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Photoalignment by azo dye nanolayers can provide high alignment quality for large-area liquid crystal devices. Application of this technology to active optical elements for signal processing and communications is a hot topic of photonics research. In this article, we review recent demonstrations and performance of liquid crystal photonic devices, discuss the advantages of the proposed technology, and identify challenges and future prospects in the research field of photoaligned multi-domain liquid crystal structures. We believe that the developments discussed here can provide directions for future research and potential opportunities for applications of liquid crystal devices based on multi-domain photoalignment. © 2024 by the authors.

In this article, we present the procedure of preparation of flexible electronic paper with a photosensitive azo dye layer as the key element for changing the orientation of the polarization plane. The main steps of the technology for the fabrication of flexible e-paper are reported. The possible production of Digital Mirror Devices and the roll-to-roll process is discussed. Images on flexible e-paper are demonstrated, including bank card options. The advantages of optically rewritable e-paper technology in comparison with the e-ink usually used for this purpose are highlighted. Potential applications of flexible optically rewritable e-paper include price tags for supermarkets, indoor and outdoor advertisements, smart card labels, etc. © 2023 by the authors.

With the advancement of information display technologies, research on liquid crystals is undergoing a tremendous shift to photonic devices. For example, devices and configurations based on liquid crystal materials are being developed for various applications, such as spectroscopy, imaging, and fiber optics. One of the problems behind the development of photonic devices lies in the preparation of patterned surfaces that can provide high resolution. Among all liquid crystal alignment techniques, photoalignment represents a promising non-contact method for the fabrication of patterned surfaces. In this review, we discuss the original research findings on electro-optic effects, which were mainly achieved at the Department of Electronic and Computer Engineering of the Hong Kong University of Science and Technology and the collaborating research laboratories.

Optical security films have gained a lot of attention due to its potential application in authentication services. Electrode-free optical devices are in demand along with electrode devices due to its secure access to the decryption key and ability to hide information from public access. Our concern in this study is the examination of electrode-free optical security film. In such devices, hidden information can be viewed in black/white contrast or controlled birefringent colours, which are produced by patterned phase plate and polariser. In particular, we develop the model of stratified anisotropic media for theoretical description of reflective characteristics of optical security films. Our theoretical results are compared with experimental observations. Blue and yellow birefringent colours of supertwisted nematic liquid crystal cell are achieved by using theory of optical characteristics in anisotropic media and partial coherence. This theoretical analysis suggests new openings in the optimisation of liquid crystal security films.

The ability to establish significant phase modulation at low applied field provides a promising route toward polarization control and wavefront shaping for liquid-crystal-based devices. Owing to the polarization-selective reflectivity of chiral liquid crystals (CLCs), reflective wavefront shaping via geometric phase is demonstrated when a circularly polarized light is Bragg reflected by a spatially orientated chiral layer. Nowadays, photoresponsive CLCs have attracted extensive attention due to their exotic feature that endows the CLC devices with the capability of electric-free remote control. Despite the mature photoresponsive CLC materials and the sophisticated reflective geometric phase devices, a light-induced topological optical phase modulation for a transmissive wave exiting a CLC cell remains elusive. Here, with the employment of a photosensitive chiral dopant, a hybrid aligned photoresponsive CLC system, demonstrating the simultaneous light modulation of topological geometric phase and dynamic phase via helical pitch manipulation is established. The continuous dual-phase modulation engenders a smooth optically controllable diffraction efficiency (i.e., from 90%) of the proposed light-reconfigurable CLC geometric phase optical element prototypes with multistable diffractive behavior, thus launching a paradigm shift for the application of novel liquid-crystal photonic devices in the field of all-optical polarization and spin processing.

Interactions between liquid crystal molecules and target analytes open up various biosensing applications for quick screening and point-of-care applications. In this review, we categorized biosensors by type, depending on the liquid crystal mesophase, and considered several applications for the detection of biomolecules, point-of-care diagnostics and environmental monitoring. We also discuss interactions between polarized light and target pathogens dispersed in biological fluids, which result in the change of the polarization state. An array of the Stokes parameters can be compared with the pattern, and a proper pathogen can be manifested. We suggest that a combination of a micropolarizer array and a complementary metal oxide semiconductor sensor is an optimal setup for the detection of pathogens. Herein, we discuss the working principles of liquid crystal biosensors and their fabrication principles. In addition, relevant theoretical and practical issues related to liquid crystal biosensors are outlined. In general, this review gives an in-depth survey of the research on liquid crystal-based sensors, making it easier for researchers to locate their niche and make contributions to this subject from multiple viewpoints. © 2023 by the authors.

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. © 2022 Informa UK Limited, trading as Taylor & Francis Group.

In this review we describe the reversible photoalignment effect imposed on the director in nematic liquid crystals that provides an approach for fabrication of advanced optically addressed devices. Several new concepts have been developed to render photosensitive materials during the past decade. Functional soft azo dye compounds exhibiting distinct functionalities in response to polarized light are highly desirable for fabrication of optically rewritable electronic paper. An optically rewritable element base using simple and inexpensive materials can potentially enable the development of novel environmentally friendly, paper-like gadgets with improved functionality over regular electronic paper. We argue that an optically rewritable technique is relevant for some applications, where conventional paper might be irrelevant. In particular, we have tested and discussed several techniques of color and 3D image formation. This strategy for fabrication of novel devices offers versatile methods for visualization. We also show that the intensity modulation of the irradiation light has a potential to generate improved grayscale visualization. This principle is based on the statistical distribution control of photosensitive azo dye molecules, driven by the incident polarized light. Additionally, we discuss the functional characteristics of the developed prototypes.

This chapter focuses on the progress in developing bistable Optically Rewritable Liquid Crystal Display (ORW LCDs) based on photoalignment technology over the past few years. It reviews photoalignment technology, which is essential for understanding ORW technology. The chapter presents the features of ORW LCDs including rewriting speed, gray levels and full-color generation, and flexible demonstration. It focuses on a typical photoalignment material, namely, sulfonic azo-dye SD1. The front polarizer is still the main disadvantage for ORW LCDs since it has to be removed for each erasing and rewriting, which is inconvenient or impractical in most applications. Flexible flat panel display technologies offer many potential advantages, such as very thin profiles, lightweight, the ability to flex, curve, conform, and fold, etc. Indium tin oxide electrodes are also unnecessary for ORW LCDs in the absence of applied voltage.

Time-limited exposure of electrode-free optically rewritable electronic paper is essential for applications, but performance characteristics can be modest. In this study, we investigate whether image writer mode can improve kinetics of the photosensitive layer order parameter, which is described by the rotational diffusion model. The model that we developed allows adjusting the transmittance spectra of polarizers. We have found that fairly uniform transmittance spectrum of polarizers enables to obtain competitive contrast ratio of 8:1 within 10 s of exposure. In comparison with long-time exposure, the reflectance coefficient is reduced by 24%. Computer-generated image is consistent with the experimental observations. © 2022 Author(s).

A comprehensive theory of light-reflective characteristics and experimental technique of liquid crystal layer thickness control for flexible optically rewritable electronic paper is presented. Cylindrical pillars were used to control the gap between flexible substrates. The introduced prototype of optically rewritable electronic paper has shown very promising performance. In this regard, we report theoretical results of structural photosensitive alignment of nematic liquid crystals on flexible substrate. The focus of theoretical study is on understanding the self-assembled complex structure, governed by the interplay between surface anchoring and liquid crystal elasticity. Mueller matrix spectroscopic ellipsometry was used to study light-reflecting characteristics and polarization properties of the twisted nematic film.

We propose a period control method of liquid crystal polarization grating (LCPG) based on an nterference-free and single exposure process. By adjusting three parameters of exposure setup, including incident angle of exposure beam, wedge angle of birefringent prism and tilt angle of the sample, polarization distribution of the exposure beam is changed. The spatially variant polarization of the exposure beam is transferred to liquid crystal (LC) molecules by an azo-dye photo-sensitive layer. Consequently, the LCPG with the target period is obtained. The proposed method has high flexibility and a wide range of period adjustment covering several microns to more than thousands of microns according to calculated results. Experimental results fit well with calculations. The LCPGs with different values of period from 4.5µm to more than 200µm have been realized experimentally. The proposed interference-free method would accelerate the application of LCPGs with a robust and simple fabrication process. © 2022 Optica Publishing Group

The self-assembly of silver nanowires (AgNWs) with a high aspect ratio is highly desirable for practical applications; however, several challenges still exist. Herein, a non-contact, simple, large-scale, micro-domain photo-alignment technology is proposed to align AgNWs in a liquid crystal polymer (LCP) film, resulting in a high order parameter (above 0.9). The AgNWs are precisely controlled to have planar alignment and twisted nematic structures in micro-multi-domains. The AgNW/LCP precursor is frozen to shape the film via ultraviolet crosslinking, allowing it to be easily peeled off and transferred to various flexible and other functional substrates. The aligned AgNWs can be used to improve the mechanical properties of LCP films, as they exhibit high elongation-at-break and high strength-at-break. The AgNW/LCP film can also be further programmed by micro-multi-domain photo-alignment for use in flexible and wearable devices. © 2022 Wiley-VCH GmbH.

Time-limited exposure of electrode-free optically rewritable electronic paper is essential for applications, but performance characteristics can be modest. In this study we investigate whether image writer mode can improve kinetics of the photosensitive layer order parameter, which is described by the rotational diffusion model. The model that we developed allows adjusting the transmittance spectra of polarisers. We have found that fairly uniform transmittance spectra of polarisers enable to obtain competitive contrast ratio of 8:1 within 10 s of exposure. In comparison with the long-time exposure, the reflectance coefficient reduces by 24%. Computer-simulated image is consistent with the experimental observations. © 2021 Informa UK Limited, trading as Taylor & Francis Group.

Photoalignment on azo dye films is a simple and efficient way to achieve anisotropic ordering of molecules, which is promising to achieve ultra-thin high-performance dichroic polarizers. Besides, the photoalignment technique shows a unique advantage on patterned alignment applications, and enables the realization of advanced optical elements such as liquid crystal (LC) Dammann grating, LC Fresnel lens, holographic polarizers and bifocal optical-vortex lens. © 2022 SPIE.

A composite photoalignment layer for aligning liquid crystal molecules includes: a monomeric material; a photoinitiator or a thermal initiator; and an azo dye material. A method for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a photoinitiator or a thermal initiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; exposing the thin film to polarized light; and, with a thermal initiator, heating the thin film to polymerize the monomeric material and form a solid thin film.