Metasurface AR Near-Eye Display

Background

Augmented reality (AR) was first proposed in 1990 by former Boeing researcher Tom Caudell. Over the past three decades, with the advancement of computing power in mobile electronics, its applications have become increasingly widespread. AR is an interactive experience that combines real-world scenes with computer-generated content. This content can span multiple sensory modalities, including vision, hearing, and touch. In this way, AR alters people's ongoing perception of the real environment, seamlessly interweaving it with the physical world and resulting in an immersive experience of the real environment. An ideal AR display system must simultaneously achieve lightweight, high portability, and high image quality.

However, current AR display systems rely on a combination of traditional refractive, reflective, and diffractive optical elements. Due to physical limitations, these traditional optical elements offer limited light field modulation capabilities, are bulky, and exhibit significant color dispersion. Consequently, they cannot simultaneously achieve the compact size and excellent display performance required for AR display systems, including a wide field of view (FOV), high color accuracy, and a large eye box. In recent years, metasurface technology has been able to manipulate the amplitude, phase, and polarization of incident light through customized meta-atoms, mimicking the functions of traditional refractive, reflective, or diffractive optical elements. Metasurface technology exhibits unique advantages, such as compact size and flexible light field manipulation, and is widely considered to overcome some of the limitations of current AR display systems.

Research on metasurface-based AR near-eye displays has made continuous progress in recent years. Professor Zhang Cheng of Huazhong University of Science and Technology led a review of metasurface-based AR display technologies, analyzing in detail three mainstream metasurface devices—metalenses, metacouplers, and metaholograms—and their potential applications in various AR display formats. The researchers explain the physical principles, design schemes, and characteristics and advantages of these three devices in related AR display systems. The review, titled "Metasurface-enabled Augmented Reality Display: A Review," was published in Advanced Photonics, Issue 3, 2023.

A Brief History of Metasurface AR Displays

Based on the different forms of core optical components, current mainstream AR display solutions can be roughly divided into four categories: traditional optical solutions, freeform optical solutions, holographic optical solutions, and optical waveguide solutions. An AR display system is typically described by several performance factors, including field of view (FOV), eye box, angular resolution, and focal cue. By introducing metasurface devices, including mainstream metalenses, metasurface couplers, and metasurface holographic devices, researchers are gradually breaking through the performance limitations of traditional optical devices. These devices can effectively replace traditional optical components and have the potential to be widely used in emerging AR display systems to improve their compactness and display performance. Figure 1 illustrates the potential role of these three types of metasurface devices in waveguide AR display systems.

As early as 2011, Professor Capasso's research team at Harvard University used V-shaped metal nanoantennas to induce plasmon resonance, achieving phase abrupt changes. Based on this, they constructed a metasurface with a phase gradient and experimentally observed anomalous light beam deflection. In 2012, Professor Zhou Lei's research team at Fudan University used H-shaped metal nanoantennas to create a phase gradient, compensating for the wave vector difference between the two different electromagnetic modes, namely, the propagation wave and the surface wave. This allowed efficient coupling of the propagation wave to the surface wave through a metasurface. In 2013, Professor Shalaev's research team at Purdue University used V-shaped metal nanoholes to simultaneously manipulate the amplitude and phase of transmitted light, constructing an ultrathin metasurface hologram. Measuring only 30 nm thick, it was the thinnest holographic element known at the time. In 2018, the research teams of Academician Zhu Shining at Nanjing University and Professor Tsai Dingping at National Taiwan University proposed an achromatic metalens composed of GaN resonant units that covers the entire visible light band, achieving an average efficiency of 40% and enabling full-color imaging. Metalenses achieve beam focusing by introducing phase shifts through metaatoms. Compared to traditional lenses, they offer higher numerical aperture (NA), more compact dimensions, and richer imaging capabilities. Therefore, they hold great promise for application in AR display systems, which place high demands on both system size and image quality. In 2018, a research team at Seoul National University in South Korea used anisotropic metasurface units to construct a metalens with high numerical aperture, large device size, and wide operating bandwidth, achieving a compact, large field of view, and high-resolution color AR display.

Current waveguide AR display systems face two major limitations: a small field of view and large chromatic aberration. Metasurface couplers, which have been widely explored in recent years, offer greater design freedom and more powerful light field modulation capabilities than traditional couplers, offering an effective approach to overcoming these limitations. Polarization-sensitive metasurface couplers can selectively couple light of a specific polarization state into or out of a waveguide, thereby expanding the field of view of waveguide displays through polarization multiplexing. Polarization multiplexing can also be used to achieve stereoscopic AR displays. In 2021, a collaborative research team from the University of Michigan and Huazhong University of Science and Technology constructed a metasurface coupler using an elliptical array of gold nanorods, achieving unidirectional waveguide coupling determined by the chirality of the incident light. Leveraging this property, a waveguide AR display system was constructed that projects two parallax images (different perspectives of a regular octahedron) carried by oppositely chiral circularly polarized light into different eyes, achieving parallax stereoscopic AR display.

Furthermore, metasurface holographic devices encode holographic image information through subwavelength structural units, enabling more flexible and diverse light field manipulation of incident light. Compared to traditional holographic devices, metasurface holographic devices can generate two-dimensional and three-dimensional holographic images with higher spatial resolution and wider divergence angles, all with a smaller structural size and higher efficiency. Metasurface holographic devices can serve as miniature image sources for AR display systems, providing high-quality monochrome or color holographic images. Researchers have utilized metasurface holographic devices, combined with various optical elements as a projection system, to achieve AR displays with diverse functional features.

Regarding materials and fabrication processes, metasurface devices for AR displays typically utilize dielectric materials with low optical loss in the visible light band. To effectively manipulate the phase of incident light, dielectric materials with a high refractive index (n approximately 2.0 or higher) are preferred for metasurface construction. Common materials include titanium dioxide (TiO2), hafnium oxide (HfO2), gallium nitride (GaN), and silicon nitride (SiNx). The structural patterns of metasurfaces can generally be produced by deep ultraviolet (deep-UV) or electron beam (e-beam) lithography. In recent years, researchers have also developed some unconventional metasurface processing technologies to achieve high-yield, large-area, and high-aspect-ratio metasurface device processing. Damascene lithography fills the constituent materials into nanopores to form columnar structures without relying on an etching process. Therefore, it is suitable for processing metasurface devices composed of materials that are difficult to etch (Figure 5a). Furthermore, nanoimprint lithography (NIL), which creates nano- to micron-scale structures through mechanical molding under heating or UV irradiation, is an effective method for low-cost and high-volume large-area metasurface device fabrication.

Conclusion

Thanks to their powerful light field modulation capabilities and small structural dimensions, metasurface devices are enabling new AR display systems with improved image quality, more advanced imaging functions, lower weight, and more compact dimensions. However, to ultimately achieve the commercialization and widespread application of metasurface AR displays, a number of challenges must be overcome. For example, broadband achromatic metalenses are limited by their low NA values and small device size, achromatic metasurface couplers lack a readily processable device design, and metasurface holographic devices cannot project arbitrary holographic images in real time. Meanwhile, researchers have proposed a series of solutions to address these issues, and related research is ongoing.

In summary, metasurface technology holds great potential for improving the performance of AR displays in all aspects. In addition to the research work described above, metasurfaces can also enrich the functionality of AR glasses, such as enabling eye tracking and preventing lens fogging (anti-fogging). Using metasurfaces to replace traditional optical components such as wave plates, polarizers, beam splitters, and color filters can further reduce system size. Metasurfaces have also demonstrated advantages in improving the resolution of microdisplays. For example, organic light-emitting diode (OLED) displays integrated with metasurfaces can achieve spatial resolutions exceeding 10,000 pixels per inch (PPI). Therefore, metasurfaces are expected to become one of the core technologies for future AR displays and related applications, playing a variety of roles in AR display devices.