Photoalignment Materials Research Articles

Spin-decoupled and phase-multiplexed liquid crystals element enabling intensity-controllable holography based on rewritable photoalignment SD1

ABSTRACT Azobenzene photoalignment (PA) material SD1 is of significance in fabricating planar liquid crystals (LC) elements because of their high sensitivity and rewritability. Herein, the influence of exposure energy density on the rewriting process of SD1 is studied via the analysis of the alignment quality of the LC layer. Based on such unique erasability of SD1, a multi-exposure approach of preparing the spin-decoupled LC devices is presented experimentally, confirming multiple phase structures can be integrated into a single component. the relative weight for the two kinds of orientation structures can be controlled by exposed energy density, successfully enabling the intensity-editable holographic functionality. Our work might offer an entirely new perspective on studying the rewriting mechanism of SD1 stuff and open an effective avenue for developing the LC-based integrated apparatus.

Large aperture and defect-free liquid crystal planar optics enabled by high-throughput pulsed polarization patterning.

The liquid crystal (LC) geometrical phase optics, which is realized by the high-resolution control of the optical axis orientation in transparent micrometer-thin polymer films, is emerging as a next generation of planar optics. It features pronounced optical properties and stimuli-responsive behaviors, which could introduce appealing and new possibilities for photonic purposes. The development of fabrication techniques producing elements with large aperture sizes and arbitrarily varying molecular orientation is of significance in terms of practical utility. Here, we propose the pulsed polarization patterning technique to create large-aperture and defect-free LC geometrical phase elements. We investigated the capability of the azo photo-alignment material responding to nanosecond laser pulses and the corresponding anchoring behaviors to LCs. The threshold was reduced to one fourth of that under the continuous wave recording. The patterning resolution was found to be enhanced to around 0.71 µm, due to the ultra-fast interaction nature of the photo-alignment material with the polarized light field. We proposed the flying exposure mode to deliver high frequency modulated polarized laser pulses (8 kHz), with the precision stage moving in a uniform velocity for light-field stitching and the servo auto-focusing in the sample normal, enabling the stable and reliable polarization patterning for large aperture sizes. We further report on representative fabrication of LC polarization gratings with an aperture of 4 inch and 99.2% average diffraction efficiency.

Designing, generating and reconfiguring disclination interconnects in nematic liquid crystals

ABSTRACTDisclinations in nematic liquid crystals are of great interest both theoretically and practically. The ability to create and reconfigure disclinations connecting predetermined points on substrates could enable novel applications such as directed self-assembly of micro/nanoparticles and molecules. In this study, we present a novel approach to design and create disclination interconnects that connect predetermined positions on substrates. We demonstrate that these interconnects can be switched between different states by re-writing photoalignment materials with linearly polarised light and can be switched between metastable states using electric fields.

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.

Equal-intensity beam splitter fabricated by segmented half-wave plate for passive laser speckle reduction.

An equal-intensity beam splitter (EIBS) for passive laser speckle reduction is reported. The EIBS consists of a segmented half-wave plate (SHWP) with the designed orientation of the fast axis of each segment, a polarization beam splitter, and a mirror. The SHWP is fabricated using patterned photoalignment material and liquid crystal polymer. Ten laser sub-beams are generated by the twenty-one-pixelated EIBS, where the largest intensity ratio among them is 7.6. Laser temporal and spatial coherences are destroyed because of the optical path delays among the laser sub-beams. The EIBS is used to reduce laser speckle passively, and objective speckle contrast is reduced from 0.82 to 0.33 when all 10 laser sub-beams are used.

40.3: Effect of Birefringent Alignment Layer on Azimuthal Anchoring Energy Measurement

Birefringent alignment layer contributes to the optics of the liquid crystal cell, which is significant for optical method of azimuthal anchoring energy measurement. We obtained analytical equation to compute the corrected value of liquid crystal twist angle subject to alignment layer retardation. The data on azimuthal anchoring energy measurements of AtA‐2 and AtA‐0042 azo‐dye photoalignment materials are obtained: above >10−4J/m2 within the wide range of exposure doses 0.04 – 5.12 J/cm2.

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.

High anchoring photoalignment material based on new photo‐induced hole dipoles' mechanism

AbstractFlat nonprofiled surface of photoalignment AtA‐2 azo‐dye layer possesses simultaneously low molecular orientation anisotropy and strong azimuthal anchoring energy. We explain how flat surface of alignment layer orients nematic liquid crystal and show the existence of the new photoalignment mechanism based on photo‐induced hole dipole moments in the azo‐dye layer. The dependence of anchoring energy coefficient on the square of dipole moments of the alignment layer and the liquid crystal layer is formulated, grounding the idea of chemical modification of AtA‐2 azo‐dye with molecular dipole moment ~16D to obtain novel high anchoring photoalignment material. The new AtA‐0042 azo‐dye with molecular dipole moment ~27.5D is synthesized and experimentally verified. Strong azimuthal anchoring energy over >1.4 × 10−4 J/m2 is obtained for AtA‐0042 photoalignment material layer with ~40 mJ/cm2 exposure dose.

Multiple-Color Reflectors Using Bichiral Liquid Crystal Polymer Films and Their Applications in Liquid Crystal Displays.

Multiple-color reflectors using bichiral liquid crystal polymer films (BLCPFs) are investigated. The BLCPFs consist of alternate layers of two different single-pitch cholesteric liquid crystal (CLC) layers, named CLC#A and CLC#B. The thickness of each CLC layer equals its single pitch length. The optical properties in terms of reflections, reflection-wavelength ranges, and distributions of reflection spectra of the BLCPFs that result from the fixed pitch length of CLC#A along with the decrease of the pitch length of CLC#B are qualitatively simulated and investigated. The results indicate that the above optical properties of the BLCPFs depend on the LC birefringence and pitch lengths of CLC#A and CLC#B layers. The concept of fabrication method of the BLCPFs by using polymerizable CLCs and thin films of poly(vinylalcohol) or photoalignment materials is discussed. They have potential practical applications in functional color filters, asymmetrical transmission systems, etc., owing to the multiple reflection bands of BLCPFs. Moreover, the BLCPFs, which can enhance the color gamut and light-utilization efficiency of light sources/LC displays, are reported herein.

A practical guide to versatile photoalignment of azobenzenes

ABSTRACT Photoalignment refers to a technique for the definition of the orientation of liquid crystals using light. The approach is based on the use of materials that either absorb or trigger a chemical reaction with an anisotropy that depends on the polarisation of light. Azobenzenes are a well-established for aligning liquid crystals in research applications. Despite their popularity, only few details of the process are publicly available. We consider here the optimisation of the photoalignment process using the dichroic dye Brilliant Yellow. Its solubility in various solvents; the alignment quality as a function of exposure time and intensity; the influence of humidity at various stages; and the stability of the process in the presence of environmental light are investigated. Additionally, the dichroic dyes Congo Red and Chrysophenine are tested to increase the portfolio of available photoalignment materials.

52‐1: Invited Paper: Liquid Crystal Materials and Devices for Displays and Photonics

Photoalignment materials can be effectively used in LC alignment and patterning for new generations of LC devices with extremely high resolution and optical quality both in glass and plastic substrates. Ferroelectric Liquid Crystals based on photoalignment are very promising for new display and photonics devices.

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.

Azobenzene Sulphonic Dye Photoalignment as a Means to Fabricate Liquid Crystalline Conjugated Polymer Chain‐Orientation‐Based Optical Structures

AbstractThe use of a noncontact photoalignment method to fabricate in‐plane optical structures, defined by the local uniaxial ordering of liquid crystalline conjugated polymer chains, is reported. Molecular orientation is demonstrated for both green‐light‐emitting fluorene‐benzothiadiazole alternating copolymer F8BT and F8BT/red light emitting complex copolymer Red‐F binary blend films deposited on a well‐known azobenzene sulphonic dye photoalignment material SD1. Absorption anisotropy ratios of up to 9.7 are readily achieved for 150 nm thickness F8BT films. Spatial pattern definition, afforded by masking the UV polarized light exposure of the photoalignment layer, allows the fabrication of optical structures with a resolution down to the micron scale. The alignment process is further extended to enable the serial, independent orientation of films deposited on top of each other and to permit the molecular orientation to follow curvilinear patterns. In the former case, this allows F8BT bilayer structures to be fabricated that show even higher absorption anisotropy ratios, up to ≈12, close to the theoretical limit for the previously deduced ≈22° optical transition dipole moment angle relative to the chain axis.

The Study of Chromaticity Optimization for Photo Alignment IPS LCD

The new methods of optimization of chromaticity for IPS mode have been presented. We have investigated the effect of the alignment angle on the panel chromaticity base on a new photo alignment material composition. The chromaticity of panel also can be improved by optimizing the angle between LC initial orientation and pixel metal electrode for red, green and blue pixel, respectively.

High performance and low level of image sticking fringe field switching mode liquid crystal display with photo-alignment layers

A novel photo-alignment fringe field switching mode liquid crystal display (PA-FFS-LCD) with a wide nematic temperature range from −40 °C to 95 °C was developed. The new developed PA-FFS-LCD shows fast gray-to-gray response time (<20 ms at 25 °C and < 300 ms at −30 °C), wide viewing angle, and high contrast ratio about 1600 owing to use an isomerization-type photo-alignment material. Image sticking was not recognized owing to a suitable arrangement of pixel and common electrodes.

P‐205: Late‐News Poster: LED Induced Vertical Alignment Surface for High Performance Liquid Crystal Displays

Photoalignment is one of the best candidates of the proven market for multi domain display. It allows nearly free pattern to the liquid crystal alignment direction compared to the conventional rubbed polyimide. Most of the available photoalignment materials require polarized deep UV irradiation with a finite dosage. With the consideration of the current limitation with such a small working window, a vertical photoalignment surface by non‐polarized LED light irradiation with large wavelength is proposed and demonstrated. The alignment direction of proposed surface is generated by the inclined non‐polarized light. The measurement results show the alignment properties of this proposed alignment surface are comparable to conventional polyimide. It has stable and uniform pretilt angle, high polar anchoring energy and fast response time. As a result, the proposed alignment layer is robust and especially suitable for large‐area liquid crystal displays which require high performance with stable pretilt angles and multi‐domain purposes.

Cycloidal diffractive waveplates fabricated using a high-power diode-pumped solid-state laser operating at 532 nm

Diffractive waveplates (DWs) are highly efficient optical components realized by means of a polarization holography setup that makes use of UV/blue laser sources. It is more convenient to perform the holographic recording process with green lasers (e.g., continuous wave operating at 532 nm) because they offer compactness, efficiency, and high power. Unfortunately, the photo-alignment materials used for DW fabrication exhibit limited sensitivity at 532 nm. Herein, we report the realization of cycloidal diffractive waveplates (CDWs) by employing a polarization holographic setup using a high-power, diode-pumped solid-state laser operating at 532 nm. The enabling factor is the use of a photo-alignment material with a broad absorption band extending into the visible part of the spectrum. Samples are characterized in terms of morphological and optical properties. They exhibit a well-defined morphology along with very high diffraction efficiency (≈97%). The possibility to realize CDWs with larger apertures is underway, enabling the realization of a new generation of optical components for integration in modern applications.

Transflective spin-orbital angular momentum conversion device by three-dimensional multilayer liquid crystalline materials.

In this paper, a liquid crystal device for generating transflected optical vortices with high efficiency based on Pancharatnam-Berry phase is devised and demonstrated experimentally. In the experiment, both photo-alignment material and polymer-alignment material are used for assembling three-dimensional distributed liquid crystal polymer and cholesteric liquid crystal. Through the interaction between the incident light and the device, both transmitted light and reflected light get spin-orbital angular momentum conversion. Moreover, the amount of transmitted and reflected beams can be modulated by the input polarization. In our proposal, the device is dual functional, low-cost and simple in manufacturing process.

29‐2: Advanced Photo‐alignment Material for Both Photo and Rubbing‐alignment Methods

We propose the advanced photo‐alignment material with improved film hardness suitable for both photo‐alignment and rubbing‐alignment method. Using advanced photo‐alignment material, excellent performance can be obtained using not only photo‐alignment but also rubbing alignment.

High Photoinduced Ordering and Controllable Photostability of Hydrophilic Azobenzene Material Based on Relative Humidity

Azobenzene materials provide an effective way for liquid crystal (LC) alignment besides traditional rubbing technology. A strong relationship between relative humidity (RH) and the photoalignment quality of hydrophilic azobenzene dye brilliant yellow (BY) has been investigated. Good photoalignment quality can only be ensured at about 40% RH or below. On the other hand, the photostability of the alignment layer is also influenced dramatically by RH. The rewritability can be guaranteed at extremely low RH (≤10%). It is gradually lost with increasing RH, and the alignment layer becomes photostable against further light exposure when at 40% RH or above. Therefore, the BY photoalignment layer can be tuned from rewritable to photostable by simply adjusting RH, and thus multistep photopatterned alignments can be obtained and reserved based on this method. Similar properties are also expected for other hydrophilic azobenzene photoalignment materials, where the specific RH values may vary correspondingly. The reason is due to the strong intermolecular interaction and J-aggregate formation of BY molecules with water insertion. Moreover, the lyotropic LC formed by J-aggregated BY molecules in aqueous solution is reported here.