Augmented reality (AR) displays, as the next generation platform for spatial computing and digital twins, enable users to view digital images superimposed on real-world environment, fostering a deeper level of human-digital interactions. However, as a critical element in an AR system, optical combiners face unprecedented challenges to match the exceptional performance requirements of human vision system while keeping the headset ultracompact and lightweight. After decades of extensive device and material research efforts, and heavy investment in manufacturing technologies, several promising waveguide combiners have been developed. In this review paper, we focus on the perspectives and challenges of optical waveguide combiners for AR displays. We will begin by introducing the basic device structures and operation principles of different AR architectures, and then delve into different waveguide combiners, including geometric and diffractive waveguide combiners. Some commonly used in-couplers and out-couplers, such as prisms, mirrors, surface relief gratings, volume holographic gratings, polarization volume gratings, and metasurface-based couplers, will be discussed, and their properties analyzed in detail. Additionally, we will explore recent advances in waveguide combiner design and modeling, such as exit pupil expansion, wide field of view, geometric architectures of waveguide couplers, full-color propagation, and brightness and color uniformity optimization. Finally, we will discuss the bottlenecks and future development trends in waveguide combiner technologies. The objective of this review is to provide a comprehensive overview of the current state of waveguide combiner technologies, analyze their pros and cons, and then present the future challenges of AR displays.

Metalenses consist of nanostructures that locally control the optical phase. They offer many degrees of freedom for manipulating a wavefront, which gives a number of advantages over bulk lenses, such as the straightforward elimination of spherical aberrations and an ultrathin dimension. Here, we compare the phase profiles of metalenses made of different dielectric materials and note the advantage of high refractive index materials. Higher refractive index materials such as silicon afford more degrees of freedom in terms of design and fabrication and are the basis for high-performance metalenses, even in the visible. Nevertheless, the imaging performance of single-element metalenses is still limited by coma and chromatic aberrations. This limitation is exacerbated by high numerical apertures and large areas. We review the challenges and trade-offs between numerical aperture, field of view, coma, chromatic aberration, and size. We also evaluate different phase engineering approaches to address these problems. We believe this review will help guide future developments in high-performance metalenses toward wide-field and high-resolution imaging, enabling scientific high-end miniature imaging systems.

Virtual reality (VR) and augmented reality (AR) have found widespread applications in education, engineering, healthcare, and entertainment. However, these near-eye displays are often bulky and heavy, and thus are not suitable for long-term wearing. Metalenses, with an ultra-thin formfactor, subwavelength modulation scale, and high modulation flexibility, are promising candidates to replace the conventional optics in AR display systems. In this work, we proposed and fabricated a novel reflective dielectric metalens-visor based on Pancharatnam-Berry phase with see-through capability. It achieves diffraction-limited focusing behavior for the reflected red light, while keeping a good transmission spectrum in the visible region. Hence, this single piece metalens-visor can perform the function of two integrated elements simultaneously: an eyepiece and an optical combiner, which in turn greatly reduces the weight and the size of an AR display. We have implemented a proof-of-concept AR display system employing the metalens-visor, and experimentally demonstrated color AR images with good image quality. This work reveals the great potential of multi-functional metasurface devices which enables optical integration in interdisciplinary applications including wearable displays, biological imaging, and aeronautic optical instruments.

Virtual reality (VR) and augmented reality (AR) are revolutionizing our lives. Near-eye displays are crucial technologies for VR and AR. Despite the rapid advances in near-eye display technologies, there are still challenges such as large field of view, high resolution, high image quality, natural free 3D effect, and compact form factor. Great efforts have been devoted to striking a balance between visual performance and device compactness. While traditional optics are nearing their limitations in addressing these challenges, ultra-thin metasurface optics, with their high light-modulating capabilities, may present a promising solution. In this review, we first introduce VR and AR near-eye displays, and then briefly explain the working principles of light-modulating metasurfaces, review recent developments in metasurface devices geared toward near-eye display applications, delved into several advanced natural 3D near-eye display technologies based on metasurfaces, and finally discuss about the remaining challenges and future perspectives associated with metasurfaces for near-eye display applications.