A team from the University of Rochester’s Institute of Optics imprinted freeform optics with a metasurface, in a combination of optical technologies that charts a course for a new fabrication method for AR/VR glasses. The metasurface, a silver nanoscale structure on a thin metallic film, conformed to the freeform shape of the optics used in the work, which allowed the researchers to develop an optical component that they call a “metaform.”
The metaform defies 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.
“Metasurfaces are also called ‘flat optics,’ so writing metasurfaces on freeform optics is creating an entirely new type of optical component,” said Jannick Rolland, the Brian J. Thompson Professor of Optical Engineering and 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,” Rolland said. Those components include sensors and mobile cameras.
The new device uses a freespace optical combiner to direct visible light that enters the AR/VR glasses to the eye. However, when the combiner is part of freeform optics that curve around the head to conform to an eyeglass front, not all of the light is directed to the eye.
The researchers reported that their first successful demonstration took many years to complete and did not happen until they leveraged 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,” said Daniel Nikolov, an optical engineer in Rolland’s research group and lead author on a paper describing the work.
Another obstacle was bridging from macroscale to nanoscale, Rolland said; the actual focusing device measures about 2.5 mm across. That measurement is 10,000× 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 said. An optical engineer in Rolland’s group ultimately resolved the bottleneck (the inability to directly specify metasurfaces in optical design software) by using different software programs to achieve an integrated metaform device.
Fabrication was another challenge. The team used electron-beam lithography, in which beams of electrons 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 it forced the team to develop new fabrication processes. The researchers used a JEOL electron-beam lithography (EBL) machine at the University of Michigan’s Robert H. 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 the height at which each of the nanostructures needed to be fabricated.
The research was supported in part by a fellowship to Nikolov from the Link Foundation and funding from the Center for Emerging and Innovative Sciences at the University of Rochester.
The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe5112).