liquid crystals (LCs) are recognised as an emerging type of responsive and functional materials that exhibit diverse technological applications. The spatial arrangement or alignment of LC molecules creates an optical anisotropy, which influences the propagation of light. This study presents an automated image processing algorithm designed to quantitatively assess LC alignment. The algorithm calculates the image structure tensor and computes a score ranging from 0 to 1 to represent the uniformity of LC alignment. The tool was verified on both artificially generated images of lines with pre-defined orientations and then tested on microscope images of LCs. For the latter, test samples of deformed helix ferroelectric LCs were constructed using three different alignment methods. Analysis of microscopic images of samples prepared utilising photoalignment, polymer rubbing, and oblique evaporation alignment techniques, determined average alignment scores (mean ± standard error of the mean) of 0.44 ± 0.03, 0.26 ± 0.1, and 0.17 ± 0.04, respectively, in comparison to commercially sourced control samples with a score of 0.64 ± 0.03 (n=5 each). This study paves the way for automated objective assessment of LC alignment, a critical aspect to ensure the optimal performance, reliability, and efficiency of various devices and sensors relying on LC technology.

Photoaligned Tunable Liquid Crystal Lenses with Parabolic Phase Profile

We present a theoretical model of a cylindrical tunable liquid crystal lens based on the modulation of anchoring energy. The latter can be easily obtained using photoalignment techniques. The liquid crystal cell we propose exhibits strong anchoring at the top substrate and anchoring energy with a parabolic profile at the bottom substrate. The model describes the dependence of the focal length on the applied voltage and presents a theoretical study of the lens aberrations. The results obtained are of general relevance and can be used to optimize the performances of every type of liquid crystal lens with a parabolic profile.

Geometric-phase-based axicon lens for computational achromatic imaging.

Conventional optical imaging systems usually utilize several lenses within a precise assembly to eliminate chromatic aberration, which increases the difficulty of system integration. In recent years, with the rapid development of metasurfaces and liquid crystals (LCs), planar optical elements provide feasible solutions to realize flexible light manipulation and lightweight systems. However, there also exists chromatic aberration, which can be corrected but at the cost of a complex device design. Here, a geometric-phase-based axicon lens is utilized to correct chromatic aberration across a broadband wavelength with the assistance of a post-process algorithm. The axicon lens is fabricated through arranging orientations of liquid-crystal molecules with a standard photoalignment technique, and it produces an approximately invariant point spread function (PSF) at several discrete wavelengths, which is used as the prior information to extract the object in the blurred image. In the experiment, the reconstruction quality is significantly improved after the post-process algorithm. We expect our work to provide further development to reduce the dispersion with both the device design and the computational image technique.

Photoalignment and Photofixation of Chromonic Mesophase in Ionic Linear Polysiloxanes Using a Dual Irradiation System

Photoalignment technology enables macroscopic alignment of liquid crystalline molecules and their aggregates in a non-contact process by irradiating photo-responsive liquid crystalline compounds with linearly polarized light. Because photoalignment techniques prevent dust generation and uneven stretching, and accomplish fine and complex patterning, they are involved in the practical process of fabricating display panels, and continue to be applied in the research and creation of various anisotropic materials. Brilliant yellow (BY), a chromonic liquid crystal, has attracted considerable attention as the photoalignment sublayer in recent years, because of its ability to induce a high dichroic nature among many photo-responsive liquid crystalline materials. However, its dichroism is not maintained after prolonged exposure to a humid environment because of its intrinsic strong hygroscopicity of ionic BY molecules. In this study, to overcome this drawback, the photoalignment and successive photo-fixation of the BY columnar phase is proposed using UV-curable ionic polysiloxane as a matrix. Visible light was used for the photoalignment of the BY columnar phase, and UV light for photo-fixation. Consequently, the columnar chromonic phase is found to retain its orientation even after 4 h of exposure to a highly humid environment.

Customized Design of Aperiodic Liquid Crystal Grating for Generation of Multiple Optical Patterns

Liquid crystal (LC) grating has drawn extensive attention due to its merits, including tunability, easy preparation process, cost effectiveness, and high diffraction efficiency. However, limited efforts have been made to explore LC grating for the generation of customized diffraction patterns, despite the fact that the patterning size of LC molecular director orientation has been tremendously reduced even to subwavelength scale via high-precision photoalignment techniques. Here, we propose a versatile method based on the binary particle swarm optimization algorithm to efficiently design aperiodic LC gratings for customized optical patterns generation, including needle beam, multi-foci, and hollow beam. The optical performance of the LC gratings was simulated using both the vectorial Rayleigh–Sommerfeld method (with discrete zero or π phase, in this article) and the actual director orientations. These results matched each other well, which proves the effectiveness of the optimization method based on discrete phase. This study paves the way for designing LC gratings with engineered functionalities, which may find applications in optical tweezers, laser delivery systems, and laser scanning imaging.

Toward Arbitrary Spin‐Orbit Flat Optics Via Structured Geometric Phase Gratings

AbstractReciprocal spin‐orbit coupling (SOC) via geometric phase with flat optics provides a promising platform for shaping and controlling paraxial structured light. Current devices, from the pioneering q‐plates to the recent J‐plates, provide only spin‐dependent wavefront modulation without amplitude control. However, achieving control over all the spatial dimensions of paraxial SOC states requires spin‐dependent control of corresponding complex amplitude, which remains challenging for flat optics. Here, to address this issue, a new type of flat‐optics elements termed structured geometric phase gratings is presented, that is capable of conjugated complex‐amplitude control for orthogonal input circular polarizations. By using a microstructured liquid crystal photoalignment technique, a series of flat‐optics elements is engineered and their excellent precision in arbitrary SOC control is shown. This principle unlocks the full‐field control of paraxial structured light via flat optics, providing a promising way to develop an information exchange and processing units for general photonic SOC states, as well as extra‐/intracavity mode convertors for high‐precision laser beam shaping.

Dual photopatterning of rotational fingerprint superstructures

Controlling architecture of hierarchical microstructures in liquid crystals (LCs) plays a crucial role in the development of novel soft-matter-based devices. Chiral LC fingerprints are considered as a prospective candidate for various applications; however, the efficient and real-time command of fingerprint landscapes still needs to be improved. Here, we achieve elaborate rotational fingerprint superstructures via dual photopatterning semifree chiral LC films, which combine the photoalignment technique and a dynamic light patterning process. An intriguing spatial-temporal rotational behavior is presented during the patterning of chiral superstructures. This work opens new avenues for the applications of chiral LCs in soft actuators, sensing, and micromanufacturing.

Chiral liquid crystal based holographic reflective lens for spectral detection.

Flat optics based on chiral liquid crystal (CLC) can be achieved using holographic polarization recording with the help of a photoalignment technique to vary the orientation of the optical axis in a thin CLC layer. A variety of reflective diffractive optical components with high efficiency and polarization selectivity can be realized employing this technique. In this work we discuss the use of CLC diffractive lenses in a spectrometer. The functionalities of two mirrors and a linear grating used in a traditional spectrometer are combined into a single holographic CLC component. Circularly polarized light entering through the slit can be reflected and projected onto a linear detector by the CLC component, with over 90% efficiency. This excellent optical functionality can be achieved with a micrometer thin CLC layer, offering the opportunity for device integration.

Heliconical Cholesterics Endows Spatial Phase Modulator with an Electrically Customizable Working Band

AbstractMultidimensional and large‐scale parallel manipulation of light, especially on‐demand tailoring of the working frequency and spatial phase front, is highly pursued in modern optics. Here, broadband tunable planar optics is demonstrated by electrically driving the nanohelix of photopatterned heliconical cholesterics. By preprogramming the initial orientation of the helixes using a dynamic‐mask photoalignment technique, spatial geometric phases can be arbitrarily encoded to the reflected light in a reconfigurable way. Due to the reversible electrically variant pitch of the heliconical superstructures, the reflective Bragg band can be precisely selected in the range from 380 to 1550 nm. In addition to wavelength selection and geometric phase modulation, spatial amplitude modulation and spin reversion can be further expected. This may offer a platform for full‐dimensional manipulation of light, including wavelength/frequency, phase, amplitude, time, and spin, thus upgrading optical information processing techniques.

Two‐Step Photoalignment with High Resolution for the Alignment of Blue Phase Liquid Crystal

AbstractThe self‐assembling properties of liquid crystal (LC) are ideal for realizing switchable optical components. The director organization is strongly determined by the alignment at the substrate interfaces and photoalignment is a versatile method based on linearly polarized UV light. In this work, a two‐step photoalignment procedure is proposed to pattern the alignment at the surface. Illumination with uniform polarization or with an interference pattern is combined to obtain stripe patterns or square patterns of photoalignment. It can indeed be confirmed that the nematic director follows the varying azimuthal orientation. By increasing the angle between the two interfering beams, photoalignment with sub‐micrometer resolution is obtained, compatible with the dimensions of blue phase (BP) LC unit cell. Homogeneous domains of BP II with (100) or (110) crystal orientation are obtained and their Kossel patterns are recorded. The two‐step photoalignment technique allows to create patterns with high resolution and to control the orientation of BP LC, which is promising for photonic applications that require single domains.

Holo-imprinting polarization optics with a reflective liquid crystal hologram template

Liquid crystal polarization optics based on photoalignment technique has found pervasive applications in next-generation display platforms like virtual reality and augmented reality. Its large-scale fabrication, however, remains a big challenge due to the high demands in small feature size, fast processing speed, and defects-free alignment quality during the photoalignment process, especially for large-angle reflective devices. Here we propose a new concept of holo-imprinting based on non-contact replication of polarization pattern with a reflective liquid crystal hologram as a template. Our theoretical analysis and experimental results validate the possibility of generating a high-quality polarization pattern exploiting the self-interfering beams of reflective holograms. The method can be extended to numerous devices, from transmissive to reflective, from small angle to large angle, and from grating, lens, to freeform optics. Its widespread impact on the fabrication of liquid crystal polarization optics for advanced display and imaging systems is foreseeable.

4-domain twisted liquid crystal micropolarizer array for visible linear polarization imaging

In this paper, a 4-domain twisted liquid crystal micropolarizer (twisted-LCMP) array was designed and fabricated enabling linear polarization imaging in the visible band. It contains a pixelized twisted-LC layer and a polarizer. The optimized twist angles were designed as ±22.5° and ±67.5° for best extinction ratio. A large birefringence LC material was used to fulfill the Mauguin condition in a wider visible band. Using a digital micromirror device (DMD) lithography system, the twisted-LCMP array was fabricated precisely using the photoalignment technique. It exhibited excellent optical performance, which could meet the requirements for polarization imaging. The measurement error for degree of linear polarization (DoLP) and angle of polarization (AoP) were less than 1.15% and 0.65°. The proposed twisted-LCMP array has great potential to be integrated directly into a camera for real-time linear polarization imaging.

Optically Rewritable Liquid Crystal Displays: Characteristics and Performance

Recent achievements in the photoalignment technique for fabrication of optically rewritable electronic paper with high performance characteristics are surveyed with emphasis on temporal constraints on the exposure process. The possibility of creating electrode-free electronic paper has very important practical aspects. However, many existing studies do not include sufficient analysis on how to achieve acceptable reflective characteristics within short exposure time. In order to achieve this goal, we have applied the rotational diffusion model. We find that the parameters of the diffusion model can be adjusted to get acceptable light-reflecting characteristics within 10 s of exposure. In comparison with the long-time exposure, the reflectance coefficient reduces by 24%. The route to material improvements for optimized e-paper device is discussed.

Azimuthal Anchoring Strength in Photopatterned Alignment of a Nematic

Spatially-varying director fields have become an important part of research and development in liquid crystals. Characterization of the anchoring strength associated with a spatially-varying director is difficult, since the methods developed for a uniform alignment are seldom applicable. Here we characterize the strength of azimuthal surface anchoring produced by the photoalignment technique based on plasmonic metamsaks. The measurements used photopatterned arrays of topological point defects of strength +1 and −1 in thin layers of a nematic liquid crystal. The integer-strength defects split into pairs of half-integer defects with lower elastic energy. The separation distance between the split pair is limited by the azimuthal surface anchoring, which allows one to determine the strength of the latter. The strength of the azimuthal anchoring is proportional to the UV exposure time during the photoalignment of the azobenzene layer.

Advances in model-independent restoration of the liquid crystal surface anchoring potential

We extend further the theoretical and experimental studies of the actual surface anchoring potential restoration by polarization microscope technique in a wedge-shaped cell with weak surface anchoring forces filled by a chiral nematic liquid crystal (in Grandjean-Cano Wedge). To realize the theoretically predicted options of observation of large director deviation angles from the easy axes the experimental studies of director distribution in Grandjean-Cano zones were performed for differing easy axes orientations at the wedge surfaces. A weak surface anchoring at one wedge surface was obtained by a photo-alignment technique. A strong surface anchoring at the second wedge surface was also obtained by a photo-alignment technique with an enlarged irradiation dose. There were observed walls with weak jumps of the director orientation between first neighboring Grandjean-Cano zones. Some experimentally obtained polarization microscope images demonstrate partially unwound director structures in zero Grandjean-Cano zone. To ensure optimal parameters of the further experiments on the actual surface anchoring potential restoration theoretical calculations of the director distribution in individual Grandjean-Cano zones were performed for various model surface anchoring potentials. It was found that especially promising for description of the experimentally found director distribution is the D-potential. As a result of the present studies the angular range of a model-independent actual azimuthal surface anchoring potential restoration was enlarge (up to π/3) and the ways of an actual surface anchoring potential restoration in a whole angular range of its determination are discussed.

Polarized beam steering using multiply-cascaded rotating polarization gratings.

A polarized laser beam steering system using multiply cascaded rotating polarization gratings (PGs) is presented. The rotating PGs steer incident circularly polarized beams with high optical throughput because the theoretical limitation of the PG diffraction efficiency is 100%. The system also offers more rapid rotation when compared with wedge prism pairs because of the weight of the PGs. The beam steering performance was analyzed theoretically for schemes using two and four rotating PGs. The system's feasibility was demonstrated experimentally by projecting Lissajous and raster patterns using PGs fabricated from photocrosslinkable polymer liquid crystal films via a photo-alignment technique. The steered beam's diffraction efficiency and ellipticity were maintained at 83∼89% and 96∼99%, respectively, during PG rotation. This beam steering system will be applicable to light detection and ranging, optical imaging, and laser displays.

Capillary Flows of Nematic Liquid Crystal

In this paper we report the new experimental results on the rise of a liquid crystal in flat capillaries with inner photosensitive surfaces. The capillaries with different surface orientations were prepared by the use of the photo-alignment technique. Such a surface treatment makes it possible to eliminate the noncontrollable influence of a nanorelief on the wetting process, which takes place in the rubbing treatment technique previously used in similar experiments. The dynamics of the capillary rise of a nematic liquid crystal 5CB (4-cyano-4′-pentylbiphenyl) in vertical plane capillaries with photo-aligned substrates were studied for the first time. It was found that the stationary value of a contact angle weakly depends on the direction of a planar surface orientation relative to the direction of a capillary rise. It has been shown that the application of strong electric fields resulted in a decreasing of the contact angle. The results, obtained for the nematic liquid crystal, are compared with the results of an investigation of the capillary flow in a shock-free ferroelectric smectic phase.

Patterned surface alignment to create complex three-dimensional nematic and chiral nematic liquid crystal structures

ABSTRACT Combining liquid crystals (LCs) with well-designed anchoring patterns at the substrates offers tremendous potential for the development of functional electro-optic devices or stimuli-responsive actuators. Photo-alignment techniques nowadays allow an almost arbitrary control over the surface anchoring and this flexibility is used to design highly efficient flat optical LC components with different functionalities. Part of this research, dealing with nematic and chiral nematic LC between substrates with patterned azimuthal anchoring, is reviewed here. The focus is on understanding the self-assembly of complex structures, steered by an interplay between surface anchoring and LC elasticity. Additional insight into the LC bulk behaviour is obtained by comparing experimental results with numerical simulations of the director configuration. Periodic anchoring patterns with azimuthal rotation at the top and bottom substrate are studied, as well as ring-shaped alignment patterns with a 180° or 360° azimuthal rotation in a confined region in space. Different combinations of anchoring patterns at the top and bottom substrates are investigated and in addition to nematic liquid crystal (NLC), also short and long-pitch chiral nematic liquid crystal (CLC) is considered.

Electrically switchable polymer membranes with photo-aligned nematic structures for photonic applications

Abstract Composite materials containing liquid crystalline microstructures, providing fast stimuli responsive variations of birefringence at low consumption power, have great potential applications in photonics. Here, a composite material based on polymer membranes with photo-aligned nematic microstructures is proposed for the first time. For this purpose, porous polyethylene terephthalate (PET) films were covered with an azo-dye and subjected to a modified photoalignment technique to control the azimuthal orientation and anchoring strength of nematic liquid crystals (LCs) at the pore walls. Nematic internal ordering was identified using polarized optical microscopic observations assisted by theoretical simulations. The proposed approach provides spatial distribution of nematic microstructures characterized by specific internal ordering and fingerprint in near infrared spectra at an electro-optical response. These composites can be used as flexible photonic LC elements such as spatial light modulators, optical filters and lenses.

Topological defects arrays and control of electro-convections in periodically photo-aligned bent-core nematics

The applicability of photo-alignment technique to bent-core nematics (BCN) is demonstrated in this paper. In order to cope with the inherent problems of BCN such as high working temperature and large working current, a photo-alignment technique, based on two superimposed photosensitive layers of azo and reactive mesogen, is used to produce symmetric and asymmetric planar alignments. With various surface conditions, defect lines arrays are self-assembled and ac-driven electro-convection (EC) rolls are modulated. Further inspections indicate that the defect lines arrays and EC rolls are in a competitive state, which reveal that the EC rolls in BCN belong to nonstandard EC scenarios. By producing defect lines and modulating EC rolls via photo-alignment technique, we could customize patterns in liquid crystals towards new photonic devices.