Planar optical elements that can manipulate the multidimensional physical parameters of light efficiently and compactly are highly sought after in modern optics and nanophotonics. In recent years, the geometric phase, induced by the photonic spin-orbit interaction, has attracted extensive attention for planar optics due to its powerful beam shaping capability. The geometric phase can usually be generated via inhomogeneous anisotropic materials, among which liquid crystals (LCs) have been a focus. Their pronounced optical properties and controllable and stimuli-responsive self-assembly behavior introduce new possibilities for LCs beyond traditional panel displays. Recent advances in LC-mediated geometric phase planar optics are briefly reviewed. First, several recently developed photopatterning techniques are presented, enabling the accurate fabrication of complicated LC microstructures. Subsequently, nematic LC-based transmissive planar optical elements and chiral LC-based broadband reflective elements are reviewed systematically. Versatile functionalities are revealed, from conventional beam steering and focusing, to advanced structuring. Combining the geometric phase with structured LC materials offers a satisfactory platform for planar optics with desired functionalities and drastically extends exceptional applications of ordered soft matter. Some prospects on this rapidly advancing field are also provided.

Liquid-Crystal-Mediated Geometric Phase: From Transmissive to Broadband Reflective Planar Optics

An adaptive-focus lens is a device that is capable of tuning its focal length by means of an external stimulus. Numerous techniques for the demonstration of such devices have been reported thus far. Moving beyond traditional solutions, several new approaches have been proposed in recent years based on the use of liquid crystals, which can have a great impact in emerging applications. This work focuses on the recent advances in liquid crystal lenses with diameters larger than 1 mm. Recent demonstrations and their performance characteristics are reviewed, discussing the advantages and disadvantages of the reported technologies and identifying the challenges and future prospects in the active research field of adaptive-focus liquid crystal (LC) lenses.

Recent Advances in Adaptive Liquid Crystal Lenses

The photonic spin Hall effect (SHE) refers to the transverse spin separation of photons with opposite spin angular momentum, after the beam passes through an optical interface or inhomogeneous medium, manifested as the spin-depend-ent splitting. It can be considered as an analogue of the SHE in electronic systems: the light’s right-circularly polarized and left-circularly polarized components play the role of the spin-up and spin-down electrons, and the refractive index gradient replaces the electronic potential gradient. Remarkably, the photonic SHE originates from the spin-orbit interaction of the photons and is mainly attributed to two different geometric phases, i.e., the spin-redirection Rytov-Vlasimirskii-Berry in momentum space and the Pancharatnam-Berry phase in Stokes parameter space. The unique properties of the photonic SHE and its powerful ability to manipulate the photon spin, gradually, make it a useful tool in precision metrology, analog optical computing and quantum imaging, etc. In this review, we provide a brief framework to describe the fundamentals and advances of photonic SHE, and give an overview on the emergent applications of this phenomenon in different scenes.

Photonic spin Hall effect: fundamentals and emergent applications

Structures bridge the micro and macro worlds and play vital roles in material sciences and technologies. Nature is more wondrous than our imagination, which always solves complicated problems in simple manners. For instance, the iridescence in butterfly wings, peacock feathers, and opals;[1] the self-cleaning ability of lotus leaves;[2] the water-repellent legs of water striders;[3] and the dry adhesion of gecko foot hairs[4] are all resulted from exquisite structures with long-range ordered multiscale organizations; DNA can be considered as a nanoscale knotting structure for recording and reading genetic information. Such phenomena provide amounts of inspirations for the development of new functional materials via mimicking natural structures. Hierarchical architecture in the mesoscopic scale is a booming topic that triggers curiosities toward insights of fantastic natural materials. In recent years, much effort has been devoted to this field, and numerous novel artificial architectures have been presented. Thanks to the intrinsic self-assembly behavior, liquid crystal (LC) is a promising candidate for building up ideal hierarchical superstructures. The molecular self-organization of LCs enables a landscape of hierarchical architectures featured by fantastic textures in diverse phases. Nematic LC is commonly composed of rod-like molecules with their long axes along a preferred direction (Figure 1A). Different LC director distributions would introduce varied anisotropic physical properties. The orientational order introduces various anisotropic physical properties. While some other LC phases allow the rational control of assembly behavior and cause the formation of remarkable hierarchical superstructures. Moreover, advantages of widely tunable feature size, extra-field tunability as well as rapid and cost-effective formation are exhibited. Smectic LCs (SLCs) are characterized by an ordered lamellar structure (Figure 1B).[5] Within each layer, molecules align parallel to each other, with their long axes parallel or incline to the layer normal. Under antagonistic anchoring conditions (i.e., the two interfaces of the SLC film are introduced with a planar alignment and a homeotropic alignment, respectively), parallel layers periodically wrap around defect lines to form different defect arrays.[6] The cholesteric LC (CLC) is an LC phase where rod-like molecules self-assemble into a periodic helical structure (Figure 1C). This Hierarchical architectures are of vital importance in materials science and nanotechnology. Among various building blocks, liquid crystals (LCs) have attracted particular attention due to their excellent controllability of selfassembly behavior and the resultant physical properties. In recent years, interest in light-activated LC hierarchical superstructures has emerged for a wide range of applications beyond LC displays. This report consists of two main parts: the light-activated LC hierarchical architectures and their corresponding photonic applications. In the first part, different smectic layer curvature controlling methods are introduced to produce periodic defect arrays. By taking the advantages of photoalignment techniques and light-driven molecular motors, cholesteric LC helical superstructures are highly manipulated, including pitch tuning, helix rotation, and chirality inversion. As a mutually concerned scientific issue, the photoresponsive properties of blue phase (BP) hierarchical superstructures are also introduced, enabling unique BP materials and devices driven by a facile light stimulation. Moreover, traditional optics (microlens and beam steering) and advanced photonics (specific optical field generation and LC lasers) are reviewed in the second part. It is believed that the developments introduced here can open a door for the concept of “smart optical materials” based on the self-assembled soft LC superstructures.

Light-Activated Liquid Crystalline Hierarchical Architecture Toward Photonics

We demonstrate a physical model of photoalignment and photopatterning based on rotational diffusion in solid azo-dye nanolayers. We also highlight the new applications of photoalignment and photopatterning in display and photonics such as: (i) liquid crystal (LC) E-paper devices, including optically rewritable LC E-paper on flexible substrates as 3D E-paper, as well as optically rewritable technology for photonics devices; (ii) photonics LC devices, such as LC Switches, polarization controllers and polarization rotators, variable optical attenuators, LC filled photonic crystal fiber, switchable diffraction grating; (iii) patterned micro-polarizer array using photo-alignment technology for image sensor; (iv) electrically tunable liquid crystal q-plates; (v) electrically switchable liquid crystal Fresnel lens; (vi) liquid crystal optical elements with integrated Pancharatnam-Berry phases. We are sure, that in the field of (LC), the main point is no longer display research, but new photonic applications of LC are emerging in telecommunication, fiber optical communication systems, sensors, switchable lenses, LC light converters and other LC photonics devices.

Photoaligning and Photopatterning: New LC Technology

We propose theoretically and verify experimentally a method of using electrically tunable liquid crystal q-plate and wave plate for generating arbitrary vector vortex beams on a hybrid-order Poincare sphere (HyOPS). The generated vector vortex beam is verified and shows decent agreement with the prediction. This method brings many advantages, such as high conversion efficiency, good electrical controllability, and integration. This system can provide fundamental optical system support for various structured beam applications.

Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere based on liquid crystal device.

In this paper, we proposed and fabricated a liquid crystal (LC) lens with spatially separated focuses via liquid crystal photoalignment technology. The novel lens is an integration of the polarisat...

Liquid crystal Pancharatnam-Berry phase lens with spatially separated focuses

In this Letter, transverse and longitudinal liquid crystal bifocal lenses (LCBLs) are proposed to continuously control the relative intensity of two foci through a simple polarization control. The modulation of a LCBL comes from the geometric phase control and is designed through the principle of holography, where the object wave is a light field from two foci respectively formed by the left-circular polarized (LCP) and right-circular polarized (RCP) light, and the reference wave is the incident plane wave. Constructed millimeter-scale LCBLs are verified experimentally, and the foci are precisely formed at the preset plane. Besides, the relative intensity can be easily controlled with different weights of LCP and RCP light. The proposed strategy overcomes the shortcomings of previous bifocal lenses, such as a complex design method, a long optimization time, and an unchangeable relative intensity, and it is expected to find potential applications in parallel optical processing and optical interconnections.

Liquid crystal bifocal lens with adjustable intensities through polarization controls.

Liquid crystal (LC) circular polarization gratings (PGs), also known as Pancharatnam–Berry (PB) phase deflectors, are diffractive waveplates with linearly changed optical anisotropy axes. Due to the high diffraction efficiency, polarization selectivity character, and simple fabrication process, photoalignment LC PGs have been widely studied and developed especially in polarization management and beam split. In this review paper, we analyze the physical principles, show the exposure methods and fabrication process, and present relevant promising applications in photonics and imaging optics.

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

In this Letter, a method of stacking two opposite-handed cholesteric liquid crystals (CLCs) with the same pitches and the same surface azimuthal angles to generate vector beams from homogeneous linear polarization is proposed. By this means, vector beams with arbitrary orders can be generated based on point-to-point photo-alignment technology. In our experiment, radially polarized, azimuthally polarized, and high-order vector beams with high efficiency have been generated. The device operates light polarization within the Bragg reflection polychromatic bandgap. Our proposal also provides the possibility to generate reflective hybrid-order Poincare beams if these two CLC layers are impressed with different topological charges. It has great potential for reflective photonics applications in high-resolution imaging, focusing, and information processing.

Yide Yuan

Papers

Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere based on liquid crystal device.

Liquid crystal Pancharatnam-Berry phase lens with spatially separated focuses

Liquid crystal bifocal lens with adjustable intensities through polarization controls.

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

Bragg reflective polychromatic vector beam generation from opposite-handed cholesteric liquid crystals