"Nonlocal" Metasurface Enables High-Performing AR Eyeglasses

With conventional AR glasses, there is a trade-off in terms of quality and brightness between the external scene that a wearer actually sees and the contextual information that the wearer wants to visualize. Early solutions used multiple bulky optical components that were partially reflective and partially transmissive to mix the real-world and contextual scenes, with the result of a dimmed and distorted vision of both scenes.

Though more recent AR head-mounted-display eyeglasses were patterned with diffractive gratings —with wavelength-size spacing that deflects contextual information from a miniprojector beside the glasses to the viewer’s eye — these eyeglasses still dim and distort the external scene because. Real-world light passing through the glass inevitably gets scattered and dispersed by the gratings. The distortions get worse when several sets of overlapping gratings must be used to handle multiple distinct colors from the miniprojector.

AR-display glass must be highly transparent over almost the entire visible spectrum, allowing for unattenuated and undistorted vision of the outside world, and also function as a highly efficient lens that focuses light from a miniprojector into the human eye to form a visual context accompanying the external real-world scene.

Researchers at Columbia University School of Engineering and Applied Science (Columbia Engineering) have now invented a glass that shows promise for applications in AR glasses. Led by associate professor of physics and applied mathematics Nanfang Yu, the team created a flat optical device that focuses only a few selected narrowband colors of light while remaining transparent to nonselected light over the vast majority of the spectrum.

“We’ve built a very cool flat optical device that appears entirely transparent — like a simple piece of glass — until you shine a beam of light with the correct wavelength onto it, when the device suddenly turns into a lens,” said Yu, a leader in nanophotonics research. “To me this is optical magic.”

Illustration showing the operation of an augmented reality headset with multifunctional nonlocal metasurfaces as optical see-through lenses. Courtesy of Nanfang Yu, Stephanie Malek, Adam Overvig/Columbia Engineering.

Yu’s group develops flat optical devices based on metasurfaces. A key feature of metasurfaces is that the 2D nanostructures of which they are made, light “scatterers” known as optical antennas, are all different optically. The light that they scatter can have different amplitude, phase, or polarization, so that metasurfaces can introduce a spatially varying optical response that can control light in extremely flexible ways. As a result, metasurfaces make it possible to realize functionalities that conventionally require 3D optical components or devices with a much larger footprint, such as focusing or steering light beams, or switching optical signals on integrated photonic chips.

Yu’s team created a “nonlocal metasurface” that can manipulate lightwaves in distinct ways at distinct targeted wavelengths, while leaving light at untargeted wavelengths unaffected. The devices exerted both spatial and spectral control over light by selecting a color and focusing it not just at a single wavelength, but also independently at multiple different wavelengths.

For example, one demonstrated device functions both as a converging lens that focuses light at one color, and as a concave lens that disperses light at a second color, while staying transparent like an unpatterned slab of glass when illuminated with light at colors over the rest of the spectrum.

Top row: Illustration showing the operation of a wavelength-selective metalens, with 'green' light being focused, while the other colors are passed without distortion (left). Optical image of a wavelength-selective metalens composed of rectangular apertures etched into a silicon thin film (middle). Scanning electron microscope (SEM) images of the metalens at its center and edge (right). Bottom row: A series of two-dimensional far-field scans shows that focusing is most efficient at the center of the resonance, λ = 1590 nm, with the focusing efficiency dropping at the two shoulders of the resonance, λ = 1575 nm and 1600 nm, and that the focal spots become almost undetectable at wavelengths tens of nanometers away from the center of the resonance. Courtesy of Nanfang Yu, Stephanie Malek, Adam Overvig/Columbia Engineering.

The devices trace their origin to theoretical explorations by study co-author Adam Overvig, into how to manipulate symmetry in photonic crystal (PhC) slabs, such as a 2D periodic structure that is a square array of square holes defined in a thin film of silicon. PhC slabs are known to support a set of modes, the frequencies or colors of which are determined by the geometry of the slab (e.g., periodicity of the array and size of the holes). The modes are essentially a sheet of light that is spatially extended (nonlocal) along the slab but otherwise confined in the direction normal to the slab.

Introducing a symmetry-breaking perturbation to an otherwise structurally repetitive PhC slab lowers the degree of symmetry of the PhC so that the modes are no longer confined to the slab. Now, they can be excited by shining a beam of light from free space with the correct color and can also radiate back into free space.

Instead of applying a uniform perturbation over the entire PhC slab, the researchers spatially varied the perturbation, orienting the rectangular holes along different directions over the device. In this way, the surface emission from the device could have a molded wavefront in relation to the pattern of the orientation angles of the rectangles.

“This is the first time that anyone has experimentally demonstrated wavelength-selective, wavefront-shaping optical devices using an approach that is based on symmetry-breaking perturbations,” said Stephanie Malek, a doctoral student in Yu’s group and lead author of the study. Carefully choosing the initial PhC geometry allowed the team to achieve wavelength selectivity. By tailoring the orientations of the perturbation applied to the PhC, the team could sculpt the wavefront of the selected color of light and make lenses that focus light of only the selected color, according to Malek.

The wavelength-selective, wavefront-shaping “nonlocal” metasurfaces present a promising solution for AR technologies, including head-up displays on the front windshield of cars. The lens is able to reflect contextual information to the viewer’s eye at selected narrow-band wavelengths of the miniprojector while also allowing an unobstructed, undimmed, broadband view of the real world.

Top row: Illustration showing the operation of a three-function metalens doublet (left). The doublet is able to generate three distinct focal patterns (two focal lines orthogonal to each other and a star-shaped focal spot) at three different wavelengths, while staying transparent at other wavelengths. The doublet is composed of a quasi-radial metalens as a diverging element and a dual-function cylindrical metalens as a converging element. Optical images of the quasi-radial metalens and the dual-function cylindrical metalens (middle). SEM images showing the corners of the quasi-radial metalens and the dual-function cylindrical metalens (right). Bottom row: A series of 2D far-field scans showing the three focal patterns at l = 1424 nm, 1492 nm, and 1626 nm and minimal wavefront shaping over the rest of the spectrum. Courtesy of Nanfang Yu, Stephanie Malek, Adam Overvig/Columbia Engineering.

Because the wavelength-selective metasurface lenses are thinner than a human hair, they are well-suited to developing AR goggles that look and feel like comfortable and fashionable eyeglasses. The metasurfaces can also be used to substantially reduce the complexity of quantum optics setups that manipulate ultracold atoms. Because multiple laser beams at distinct wavelengths have to be independently controlled for cooling, trapping, and monitoring cold atoms, these setups can become massive.

This complexity has made it difficult for researchers to widely adopt cold atoms for use in atomic clocks, quantum simulations, and computations. Now, instead of building several ports around the vacuum chamber for the cold atoms, each with their unique beam-shaping optical components, a single metasurface device can be used to simultaneously shape the multiple laser beams used in the experiment.

The devices in this study simultaneously and independently control the wavefronts of several near-infrared beams using nanostructured silicon thin films. The team plans next to demonstrate the concept in the visible spectral range, to fully control the wavefronts of three narrowband visible laser beams using a device platform featuring low absorption loss in the visible, such as thin-film silicon nitride and titanium dioxide. It is also exploring the scalability of the wavelength-selective metasurface platform by including more than two perturbations into a single metasurface and by stacking more than two metasurfaces into a compound device.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-022-00905-6).