In recent years, augmented reality (AR) display systems have attracted extensive attention. Diffractive waveguides have become the preferred choice for commercial AR devices. However, current AR-HMDs still face challenges in simultaneously achieving a large field-of-view (FOV) and high angular uniformity. The FOV determines the user’s viewing angle range, and uniformity is a key factor affecting the optical performance of the waveguide display system. Therefore, it is of great significance to design a grating that can meet the requirements of large FOV and high angular uniformity.

Based on the propagation phase modulation principle and the effective medium theory, we designed the initial structure of the grating. We divided a grating period into M equal parts, and calculated the refractive index of each sub-period according to the equivalent refractive index theory. We also discussed the number of sub-periods in a cycle to determine the most reasonable initial structure. We used the rigorous coupled-wave algorithm (RCWA) to calculate the diffraction efficiency of the grating. We adopted the multi-objective genetic algorithm (NSGA-II) based on the Python evolutionary computing framework Geatpy. The period, height, and the width and spacing of the nanopillars were used as optimization variables to perform multi-objective collaborative optimization on the optical performance (uniformity and diffraction efficiency) under TE polarization. Considering the actual application of AR head-mounted devices, we set the horizontal FOV to 30° and the vertical FOV to 50° according to the aspect ratio of mainstream displays. We optimized the uniformity of both TE and TM polarizations, with each polarization state accounting for 50%. The metasurface grating was fabricated using processes including plasma-enhanced chemical vapor deposition (PECVD), electron-beam lithography(EBL), and inductively coupled plasma (ICP) dry etching. We observed the processed structure using a scanning electron microscope. For testing, we measured the diffraction efficiency using a power meter and a specific optical path. We also tested the FOV range of the grating by setting the incident angles and calculating the positions of the out-coupling gratings.

By simulation, we found that dividing the period into three sub-periods as the initial structure was the most appropriate. The initial structure had an average diffraction efficiency of 0.6649 and a uniformity of 70.77% within the FOV, indicating that there was room for improvement. After optimization by the genetic algorithm, we obtained a structure with an angular uniformity of 90.27% and an average diffraction efficiency of 0.72 under TE polarization, which was a 8.3% increase in diffraction efficiency and a 27.5% increase in uniformity compared with the initial structure (Fig. 4). For the case of simultaneous incidence of TE and TM polarized light, the overall uniformity exceeded 88%, and the average diffraction efficiency reached 0.56. The diffraction efficiency distribution of the full FOV was shown in Fig. 5. When the grating height h varied within ±15 nm, the average diffraction efficiency and uniformity were greater than 0.517 and 82.36%, respectively. Similar results were obtained for the variations of other parameters such as the width and spacing of the nanopillars (Table 2, Fig. 6). In the wavelength band of 600?680 nm, the lowest diffraction efficiency of different FOVs was greater than 0.4, the average diffraction efficiency was greater than 0.52, and the angular uniformity was greater than 83% (Fig. 7). The processed grating had a structure with a period uniformity, and the size error was within ±3 nm compared with the processing layout. The measured angular uniformity was 91.23%, which was consistent with the simulation results. The average diffraction efficiency was 0.366, slightly lower than the simulation result due to processing and interface reflection losses. The grating could reach a diagonal FOV of 50°, and the energy ratio of the three out-coupling patterns was 1∶0.991∶0.995, indicating uniform output energy (Fig. 12).

We designed a multi-nanopillar metasurface grating structure based on the equivalent refractive index theory and the multi-objective genetic algorithm. Under TE polarization, it achieved an angular uniformity of over 90% and an average diffraction efficiency of 0.72 in the 30° horizontal FOV. For the simultaneous incidence of TE and TM polarized lights, it maintained an angular uniformity of 88% and an average efficiency of 0.56 in the 56° diagonal FOV. The vertical nanopillar structure has excellent structural stability, overcoming the process bottleneck of traditional structures that are prone to collapse. Through processing and testing, we achieved a processing accuracy with a key size error within ±3 nm. The measured diffraction efficiency uniformity reached 91.23%, and the diagonal FOV was extended to 50°. This grating structure is expected to be applied to in-coupling and out-coupling of AR display diffractive waveguides to achieve a wide FOV and high-angular-uniformity display effect. Future work can focus on improving grating performance, improving processing technology, and expanding application scenarios.