Wide field-of-view (FOV) optics are essential components in many optical systems, with applications spanning imaging, display, sensing, and beam steering. Conventional refractive wide FOV optics often involve multiple stacked lenses, resulting in large size and weight as well as high cost. Metasurface lenses or metalenses promise a viable solution to realizing wide FOV optics without complex lens assembly. We review the various architectures of wide FOV metalenses, elucidate their fundamental operating principles and design trade-offs, and quantitatively evaluate and contrast their imaging performances. Emerging applications enabled by wide FOV metasurface optics are also discussed.

Augmented reality (AR) display, which superimposes virtual images on ambient scene, can visually blend the physical world and the digital world and thus opens a new vista for human–machine interaction. AR display is considered as one of the next-generation display technologies and has been drawing huge attention from both academia and industry. Current AR display systems operate based on a combination of various refractive, reflective, and diffractive optical elements, such as lenses, prisms, mirrors, and gratings. Constrained by the underlying physical mechanisms, these conventional elements only provide limited light-field modulation capability and suffer from issues such as bulky volume and considerable dispersion, resulting in large size, severe chromatic aberration, and narrow field of view of the composed AR display system. Recent years have witnessed the emerging of a new type of optical elements—metasurfaces, which are planar arrays of subwavelength electromagnetic structures that feature an ultracompact footprint and flexible light-field modulation capability, and are widely believed to be an enabling tool for overcoming the limitations faced by current AR displays. Here, we aim to provide a comprehensive review on the recent development of metasurface-enabled AR display technology. We first familiarize readers with the fundamentals of AR display, covering its basic working principle, existing conventional-optics-based solutions, as well as the associated pros and cons. We then introduce the concept of optical metasurfaces, emphasizing typical operating mechanisms, and representative phase modulation methods. We elaborate on three kinds of metasurface devices, namely, metalenses, metacouplers, and metaholograms, which have empowered different forms of AR displays. Their physical principles, device designs, and the performance improvement of the associated AR displays are explained in details. In the end, we discuss the existing challenges of metasurface optics for AR display applications and provide our perspective on future research endeavors.

The recently proposed extreme-ultraviolet beams with time-varying orbital angular momentum (OAM) realized by high-harmonic generation provide extraordinary tools for quantum excitation control and particle manipulation. However, such an approach is not easily scalable to other frequency regimes. We design a space-time-coding digital metasurface operating in the microwave regime to experimentally generate time-varying OAM beams. Due to the flexible programmability of the metasurface, a higher-order twist in the envelope wavefront structure of time-varying OAM beams can be further designed as an additional degree of freedom. The time-varying OAM field patterns are dynamically mapped by developing a two-probe measurement technique. Our approach in combining the programmability of space-time-coding digital metasurfaces and the two-probe measurement technique provides a versatile platform for generating and observing time-varying OAM and other spatiotemporal excitations in general. The proposed time-varying OAM beams have application potentials in particle manipulation, time-division multiplexing, and information encryption.

Spatiotemporal optical vortex (STOV) pulses can carry transverse orbital angular momentum (OAM) that is perpendicular to the direction of pulse propagation. For a STOV pulse, its spatiotemporal profile can be significantly distorted due to unbalanced dispersive and diffractive phases. This may limit its use in many research applications, where a long interaction length and a tight confinement of the pulse are needed. The first demonstration of STOV pulse propagation through a few-mode optical fiber is presented. Both numerical and experimental analysis on the propagation of STOV pulse through a commercially available SMF-28 standard telecommunication fiber is performed. The spatiotemporal phase feature of the pulse can be well kept after the pulse propagates a few-meter length through the fiber even with bending. Further propagation of the pulse will result in a breakup of its spatiotemporal spiral phase structure due to an excessive amount of modal group delay dispersion. The stable and robust transmission of transverse photonic OAM through optical fiber may open new opportunities for transverse photonic OAM studies in telecommunications, OAM lasers, and nonlinear fiber-optical research.

Orbital angular momentum (OAM) spectrum diagnosis is a fundamental building block for diverse OAM-based systems. Among others, the simple on-axis interferometric measurement can retrieve the amplitude and phase information of complex OAM spectra in a few shots. Yet, its single-shot retrieval remains elusive, due to the signal–signal beat interference inherent in the measurement. Here, we introduce the concept of Kramers–Kronig (KK) receiver in coherent communications to the OAM domain, enabling rigorous, single-shot OAM spectrum measurement. We explain in detail the working principle and the requirement of the KK method and then apply the technique to precisely measure various characteristic OAM states. In addition, we discuss the effects of the carrier-to-signal power ratio and the number of sampling points essential for rigorous retrieval and evaluate the performance on a large set of random OAM spectra and high-dimensional spaces. Single-shot KK interferometry shows enormous potential for characterizing complex OAM states in real time.