Consumers are looking for AR/VR glasses that are compact and easy to wear, delivering high-quality imagery with socially acceptable optics that don't look like “bug eyes”. University of Rochester researchers at the Institute of Optics have come up with a novel technology to deliver those attributes with maximum effect. They combined freeform optics with a nano­photonic metasurface.

The metasurface is a veritable forest of tiny, silver, nanoscale structures on a thin metallic film that conforms, in this advance, to the freeform shape of the optics – realizing a new optical component the researchers call a metaform. The metaform is able to defy the conventional laws of reflection, gathering the visible light rays entering an AR/VR eyepiece from all directions, and redi­recting them directly into the human eye. 

Nick Vamivakas, a professor of quantum optics and quantum physics, likened the nanoscale structures to small-scale radio antennas. “When we actuate the device and illuminate it with the right wavelength, all of these antennas start oscillating, radiating a new light that delivers the image we want downstream.” “Meta­surfaces are also called flat optics so writing metasurfaces on freeform optics is creating an entirely new type of optical component,” says Jannick Rolland, director of the Center for Freeform Optics. “This kind of optical component can be applied to any mirrors or lenses, so we are already finding applications in other types of components” such as sensors and mobile cameras.

The goal is to direct the visible light entering the AR/VR glasses to the eye. The new device uses a freespace optical combiner to help do that. However, when the combiner is part of freeform optics that curve around the head to conform to an eyeglass format, not all of the light is directed to the eye. Freeform optics alone cannot solve this specific challenge. That's why the researchers had to leverage a meta­surface to build a new optical component. 

“Integrating these two techno­logies, freeform and meta­surfaces, understanding how both of them interact with light, and leveraging that to get a good image was a major challenge,” says Daniel Nikolov, an optical engineer in Rolland's research group. Another obstacle was bridging "from macro­scale to nanoscale,” Rolland says. The actual focusing device measures about 2.5 millimeters across. But even that is 10,000 times larger than the smallest of the nano­structures imprinted on the freeform optic.

“From a design standpoint that meant changing the shape of the freeform lens and distributing the nano­structures on the lens in a way that the two of them work in synergy, so you get an optical device with a good optical performance,” Nikolov says. This required Aaron Bauer, an optical engineer in Rolland's group, to find a way to circumvent the inability to directly specify meta­surfaces in optical design software. In fact, different software programs were used to achieve an inte­grated metaform device.

Fabri­cation was daunting, Nikolov says. It required using electron-beam litho­graphy, in which beams of electrons were used to cut away sections of the thin-film metasurface where the silver nano­structures needed to be deposited. Writing with electron beams on curved freeform surfaces is atypical and required developing new fabri­cation processes.

The researchers used a JEOL electron-beam litho­graphy (EBL) machine at the University of Michigan's Lurie Nano­fabrication Facility. To write the metasurfaces on a curved freeform optic they first created a 3D map of the freeform surface using a laser-probe measuring system. The 3D map was then programmed into the JEOL machine to specify at what height each of the nano­structures needed to be fabricated.

“We were pushing the capa­bilities of the machine,” Nikolov says. Fei Cheng, a post­doctoral associate in the Vamivakas group; Hitoshi Kato, a JEOL represen­tative from Japan, and the Michigan staff of the nano­fabrication lab, colla­borated with Nikolov on achieving successful fabri­cation “after multiple iterations of the process.” “This is a dream come true,” Rolland says. “This required integrated teamwork where every contri­bution was critical to the success of this project.” (Source: U. Rochester)

Reference: D. K. Nikolov et al.: Metaform optics: Bridging nanophotonics and freeform optics, Sci. Adv. 7, eabe5112 (2021); DOI: 10.1126/sciadv.abe5112

Link: Institute of Optics, University of Rochester, Rochester, USA