Aberration-free pupil steerable Maxwellian display for augmented reality with cholesteric liquid crystal holographic lenses

Rotating twisted templates for imprinting polarization gratings with a sub- to dozen-micron period

In this Letter, we report and experimentally demonstrate what is to our knowledge a novel scheme for imprinting polarization gratings (PGs) with a pair of templates. Compared with the traditional method that a single template can only imprint PG with a single period, cascading two templates can control the period of imprinted PG at will. However, the low diffraction efficiency is inevitably caused by cascading two templates. Therefore, a rigorous coupled wave analysis (RCWA) is adopted to design a multi-twisted template to address this challenge. As a proof of concept, two multi-twisted templates with a period of 1.6 μm were fabricated, and PGs with a large period range from 0.4 to 48.6 μm were successfully imprinted. The proposed scheme is expected to enable rapid, robust, and high-quality mass production of beam steering, large-angle deflectors, and diffractive optical couplers.

Colorful multi-plane augmented reality display with dynamically tunable reflective Pancharatnam-Berry phase lens

Reflective cholesteric liquid crystal (CLC) Pancharatnam-Berry phase lens (PBL) devices have attracted significant attention in augmented reality (AR) display due to their wide spectral and angular response bandwidths, high diffraction efficiency, and polarization selectivity. However, currently reported CLC reflective PBLs are either limited by monochrome display or suffers from complicated design for colorful display. Herein, we demonstrate a colorful multi-plane AR display system with dynamically tunable reflective PBL. The reflective PBL is fabricated by polymer-stabilized cholesteric liquid crystal (PSCLC) that provides dynamical and continuous tunability of color and focal length by direct current (DC) voltage. A proof-of-concept colorful multi-plane AR device is demonstrated, where over 90% diffraction efficiency at desired wavelength has been obtained. The proposed simple, compact, and light AR display system capable of color-imaging with multi-depth shows great application potential in the vehicle-mounted head-up display (HUD).

Simultaneous chiral polarization and edge enhancement imaging enabled by a single geometric-phase-based element

In this Letter, we propose a multifunctional imaging system enabled by a single geometric-phase-based liquid crystal (LC) element, which integrates chiral polarization and edge enhancement imaging. The element is located at the frequency domain plane in a 4F imaging system, and the phase profile of the element consists of a fork grating in the x direction and a grating in the y direction, which provide edge enhancement and chiral polarization imaging capabilities. Benefiting from the tunable property of the LC, the system can be switched from a polarization and edge imaging mode to the normal conventional imaging mode which is capable of conveniently acquiring the needed image information. Experiments demonstrate that the system can easily achieve multifunctional and switchable imaging, which agrees well with our design, and our LC element can work in the broadband spectrum because of the geometric phase modulation. The multifunctional strategy used here can effectively avoid the need to increase the size of the original microscopic system and the need for additional mechanical rotation of components. We believe that the proposed system with the additional advantages of electric control and tunability can find applications in biological imaging, medical detection, and optical computing.

Optical imprinting subwavelength-period liquid crystal polarization gratings with dual-twist templates

In this Letter, we report a dual-twist template imprinting method to fabricate subwavelength-period liquid crystal polarization gratings (LCPGs). In other words, the period of the template must be reduced to 800 nm-2 µm, or even smaller. To overcome the inherent problem that the diffraction efficiency shrinks as the period decreases, the dual-twist templates were optimized by rigorous coupled-wave analysis (RCWA). With the help of the rotating Jones matrix to measure the twist angle and thickness of the LC film, the optimized templates were fabricated eventually, and the diffraction efficiencies were up to 95%. Therefore, subwavelength-period LCPGs with a period of 400-800 nm were imprinted experimentally. Our proposed dual-twist template provides the possibility for fast, low-cost, and mass fabrication of large-angle deflectors and diffractive optical waveguides for near-eye displays.

In-Depth Review of Augmented Reality: Tracking Technologies, Development Tools, AR Displays, Collaborative AR, and Security Concerns

Augmented reality (AR) has gained enormous popularity and acceptance in the past few years. AR is indeed a combination of different immersive experiences and solutions that serve as integrated components to assemble and accelerate the augmented reality phenomena as a workable and marvelous adaptive solution for many realms. These solutions of AR include tracking as a means for keeping track of the point of reference to make virtual objects visible in a real scene. Similarly, display technologies combine the virtual and real world with the user's eye. Authoring tools provide platforms to develop AR applications by providing access to low-level libraries. The libraries can thereafter interact with the hardware of tracking sensors, cameras, and other technologies. In addition to this, advances in distributed computing and collaborative augmented reality also need stable solutions. The various participants can collaborate in an AR setting. The authors of this research have explored many solutions in this regard and present a comprehensive review to aid in doing research and improving different business transformations. However, during the course of this study, we identified that there is a lack of security solutions in various areas of collaborative AR (CAR), specifically in the area of distributed trust management in CAR. This research study also proposed a trusted CAR architecture with a use-case of tourism that can be used as a model for researchers with an interest in making secure AR-based remote communication sessions.

Lensless phase-only holographic retinal projection display based on the error diffusion algorithm

Holographic retinal projection display (RPD) can project images directly onto the retina without any lens by encoding a convergent spherical wave phase with the target images. Conventional amplitude-type holographic RPD suffers from strong zero-order light and conjugate. In this paper, a lensless phase-only holographic RPD based on error diffusion algorithm is demonstrated. It is found that direct error diffusion of the complex Fresnel hologram leads to low image quality. Thus, a post-addition phase method is proposed based on angular spectrum diffraction. The spherical wave phase is multiplied after error diffusion process, and acts as an imaging lens. In this way, the error diffusion functions better due to reduced phase difference between adjacent pixels, and a virtual image with improved quality is produced. The viewpoint is easily deflected just by changing the post-added spherical phase. A full-color holographic RPD with adjustable eyebox is demonstrated experimentally with time-multiplexing technique.

Holographic near-eye display system with large viewing area based on liquid crystal axicon

In this paper, a liquid crystal axicon based holographic near-eye display system with large viewing area is proposed. The viewing area of the system is extended by implementing the liquid crystal axicon. The error diffusion algorithm is used to calculate the computer-generated hologram (CGH). When incident on the liquid crystal axicon placed at the back focal plane of Fourier lens, the reconstruction light modulated by the CGH is deflected into two directions resulting in a viewing area extension. Meanwhile, to illustrate the potential of the proposed system, two-dimensional viewing area extension is demonstrated. It combines the frequency spectrum shift with the proposed system and achieves a double expansion of the horizontal viewing area and three-times expansion of the vertical viewing area. Feasibility of the proposed system is verified by optical experiments. The proposed system has potential applications in holographic augmented reality (AR) display.

Advanced liquid crystal devices for augmented reality and virtual reality displays: principles and applications

Liquid crystal displays (LCDs) and photonic devices play a pivotal role to augmented reality (AR) and virtual reality (VR). The recently emerging high-dynamic-range (HDR) mini-LED backlit LCDs significantly boost the image quality and brightness and reduce the power consumption for VR displays. Such a light engine is particularly attractive for compensating the optical loss of pancake structure to achieve compact and lightweight VR headsets. On the other hand, high-resolution-density, and high-brightness liquid-crystal-on-silicon (LCoS) is a promising image source for the see-through AR displays, especially under high ambient lighting conditions. Meanwhile, the high-speed LCoS spatial light modulators open a new door for holographic displays and focal surface displays. Finally, the ultrathin planar diffractive LC optical elements, such as geometric phase LC grating and lens, have found useful applications in AR and VR for enhancing resolution, widening field-of-view, suppressing chromatic aberrations, creating multiplanes to overcome the vergence-accommodation conflict, and dynamic pupil steering to achieve gaze-matched Maxwellian displays, just to name a few. The operation principles, potential applications, and future challenges of these advanced LC devices will be discussed.

Holo-imprinting polarization optics with a reflective liquid crystal hologram template

Liquid crystal polarization optics based on photoalignment technique has found pervasive applications in next-generation display platforms like virtual reality and augmented reality. Its large-scale fabrication, however, remains a big challenge due to the high demands in small feature size, fast processing speed, and defects-free alignment quality during the photoalignment process, especially for large-angle reflective devices. Here we propose a new concept of holo-imprinting based on non-contact replication of polarization pattern with a reflective liquid crystal hologram as a template. Our theoretical analysis and experimental results validate the possibility of generating a high-quality polarization pattern exploiting the self-interfering beams of reflective holograms. The method can be extended to numerous devices, from transmissive to reflective, from small angle to large angle, and from grating, lens, to freeform optics. Its widespread impact on the fabrication of liquid crystal polarization optics for advanced display and imaging systems is foreseeable.

Conjugate wavefront encoding: an efficient eyebox extension approach for holographic Maxwellian near-eye display

Conventional holographic display suffers from the conjugate light issue. In this Letter, we propose to efficiently extend the eyebox of holographic Maxwellian near-eye display by encoding the conjugate wavefront as the multiplication of plane wave phase with the target image. It is interesting that after being focused by the lens, the generated conjugate viewpoints also present erect virtual images with the same image quality as the signal viewpoints. Multiple plane wave encoding is used for eyebox extension, and, because of the utilization of conjugate light, the effect of eyebox extension is doubled. That is, the space bandwidth of the amplitude-type hologram is fully used. A speckless holographic image is produced in mid-air with high quality within a large depth range. The proposed display is compact and promising for the augmented reality near-eye display. Furthermore, it may inspire better solutions for the conjugate light issue of amplitude-type holography.

Augmented reality and virtual reality displays: emerging technologies and future perspectives

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

Dual-depth augmented reality display with reflective polarization-dependent lenses

Vergence-accommodation conflict (VAC) is a common annoying issue in near-eye displays using stereoscopy technology to provide the perception of three-dimensional (3D) depth. By generating multiple image planes, the depth cues can be corrected to accommodate a comfortable 3D viewing experience. In this study, we propose a multi-plane optical see-through augmented reality (AR) display with customized reflective polarization-dependent lenses (PDLs). Leveraging the different optical powers of two PDLs, a proof-of-concept dual-plane AR device is realized. The proposed design paves the way to a compact, lightweight, and fatigue-free AR display.

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

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.

Low-Complexity Pupil Tracking for Sunglasses-Wearing Faces for Glasses-Free 3D HUDs

This study proposes a pupil-tracking method applicable to drivers both with and without sunglasses on, which has greater compatibility with augmented reality (AR) three-dimensional (3D) head-up displays (HUDs). Performing real-time pupil localization and tracking is complicated by drivers wearing facial accessories such as masks, caps, or sunglasses. The proposed method fulfills two key requirements: low complexity and algorithm performance. Our system assesses both bare and sunglasses-wearing faces by first classifying images according to these modes and then assigning the appropriate eye tracker. For bare faces with unobstructed eyes, we applied our previous regression-algorithm-based method that uses scale-invariant feature transform features. For eyes occluded by sunglasses, we propose an eye position estimation method: our eye tracker uses nonoccluded face area tracking and a supervised regression-based pupil position estimation method to locate pupil centers. Experiments showed that the proposed method achieved high accuracy and speed, with a precision error of <10 mm in <5 ms for bare and sunglasses-wearing faces for both a 2.5 GHz CPU and a commercial 2.0 GHz CPU vehicle-embedded system. Coupled with its performance, the low CPU consumption (10%) demonstrated by the proposed algorithm highlights its promise for implementation in AR 3D HUD systems.