Photoalignment Of Liquid Crystals Research Articles

Stepwise Progression of Dye-Induced In Situ Photoalignment and Subsequent Stabilization for Noncontact Alignment of Liquid Crystals.

Noncontact alignment of liquid crystals (LCs) is crucial for large-area and ultrahigh definition (UHD) display manufacturing. This research presents an innovative approach to the photoalignment of LCs, aiming to overcome challenges associated with uniformity and assembly in large-sized and UHD displays. Using homogeneously dissolved, nonionic azobenzene chromophores sensitive to both visible and UV light, we demonstrate an in situ stepwise progression of dye-induced LC alignment and subsequent stabilization using reactive mesogen (RM). Both dual-wavelength and single-wavelength approaches enable stepwise interfacial modifications for LC alignment and stabilization. The dye-induced LC alignment is rewritable, allowing for the creation of various patterns and gray-level alignments. The stability of the alignment is ensured through cross-linked RM layers, providing a robust and permanent solution for LC alignment without the need for delicate mechanical treatments. Importantly, this method addresses the challenges associated with conventional photoalignments, including various dye-induced approaches and high-energy photoalignment. The proposed method exhibits high-quality electro-optical switching, azimuthal anchoring strength, and stability against thermal, radiation, and ac-field stresses, making it a promising candidate for commercial mass production, especially in the fabrication of large-sized and UHD LC displays.

Full degree-of-freedom polarization hologram by freeform exposure and inkjet printing

Since the invention of holography by Dennis Gabor, the fabrication of holograms has mainly relied on direct recording of wavefront by engraving the intensity fringes of interfering electric fields into the holographic material. The degree-of-freedom (DoF) is often limited, especially for its usage as a holographic optical element in imaging or display systems, as what is recorded is what to use. In this work, based on the emerging self-assembled photo-aligned liquid crystal, a polarization hologram with full DoF for local manipulation of optical structure is demonstrated. The ability to record an arbitrary wavefront (in-plane DoF) is achieved by freeform surface exposure, while the local adjustment of deposited liquid crystal (out-of-plane DoF) is realized by inkjet printing. The methodology for designing and fabricating such a hologram is exemplified by building a full-color retinal scanning display without color crosstalk. Here, the arbitrary wavefront modulation capability helps to eliminate the aberrations caused by mismatched exposure and display wavelengths. The local liquid crystal adjustment ability enables the suppression of crosstalk by variation of chiral pitch and film thickness to tune the peak and valley of Bragg diffraction band. The demonstrated method is expected to greatly impact the fields of advanced imaging and display, such as augmented reality and virtual reality, that require optics with an ultrathin form factor and high degrees of design freedom simultaneously.

Surface-mediated dynamic cooperative motions in azobenzene polymer films

Abstract Azobenzene (1,2-diphenyldiazene in IUPAC name) is a widely studied, simple, rodlike photochromic molecule. The trans- (E) to cis- (Z) photoisomerization of this molecule was discovered a long time ago (nearly 90 yr). Nevertheless, the allure and utility of this molecule never faded and it has not surrendered a leading position among many other organic photochromic molecules. New applications for photoswitching molecular and macromolecular systems are still being proposed extensively. By linking the azobenzene molecule to liquid crystals (LCs) and polymer systems, a number of fascinating motility functions have been newly realized. This article overviews our research activities using azobenzene-containing monolayers and LC polymer films. The topics range across surface photoalignment of LCs, both from solid surface and free surface; photoalignment of hierarchical structures such as mesohybrids and block copolymers; photomechanical motions and morphological switching in block copolymer monolayers; high-density brushes of azobenzene side chain LC polymers (SCLCPs); photo-triggered mass migrations in azobenzene SCLCP films; and polymer motions via Marangoni flow, etc. Demonstrations of these dynamic photofunctions are expected to provide great opportunities for practical smart applications.

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

AbstractPhotodimerization of coumarin derivatives in photoactive polymers under linearly polarized UV (LPUV) light irradiation is a photochemical reaction often used to prepare the surface alignment layer that can orient liquid crystals (LCs) due to the anisotropic interfacial interactions. Herein, the synthesis of a series of novel coumarin‐containing copolymers and their use for the photoalignment surface layer of LCs is reported. These copolymers have the particular feature of bearing two coumarin side groups in each monomer unit, which is designed to have a high content of the chromophores. The alignment of LCs induced by the photoalignment layers of the coumarin‐containing copolymers is investigated. It is found that the photoalignment layers can effectively induce orientation of LC molecules along the direction of the UV light polarization used to prepare the surface layers, with an order parameter of around 0.5 achieved by adjusting the preparation conditions. Moreover, the electro‐optical behaviors of LC cells made with the photoalignment layers are investigated, confirming the alignment of LCs without the use of rubbed surfaces. Optically inscribing the photoalignment layer for nonuniform and organized LC alignment is also demonstrated.

P‐12.1: Two‐dimensional Edge Detection Implementation of Liquid Crystal Computing Metasurfaces

Computing metasurfaces have received widespread attention since they were proposed, and the ability to realize mathematical operations has enabled real time image processing with new breakthroughs, such as optical edge detection. In this paper, a new method for two-dimensional edge detection is presented, and the two-dimensional liquid crystal polarization grating is designed from the theoretical derivation. The gratings with different periods are fabricated by photoalignment technology. The one-dimensional and two-dimensional imaged edge detection are experimental realized in this work. By measuring the displacement of edge detection, the experimental results with different periods are in line with theoretical formula. The low cost and easily fabricated photoaligned liquid crystal polarization gratings achieved the real time optical computation with broadband and adjustable, which can be further applied in biological image edge and other optical display related fields.

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.

Electrically tuned active metasurface towards metasurface-integrated liquid crystal on silicon (meta-LCoS) devices.

Active metasurfaces add a new dimension to static metasurfaces by introducing tunability, and this has received enormous attention from industry. Although various mechanisms have been proposed over the past few years in literature, solutions with good practicality are limited. Liquid crystal (LC)-based active metasurface is one of the most promising approaches due to the well-established LC industry. In this paper, an electrically tunable active metasurface was proposed and experimentally demonstrated using photoaligned nematic LC. The good quality of the LC photoalignment on the metasurface was demonstrated. Tunable transmission was obtained for telecommunication C band and the modulation depth in transmission amplitude of 94% was realized for 1530 nm. Sub-millisecond response time was achieved at operating a temperature of 60°C. The progress made here presents the potential of LC-based active metasurfaces for fast-switching photonic devices at optical communication wavelengths. More importantly, this work lays the foundations for the next-generation liquid crystal on silicon (LCoS) devices that are integrated with metasurfaces (meta-LCoS).

Automated photo-aligned liquid crystal elastomer film fabrication with a low-tech, home-built robotic workstation

Laboratory procedures are often considered so unique that automating them is not economically justified – time and resources invested in designing, building and calibrating the machines are unlikely to pay off. This is particularly true if cheap labour force (technicians or students) is available. Yet, with increasing availability and dropping prices of many off-the-shelf components such as motorised stages, grippers, light sources (LEDs and lasers), detectors (high resolution, fast cameras), as well as user-friendly programmable microprocessors, many of the repeatable tasks may soon be within reach of either custom-built or universal lab robots. Building on our previous work on fabrication, characterization and applications of light-responsive liquid crystal elastomers (LCEs) in micro-robotics and micro-mechanics, in this paper we present a robotic workstation that can make LCE films with arbitrary molecular orientation. Based on a commercial 3D printer, the RoboLEC (Robot for LCE fabrication) performs precision component handling, structured light illumination, liquid dispensing and UV-triggered polymerization, within a four-hour-long procedure. Thus fabricated films with patterned molecular orientation are compared to the same, but handmade, structures.

Liquid crystalʼs photoalignment for formation of phase profiles via geometric phase distribution

Basic understandings on the concept of geometric phase, also known as Pancharatnam – Berry phase, and its application to creation of photonic liquid crystal devices as thin-films of photoaligned nematic liquid crystals are presented. The significance of the strong azimuthal anchoring energy and the role of birefringence in liquid crystal photoalignment layer for formation of geometric phase gradients are shown. The dependence of phase retardation of circular polarised light passing through the half-wave phase plate on the azimuthal angle of the plate orientation is explained in details, as it gives ground to formation of geometric phase distribution of optical liquid crystal devices, working in circular polarised light. The effective refractive index is introduced for characterisation of the optical properties of linear periodic liquid crystal’s structure that forms profile of geometric phase surface. The successful implementations of photonic liquid crystal devices (polarisation diffraction grating, annular diffraction grating, q-plate, q-plate with a phase core) are analysed in terms of geometric phase distributions and the corresponding equations of profile of the phase surfaces that ensure functioning of the devices are presented.

Orthogonal separation of arbitrary vector beams from non-polarized light waves based on a patterned liquid-crystal photo-alignment.

An effective method for orthogonally separating arbitrary vector polarized beams from non-polarized incident light waves is proposed in this Letter. A tunable patterned spatial distribution of liquid-crystal (LC) molecules can be effectively constructed based on both the initial photo-alignment and the electrically controlled birefringence of nematic LC materials. The LC photo-alignment over a smooth surface without any common nano-grooves leads to a highly efficient light-wave transformation by inducing a desired initial arrangement of LC directors and then acquiring extraordinary light waves with the needed, or even arbitrary, spatial polarization. The vector polarized beams can be highly converged according to a microhole-patterned electrode and a gradient refractive index distribution of the LC layer, which is driven and adjusted by an applied signal voltage. Due to the intrinsic polarization sensitivity of nematic LC materials, the formed gradient refractive index appearance only corresponds to extraordinary light waves. The proposed approach provides a way to achieve the orthogonal separation of arbitrary vector beams from non-polarized light waves. Moreover, it can be further utilized to generate and obtain arbitrary vector beams, as well as to perform adaptive light-beam convergence or even the focusing of arbitrary vector beams, which is expected to advance the development of vector beam generation and manipulation, thereby stimulating potential applications.

Patterned Photoalignment-Based One- and Two-Dimensional Liquid Crystal Forked Gratings

Based on the technology of patterned photoalignment of polymerizable nematic liquid crystal, one- and two-dimensional fork-dislocation gratings which enable formation of singular light beams (optical vortices) have been fabricated and examined. The proposed approach to the formation of a two-dimensional optical structure presents itself a combination of two one-dimensional gratings with mutually orthogonal orientations of the grooves. Phase structures of the formed singular beams as well as their spatial and polarization characteristics have been studied experimentally. The obtained results offer new potentialities in design of optical devices and systems for transformation of phase and polarization structures of light beams.

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns.

Highly efficient optical diffraction can be realized with the help of micrometer-thin liquid crystal (LC) layers with a periodic modulation of the director orientation. Electrical tunability is easily accessible due to the strong stimuli-responsiveness in the LC phase. By using well-designed photoalignment patterns at the surfaces, we experimentally stabilize two dimensional periodic LC configurations with switchable hexagonal diffraction patterns. The alignment direction follows a one-dimensional periodic rotation at both substrates, but with a 60° or 120° rotation between both grating vectors. The resulting LC configuration is studied with the help of polarizing optical microscopy images and the diffraction properties are measured as a function of the voltage. The intricate bulk director configuration is revealed with the help of finite element Q-tensor simulations. Twist conflicts induced by the surface anchoring are resolved by introducing regions with an out-of-plane tilt in the bulk. This avoids the need for singular disclinations in the structures and gives rise to voltage induced tuning without hysteretic behavior.

Kinetics of photoinduced phase retardation in azo dye layer

ABSTRACT Photoalignment of liquid crystals with azo dyes has a high potential for display and non-display applications. In this study, we report the existence of abnormal relaxation of photoinduced retardation in azo dye layers. Our results were obtained on the experimental setup, where the phase retardation of azo dye was measured by a photoelastic modulator. This phenomenon indicates that the role of the rotational diffusion coefficient must be carefully studied. Uniaxial Boltzmann–Smoluchowski model was used for theoretical analysis of the experimental results. With the model that includes the relative order parameter, we examined its kinetics during pumping and different relaxation processes. The proportionality between the photoinduced retardation and the relative order parameter has enabled us to study the kinetics of latter one under the hypothesis of temporal dependence of the rotational diffusion coefficient. A qualitative agreement between experimental measurements and model calculations adequately describes the photoinduced relaxation kinetics. The proposed methodology of computation of the relative order parameter is unbiased for all values of the material parameters.

Electro-Optical properties of photoaligned Liquid Crystal cells prepared with obliquely irradiated Chalcogenide glasses

We report the effect of oblique irradiation of thin As2S3 chalcogenide glass films on the photoalignment of Nematic liquid crystals and their electro optical properties. After depositing 60 nm As2S3 films on ITO coated substrates, the substrates were irradiated with polarized blue light (436 nm) at angles 0°, 5°, 20° and 48° of the light propagation direction with respect to the substrate plane. Then, the substrates were assembled and filled with the liquid crystal. The device LC-20° exhibits good extinction, and lower threshold voltage when compared to all other devices. The contrast ratio of LC-20° almost double that of the LC-5° and LC-48° devices. The response time increases with the angle, so for LC-20° slightly longer than for LC-5° but shorter than that of LC-48°. It is evident from the observations that the obliquely irradiated cells show good photoalignment and enhanced electro optical properties, although no pretilt is generated due to the oblique irradiation. Therefore, contrary to the photoalignment using photosensitive polymers, chalcogenide glasses have the advantage of providing photoalignment with little sensitivity to the irradiation angle. This fact led us to propose a model for the photoinduced alignment on chalcogenide glasses based on directional structural changes, not related to oriented dipoles generation as in the case of photoalignment with polymers.

Tuning and turning of the liquid crystal alignment by photosensitive composites

ABSTRACT To improve the characteristics and quality of the photoalignment of liquid crystals has been a main goal of many relevant studies. We demonstrate in this work, following the general concept of composite materials, that the photoalignment of liquid crystal can substantially be improved when using alignment layers made from photoalignment composite materials. These composites contain at least one photoalignment material. The other components of the photoalignment composite material may or may be not photosensitive, as conventional polyimide materials, for instance. We show that by a proper choice of the components, the properties of the photoalignment materials, made by photoalignment composites, can be tailored in a broad range, resulting in higher sensitivity, and thus shorter exposure time, uniform and smooth photoalignment topography (without domains and sandy structures) resulting in high image contrast. High-quality photo-patterning of the liquid crystal alignment is demonstrated in this work with a photoalignment composite material containing one of our photoalignment materials and a conventional polyamide alignment material.

Realization of doubly inhomogeneous waveplates for structuring of light beams

Waveplates having spatially varying fast-axis orientation and retardance provide an elegant and easy way to locally manipulate different attributes of light beams, namely, polarization, amplitude, and phase, leading to the generation of exotic structured light beams. The fabrication of such doubly inhomogeneous waveplates (d-plates) is more complex, compared with that of singly inhomogeneous waveplates (s-plates) having uniform retardance, which can be easily fabricated by different means such as photoalignment of liquid crystals, metasurfaces, etc. Here, exploiting the SU(2) formalism, we establish analytically that any d-plate can be equivalently implemented using a pair of quarter-wave s-plates and a half-wave s-plate. To underline the scope of this method, we propose novel d-plates toward complex amplitude shaping and also for imparting a polarization-dependent phase profile to a scalar light beam. For these two illustrations, the corresponding three-s-plate gadget is constructed, and its functioning is validated with extensive numerical simulations. The main result and its illustrations are generic and agnostic to the way the s-plates are fabricated, and we believe they carry the potential to push the current state of the art in interdisciplinary applications involving structured light beams.

Patterned Photoalignment in Thin Films: Physics and Applications

Photoalignment of liquid crystals by using azo dye molecules is a commonly proposed alternative to traditional rubbing alignment methods. Photoalignment mechanism can be well described in terms of rotational diffusion of azo dye molecules exposed by ultraviolet polarized light. A specific feature of the irradiated light is the intensity dependent change of azimuthal anchoring of liquid crystals. While there are various mechanisms of azo dye photoalignment, photo-reorientation occurs when dye molecules orient themselves perpendicular to the polarization of incident light. In this review, we describe both recent achievements in applications of photoaligned liquid crystal cells and its simulation. A variety of display and photonic devices with azo dye aligned nematic and ferroelectric liquid crystals are presented: q-plates, optically rewritable flexible e-paper (monochromatic and color), and Dammann gratings. Some theoretical aspects of the alignment process and display simulation are also considered.

The Surface Morphology Effects on Liquid Crystal Alignment After Photo-Agitation and Crosslinking of Polyimide.

The photo-reactive activators are highly reactive radical generators upon the ultraviolet (UV) light illumination. The photo-reactive initiators produced nitrogen radical and alkyl radical after releasing carbon dioxide. The radicals could react with polyimide (PI) main chains. These reactions enforced the alignment layers and exhibited high azimuthal anchoring energy. The thickness of photo-irradiated PI alignment layers were reduced dramatically by photo-induced crosslinking, which induced surface wrinkling and roughness. The carbon dioxide gases released from the thin films produced many micro-pores, which provides tight anchoring of liquid crystal (LC) molecules. The azimuthal anchoring energy obtain by photo-alignment was better than that obtain by rubbing method with the same PI. The maximum value was 6.92×10-5 J/m². Small aliphatic hydrocarbons, such as methane and propene, were released during photo-decomposition reaction from the PI surface. The polarity of film surface became more hydrophilic. The photo-alignment of LC was perpendicular to the polarization axis of UV light. On the basis of high anchoring energy, the rough surface, hydrophilic surface, and rapid photo-reactions, the photo-alignment mechanism is proposed.

Enhanced optical edge detection based on a Pancharatnam-Berry flat lens with a large focal length.

In this Letter, we employ photo-aligned liquid crystals to record a flat lens pattern through a simple exposure method to produce a large focal length Pancharatnam-Berry (PB) lens with a film thickness of 2µm without using expensive equipment. This PB lens with a focal length of 240.6 m (at 532 nm) can transmit up to 97% across visible wavelengths and maintain high diffraction efficiency (>90%). An improved two-dimensional optical edge detection design based on this lens is proposed. We experimentally demonstrate the integrity and high efficiency of edge information that may play an important role in the application of image processing and high-contrast microscopy.

Photoinduced hole dipoles' mechanism of liquid crystal photoalignment.

Polarized light absorption in photoalignment material induces anisotropic long-range interactions that orient liquid crystals. The main physical mechanisms behind anisotropic interactions are photocrosslinking and photodestruction of polymers, and photoisomerization and photorotation of azo dyes. Investigation of AtA-2 azo dye azimuthal anchoring versus exposure dose revealed the presence of an unusually strong anchoring peak at low doses, which is beyond our understanding of the known mechanisms. Here we explain these observations and demonstrate the existence of a photoalignment mechanism based on photoinduced hole dipole moments in the azo dye layer. Strong azimuthal anchoring energy >2×10-4J/m2 is obtained with a <0.5J/cm2 exposure dose.