Photoalignment Layer Research Articles

Orientation stabilization of light-controlling layers using liquid crystal polymers

Photo-alignment technology provides an advanced optical alignment method and high alignment quality compared to conventional rubbing techniques. However, challenges of photo-stability and response speed have hindered their practical application. This work is concerned with the stability of the photo-alignment layer, with azo dyes (SD1) employed as the photo-alignment layer material. Two solutions were investigated: the passivation of SD1 films with reactive mesogen (RM) material RM257 and the utilization of polymer–azo dye composites in lieu of the photo-alignment layer. Experimental methods, including spin-coating and UV polymerization, were employed, while optimal monomer concentrations were determined by photo-stability tests. The results demonstrate that the passivation layer enhances photo-stability. Atomic force microscopy (AFM) is employed to assess film roughness and enhance film quality. Furthermore, electro-optical tests indicate that the approach has no negative impact on the performance of LCD. These findings provide a more optimized approach to stabilizing liquid crystal (LC) alignment for advanced display technologies.

P‐13.8: Stabilization of Photoalignment Layer for Liquid Crystal

P‐13.8: Stabilization of Photoalignment Layer for Liquid Crystal

Formation of polymerizable layer on a photo-alignment layer of liquid crystal cell and its image sticking characteristics

The effect for polymerizable layer on a vertical-type photo-alignment layer was investigated from the point of image sticking characteristics. Fabrication process of the polymerizable layer is followings; photo-alignment treatment is firstly carried out by exposing polarized ultraviolet (UV) from the oblique direction. After construction of the liquid crystal (LC) cell, the LC material including monomers is injected, and unpolarized UV is exposed for formation of the polymerizable layer. For avoiding pretilt angle shift during the exposure, fast rate constant of polymerization is necessary, and mixed monomers 4,4’-dimethacryloyl-oxy-biphenyl (4,4’-DMABiph) and 4,4’-dimethacryloyl-oxy-benzilketal (4,4’-DMABzk) are appropriate for obtaining small level of image sticking parameters. As the image sticking parameters, residual direct current voltage (V rDC) and difference in pretilt angle (Δtilt) before and after application of alternate current voltage were evaluated, and the results of V rDC and Δtilt indicated that 6% 4,4’-DMABzk in the mixed monomers 4,4’-DMABiph and 4,4’-DMABzk would be useful for obtaining the small level of image sticking.

Light-controllable hybrid aligning layer based on LIPSS on sapphire surface and PVCN-F film

The creation of aligning layers for the uniform orientation of liquid crystals is significant for both research and the application of liquid crystals. For all applications, the creation of aligning layers possessing controllable characteristics such as azimuthal and polar anchoring energies, easy-axis of director alignment and pretilt angle, in the same way as it is achieved by using photoaligning layers processed by light, is very important. Here, aligning properties of hybrid aligning layers created on the basis of sapphire surfaces additionally coated by photoaligning layer of PVCN-F are studied. These hybrid layers possess the properties of the nano-structured sapphire layer and the photosensitive PVCN-F layer, and complement each other. The irradiation time dependence of the azimuthal anchoring energy of the hybrid layers is studied. By using certain experimental conditions during irradiation of hybrid layers, e.g., polarization of light and irradiation time, a minimum value of the azimuthal anchoring energy, close to zero, was obtained. Atomic force microscope studies of the irradiated hybrid layers were also carried out. It was found that the behavior of the contact angle of nematic droplets placed on treated sapphire surfaces are in good agreement with properties of hybrid aligning layers and parameters of structuring surface obtained from AFM images.

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.

Controlled wiring of disclination lines between patterned photoalignment layers in nematic liquid crystal

On demand creation of topological defect configurations is a relevant topic for theoretical studies but also enables new developments in the field of photonics, stimuli-responsive actuators and colloidal assemblies. Liquid crystals (LCs) provide a versatile and technologically relevant material system to study topological defects, and photoalignment enables complex surface patterning that includes the formation of defects. This work is based on periodic defect grids of +1/2 and −1/2 defects, imposed at the top and bottom surface of a LC cell. The periods at the two substrates are slightly different and the azimuthal orientation of the defects is varied, so that different types of disclination interconnections are formed in the bulk of the LC. Some disclination lines connect defects at opposing substrates, while others connect defects on the same substrate. The experimentally generated configurations are simulated with the help of finite element Q-tensor simulations in order to get a detailed understanding of the defect formation. The optical appearance in polymerizing optical microscopy images is compared to optical simulations to verify the validity of the director configurations.

Development of thin film quarter-waveplate for polarization-independent liquid crystal on silicon phase modulators

Liquid crystal on silicon (LCoS) phase modulators spatially modulate the phase of light across the panel. The orientation order and elongated molecules of nematic liquid crystals (LCs) means that traditional LCoS phase modulators are inherently polarization-dependent, resulting in either complicated polarization manipulation systems or high optical power loss for unpolarized incident light. We propose a polarization-independent LCoS (PI-LCoS) device that combines existing technologies, which allows for cost-effective fabrication. Our PI-LCoS phase modulator is suitable for a wide variety of applications in telecommunication, adaptive optics, and display technologies. We have demonstrated the feasibility of the proposed PI-LCoS device by fabricating polarization-independent LC cells using a thin-film quarter-wave plate composed of a photoalignment layer and a layer of LC polymer. We show experimentally that the proposed design can efficiently modulate the phase of light with arbitrary input polarization.

47.1: Green Technology of Photoalignment Layer Coating from H2O solution for Achromatic Polymerizable Liquid Crystal Retarder

Combination of several films from polymerizable liquid crystal (PLC) into single device system with complex properties faces a fabrication issue due to chemical compatibility and mutual alignment effect of the neighbouring layers. Eco-friendly green technology of high anchoring azo-dye photoalignment layer coating from water solvent provides efficient solution to hinder interlayer interaction of bottom PLC layer avoiding its chemical damage and providing unique photoalignment pattern for top PLC retarder. The advanced possibilities of green technology of photoalignment layers coated from H2O solution are demonstrated through fabrication of achromatic quarter-wave retarder system comprising formation of polimerizable liquid crystal layers on top of one another with arbitrary azimuthal photoalignment direction. The merge of photoaligned PLC retarders into single multi-layer anisotropic structure opens new approach to fabrication of thin-film phase devices with complex optical properties.

An anti-counterfeit technique using elastic and optical properties of liquid crystals

ABSTRACTAs the diversity of information and data sources increases, anti-counterfeiting technologies become more critical in data recording, delivering goods and managing valuables. We propose a security device that can detect the denatured state or hacking of the information using liquid crystal’s elastic and optical properties: Twist is allowed in the nematic phase, but not in the smectic phase. The optical polarisation varies in the twisted liquid crystal. The information images are engraved on the photo-alignment layer by irradiating polarised UV light in smectic phase. Several images are fabricated controlling polarisation directions. The engraved images are not visible as long as the smectic is maintained. To reveal the images, heating is necessary to nematic, where the twist is allowed. Even in nematic, we can selectively display or hide images by adjusting polariser angles. This security method can show a warning message or hide an image even if hacking occurs. Cooling back to the smectic phase distorts the image and it never returns to the initial phase. Therefore, if the texture is a distorted image, we notice an improper temperature change or product hacking. These devices are inexpensive, simple to manufacture, and effective in product security.

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.

Efficient Anisotropic Polariton Lasing Using Molecular Conformation and Orientation in Organic Microcavities

AbstractOrganic exciton‐photon polariton lasers are promising candidates for the efficient generation of coherent light at room temperature. While their thresholds are now comparable with those of conventional organic photon lasers, tuning of molecular conformation and orientation as a means to control fundamental properties of their emission and thus further enhance performance remains largely unexplored. Here, a two‐fold reduction in the threshold of a microcavity polariton laser based on an active layer of poly(9,9‐dioctylfluorene) (PFO) is achieved when 15% β‐phase conformation is introduced. In addtion, taking advantage of the liquid crystalline properties of PFO, a thin photoalignment layer is used to induce nematic alignment of the polymer chains. The resulting transition dipole moment orientation increases the Rabi energy, bringing the system into the ultra‐strong coupling regime and facilitating anisotropic polariton lasing with an eight‐fold reduction in absorbed threshold, down to 1.14 pJ (0.36 µJ cm−2) for the direction parallel to the orientation, with no emission along the orthogonal direction. This represents the first demonstration of anisotropic polariton lasing in conjugated polymer microcavities and a lower threshold than current organic vertical cavity surface‐emitting photon and polariton lasers. Dipole orientation offers new opportunities for switchable, more efficient polaritonic devices, and observation of fundamental effects at low polariton numbers.

A Mesoporous Silica Nanoparticle-Doped Photo-Alignment Layer and Liquid Crystal Layer for Optimizing the Rewriting Speed and the Response Time of Optically Driving Liquid Crystal Displays

Optically driving liquid crystal displays (ODLCDs) are widely applied in display and optical devices due to their long axis of liquid crystal (LC) molecules that can be tuned by a photo-alignment layer under exposure polarized light. However, their use remains challenging due to their long rewriting time and response time. In this work, the rewriting time and the response time of an ODLCD depending on mesoporous silica nanoparticles (MSNs) doped in azo-dye (SD1) and LC 5CB were studied. Among the different concentration ratios of SD1-MSNs (1-0 to 1-0.1), a ratio of 1-0.07 was optimal, decreasing the rewriting time by 40 s (from 69.1 to 29.6 s). Meanwhile, the response time was improved 10 times with MSNs doped into 5CB.

Analysis of light diffraction by azobenzene-based photoalignment layers.

Photoalignment materials, such as the azobenzene-based PAAD series studied here, are becoming increasingly important in liquid crystal-based optical devices and displays. Yet their properties and, in particular, their response to light, are still not fully understood. We investigate, experimentally and theoretically, the photoinduced birefringence, the order parameter and the formation of surface relief gratings, as well as the diffraction caused by them. We show that some of the azobenzene PAAD materials are suitable for the formation of surface relief gratings with high modulation depth, while others exhibit strong photoinduced birefringence. The two effects are inversely correlated: the stronger the surface relief grating is, the weaker is photoinduced birefringence. Analytical formulas based on the Raman-Nath approximation and numerical simulations of Maxwell's equations are used to quantify the diffraction caused by the induced diffraction gratings, showing excellent agreement between theory and experiment.

Photo‐isomerization, photodimerization, and photodegradation polyimides for a liquid crystal alignment layer

AbstractWe review the important features of LC orientation for the alignment layer, namely, production method, alignment mechanisms, and required properties. Photoalignment, which could eliminate the disadvantages of the rubbing method is explored. Applicable materials and techniques for photoalignment are also examined. Many materials for photoalignment layers have been proposed, but polyimide‐based materials are commonly used because of their transparency and insulation properties, which are needed for optical electronic devices. Furthermore, photoisomerization, photodimerization, and photodecomposition as photoalignment methods are reviewed, and specific photoreactive groups and alignment mechanisms are used to induce LC alignment. In summary, polyimides cause photodecomposition and have the following disadvantages; (1) requirement of relatively short‐wavelength ultraviolet (UV) irradiation, (2) difficulty in reorienting polymer chains owing to their rigid polymer matrix, and (3) low‐photo reactivities. The solutions for each disadvantage are already proposed, but the required properties should be determined during material design. Polyimide‐based materials introduced with the azobenzene and cinnamate groups are proposed and found to have the ability of fulfilling the required properties. The mechanism of polyimide photodecomposition is not fully understood. It likely has the advantage in terms of LC alignment stability because of the absence of E/Z isomerization. Therefore, the alignment mechanism and availabilities should be further explored.

Improvement of image sticking on a polymer-sustained-alignment liquid crystal displays with photo-alignment layers

ABSTRACT New polymer-sustained-alignment liquid-crystal-displays (PSA-LCDs) with photo-alignment layers are proposed in this study. The fabrication process of the new PSA-LC cell is followings; a photo-alignment treatment is firstly conducted by exposing polarised UV from oblique direction to the photo-alignment layer. After construction of the LC cell having monomers in a LC layer, unpolarised UV is exposed for forming a polymer layer. During the exposure of the unpolarised UV, application of electric-field is not necessary because the pretilt angle is determined by the photo-alignment treatment. In order to avoid a shift of the pretilt angle during the unpolarised UV exposure, a relatively fast rate constant of polymerisation is important. Furthermore, residual-direct-current-voltage (V rDC), which is one of image sticking parameters, is also affected by the rate constant of the polymerisation, and the V rDC becomes small with increasing the rate constant. In contrast, the shift of the pretilt angle (Δtilt), which is another image sticking parameter, depends on molecular structures of the monomers, and the Δtilt becomes small for the monomers with linear structure between a core unit and polymerisable units. The results indicated that the monomer 2,7-dimethacryloyl-oxy-phenanthrene would be suitable for fabrication and small level of the image sticking of the PSA-LC cells with the photo-alignment layers.

Spatial separation of azimuthally and radially polarized beams from non-polarized light waves based on the electrically controlled birefringence effect.

Based on the electrically controlled birefringence effect in liquid crystal materials, an effective method for spatially separating azimuthally and radially polarized beams from non-polarized incident light waves is proposed. The radially polarized beam was highly converged by using a microhole-patterned electrode and a planar photo-alignment layer to shape the initial liquid-crystal radial alignment and a gradient refractive index distribution with central axial symmetry after applying a voltage signal. Due to the intrinsic polarization sensitivity of nematic liquid-crystal materials, the shaped gradient refractive index only applies to extraordinary light waves, which then converge into a spot. Thus, the azimuthally and radially polarized beams are effectively separated. The proposed method demonstrates some advantages, such as low cost, miniaturization, and easy fabrication and integration with other functional devices. Thanks to the wideband electrically controlled birefringence of liquid-crystal materials, this light-wave manipulation to spatially separate azimuthally and radially polarized beams can also be performed over a wide wavelength range.

Disclination lines in nematic liquid crystal between a structured photoalignment layer and a homeotropic alignment layer

Topological defects in nematic liquid crystals (NLCs) are an interesting material system to study both from a theoretical point of view and for applications in photonics, soft-robotics and colloidal assembly. By using a photo-aligned template at one or both confining substrates, different disclination interconnections in the bulk can be readily stabilized. We imposed a periodic defect grid of 1/2 and −1/2 defects at the surface with the help of photoalignment and combined this with a uniform homeotropic counter substrate. This alignment configuration was used to study the optical properties and the relative stability of different types of disclinations in detail. The arrays of disclination lines are investigated by polarizing optical microscopy for different orientations of the polarizers and for different applied voltages. A theoretical analysis is performed to understand the relative frequency of occurrence for distinct types of disclinations and numerical simulations are used to verify the difference in optical appearance. The unequal elastic constants for splay, twist and bend strongly influence both properties and make it possible to identify different types of disclination lines form their optical appearance.

High-throughput and controllable manufacturing of liquid crystal polymer planar microlens array for compact fingerprint imaging.

The microlens array (MLA) with a small geometric footprint and unique performances, is the key enabler to push the development of photonic devices toward miniaturization, multi-function and large-scale integration. However, the realization of 100% fill-factor (FF) MLAs with high controllability and its mass manufacturing without complex steps has always been a difficult issue. Here, we propose an efficient, highly flexible and low-cost manufacturing approach for MLAs with a high FF via snapshot polarization patterning. The digitalized linear polarization pattern was distributed across the photo-alignment layer with both high efficiency and accuracy, enabling large-area liquid crystal MLA with parameter controllability from element to element. The MLA manufacturing process does not involve developing, etching and deposition steps and is suitable for industry up-scaling. We further proposed a novel compact compound-eye imaging system for biometrics with the obtained MLAs. The 100% FF MLA enables high light utilization efficiency and low background crosstalk, yielding compact biometrics indentation with high recognition accuracy. The realization of such planar optics would lead to a plethora of different miniaturized multiaperture imaging systems in the future.

Two-color optically addressed spatial light modulator as a generic spatiotemporal system.

Nonlinear spatiotemporal systems are the basis for countless physical phenomena in such diverse fields as ecology, optics, electronics, and neuroscience. The canonical approach to unify models originating from different fields is the normal form description, which determines the generic dynamical aspects and different bifurcation scenarios. Realizing different types of dynamical systems via one experimental platform that enables continuous transition between normal forms through tuning accessible system parameters is, therefore, highly relevant. Here, we show that a transmissive, optically addressed spatial light modulator under coherent optical illumination and optical feedback coupling allows tuning between pitchfork, transcritical, and saddle-node bifurcations of steady states. We demonstrate this by analytically deriving the system's dynamical equations in correspondence to the normal forms of the associated bifurcations and confirm these results via extensive numerical simulations. Our model describes a nematic liquid crystal device using nano-dimensional dichalcogenide (a-As 2S 3) glassy thin films as photo sensors and alignment layers, and we use device parameters obtained from experimental characterization. Optical coupling, for example, using diffraction, holography, or integrated unitary maps allows implementing a variety of system topologies of technological relevance for neural networks and potentially Ising or XY-Hamiltonian models with ultralow energy consumption.

Double-twisted nematic director configurations in cylindrical capillaries with a photocontrollable angle of twist.

The orientational configurations of thermotropic nematic liquid crystal in cylindrical capillaries with nondegenerated planar surface anchoring are investigated. The boundary conditions were determined by a photoaligning coating on the inner wall of the capillary treated with a linearly polarized UV light while rotating the capillary around its long axis, thus providing the easy alignment axis perpendicular to the polarization direction of illuminating light. By changing the angle between the incident light polarization and the capillary axis, this procedure allows us to realize axially symmetric twisted structure with any angle of twist, ranging from a trivial axial alignment with zero twist (when the UV polarization is perpendicular to the capillary axis) to the 180^{∘}-twisted configuration (when the UV polarization is along the capillary). These two extreme configurations together with an intermediate configuration induced by UV light polarized at an angle of 45^{∘} to the capillary axis were produced and their director profiles were studied using polarizing microscopy techniques. UV light intensity and polarization distribution over inner capillary surface covered with photoaligning layer were analyzed using optical ray tracing calculations. An analytical model of the observed configurations of the liquid crystal director field is proposed, which takes into account the finite anchoring on the capillary surface. By comparing the theoretically calculated and experimentally estimated director profiles, the liquid crystal anchoring energy on the photoaligned capillary surface was estimated.