Augmented reality (AR) systems hold transformative potential across numerous fields, including entertainment, education, communication, and training. However, widespread adoption is hampered by limitations in current AR display technology. Existing systems often rely on bulky projection optics, resulting in uncomfortable and unstylish devices. Furthermore, the accurate portrayal of three-dimensional (3D) depth cues remains a significant challenge, leading to the vergence-accommodation conflict and reduced perceptual realism. Waveguide image combiners offer a promising solution due to their compact form factor, but current designs are limited to 2D images and still require substantial projection optics. Holographic principles offer the potential for creating ultra-thin, high-quality 3D displays. Previous attempts to integrate digital holography into AR displays have fallen short of achieving the desired compactness and image quality. This research aims to overcome these limitations by developing a novel AR display system that combines a lensless holographic light engine with a meticulously designed metasurface waveguide.

The authors reviewed existing AR display systems, highlighting the limitations of bulky projection optics and the vergence-accommodation conflict inherent in 2D displays. They examined waveguide combiners as a promising solution for compact AR glasses but noted their limitations in 3D imaging. Previous research on holographic displays was also explored, acknowledging promising attempts to integrate holography into AR configurations, but these attempts did not yield the compact form factors and high-quality 3D image quality required for practical applications. The authors highlight the potential of metasurfaces for improving diffraction efficiency, spectral selectivity, and transmittance compared to conventional refractive and diffractive optical elements in AR applications. The paper also mentions previous research in waveguide holography for virtual reality (VR), acknowledging the poor image quality as a significant barrier to adoption.

The researchers developed a novel AR display system pairing a lensless holographic light engine with a metasurface waveguide. This system is designed for full-color optical-see-through (OST) AR applications. The key innovation lies in the use of inverse-designed metasurface grating couplers. These couplers are optimized to maximize diffraction efficiency, spectral selectivity, and angular uniformity. The high refractive index (n > 1.8) of the glass material ensures total internal reflection (TIR) for all visible wavelengths, enabling a single-layer coupler design for compactness. Chromatic dispersion is corrected at the system level through the geometric design of the waveguide and k-vector matching of the input and output couplers. A dispersion-compensating waveguide geometry is engineered by precisely controlling waveguide thickness and the dimensions and distances of symmetric metasurface couplers. Rigorous coupled-wave analysis is employed to optimize the metasurface grating geometry for maximum diffraction efficiency and uniformity of angular response. The fabrication process utilizes electron beam lithography on high-index glass to minimize surface irregularities. A physically motivated waveguide propagation model is developed, incorporating the frequency-dependent transfer function of the waveguide. To account for imperfections and discrepancies between the simulated model and physical reality, learnable components in the form of convolutional neural networks (CNNs) are integrated. These CNNs are calibrated using camera feedback, enabling accurate prediction of the output of the holographic AR glasses. The training process employs a gradient descent CGH algorithm that utilizes this camera-calibrated wave propagation model. The prototype system combines the fabricated metasurface waveguide with a phase-only SLM, a fibre-coupled module with RGB laser diodes, and a high-resolution color camera for data acquisition.

The inverse-designed metasurface couplers achieved high see-through efficiency (approximately 78.4% in the visible spectrum) and uniform transmittance regardless of the angle of incidence. The experimental results demonstrate high-quality, full-color, multiplane 3D holographic images using a single OST AR waveguide. The AI-based wave propagation model significantly outperforms baseline models (free-space and physically motivated models without learning) in terms of peak signal-to-noise ratio (PSNR), achieving improvements of 3-5 dB. The prototype system successfully produced full-color 3D holographic images with accurate depth-of-field effects, mitigating the vergence-accommodation conflict. Experimental results with optically combined physical and digital content further validate the system’s performance, showcasing superior image quality compared to baseline models. The achieved field of view (FOV) of 11.7° is comparable to commercial AR systems. The relationship between waveguide thickness, SLM size, and FOV was derived, suggesting that smaller SLMs could enable further miniaturization of the device.

The co-design of a metasurface waveguide and AI-driven holography algorithms has resulted in a compact, full-color, 3D holographic OST AR display system that significantly surpasses existing waveguide-based AR displays in terms of image quality. The integration of AI-calibrated wave propagation models effectively compensates for physical imperfections and enables accurate hologram synthesis. While the current FOV is comparable to existing commercial systems, the researchers propose future work to explore methods for expanding the FOV. They also suggest further miniaturization of the waveguide through the use of smaller SLMs and integration of illumination waveguides.

This research successfully demonstrates a compact, high-quality, full-color 3D holographic augmented reality display system. The co-design of nanophotonic hardware and AI-driven algorithms addresses key limitations of existing AR technology. Future work will focus on expanding the FOV, further reducing the waveguide thickness, and optimizing the efficiency of the hologram generation algorithm for real-time operation. The presented approach has the potential to revolutionize AR technology, paving the way for truly immersive and comfortable 3D AR experiences.

The current field of view (FOV) of the system is limited to 11.7°, although this is comparable to many commercial AR systems. The hologram generation process currently takes several minutes per phase pattern, limiting real-time capabilities. The étendue of the display is limited by the space-bandwidth product of the SLM. Further research is needed to optimize the efficiency of the CGH algorithm for real-time hologram synthesis. The fabrication method, while successful, may not be easily scalable for mass production; although the authors note the applicability of their method to other manufacturing techniques.