Diffractive waveguide serves as a core optical scheme and mainstream solution for consumer augmented reality (AR) near-eye displays, featuring thin structure, expandable exit pupil and high ambient transmittance. Nowadays, lightweight, high-imaging-quality AR glasses have become a mainstream demand for wearable smart devices, and diffractive waveguide technology is widely adopted in commercial products due to its excellent optical performance and structural advantages. Traditional surface relief gratings have defects such as chromatic dispersion, narrow field of view (FOV), low coupling efficiency and uneven brightness. These defects cause severe color separation, luminance fluctuation and obvious rainbow artifacts, which hinder the development of compact single-layer full-color AR devices with high image quality. Subwavelength-structured metasurfaces achieve precise and flexible manipulation of light phase, polarization and amplitude, and provide effective approaches to address the inherent limitations of diffractive waveguides.

Based on the grating equation, k-space analysis and geometry-determined theoretical efficiency limit, researchers summarized three typical achromatic strategies. These three technical routes have different working principles and applicable scenarios for full-color waveguide design. Layer-stacked metasurfaces realized independent red-green-blue (RGB) phase modulation by vertically arranging multiple functional layers, and achieved an average diffraction efficiency higher than 89.2% under mixed RGB illumination. Nevertheless, this scheme imposes ultra-high requirements on component alignment and fabrication. The high-order diffraction matching method assigned distinct diffraction orders to three primary wavelengths to unify the in-waveguide diffraction angles. This approach fundamentally eliminates chromatic dispersion and realizes single-layer achromatic coupling with a theoretical FOV over 45°. The geometric compensation strategy does not suppress intrinsic dispersion. By optimizing waveguide thickness, multicolor beams converge accurately at the out-coupler, offering a feasible technical route for compact full-color 3D display devices.

Multi-zone in-couplers and imaginary refractive index engineering effectively narrowed the practical efficiency gap between actual performance and theoretical limits. The minimum field efficiency was increased from 20% to 25.3%. Combined with inverse topological optimization, this method suppresses angle-dependent efficiency fluctuations and balances the output uniformity of RGB light. Silicon carbide (SiC) has excellent thermal conductivity and mechanical strength, which greatly raises the upper limit of full-color FOV and fundamentally suppresses rainbow artifacts. Polarization multiplexing divides the whole field of view into two orthogonal polarization channels, achieving a horizontal FOV exceeding 80° on a monolithic waveguide. Geometric-phase-based polarization multiplexing enables various advanced functions: spin-split metagratings realize low-crosstalk stereoscopic imaging without extra waveguide layers; hybrid polarization-volume holographic gratings adopt dual-focus design to alleviate vergence-accommodation conflict; polarization steering components combine segmented fields of view to double the total FOV without increasing waveguide thickness.

Comprehensive comparison shows that combining high-order diffraction with high-index SiC serves as the optimal scheme for compact single-layer, large-FOV full-color waveguides, which can effectively eliminate rainbow artifacts. Polarization multiplexing promotes system function expansion and optimizes user experience. At present, laboratory research results of metasurface waveguides are relatively abundant, but there is still a large gap between laboratory prototypes and mass-produced commercial products. Currently, all existing technical routes face three major industrialization bottlenecks: insufficient precision of large-area nanoimprint and deep-UV lithography lowers mass production yield; the overall efficiency of high-order gratings fails to satisfy the demands of practical display applications; the integration of tunable metasurfaces and waveguides remains in the early stage of development. Future research will focus on three key directions: inverse design optimization based on physics-driven neural networks, integrated fabrication of wide-bandgap SiC materials, and the development of multi-dimensional tunable metasurfaces for dynamic FOV and focal plane regulation. Metasurface diffractive waveguides have evolved from discrete component optimization to full light-field manipulation, and will support the development of next-generation spatial computing optical hardware.