“Image” is everything in the $ 20 billion AR / VR glasses market. Consumers are looking for eyeglasses that are compact and easy to wear, offering high quality images with socially acceptable optics that do not look like “insect eyes”.
Researchers at the University of Rochester’s Institute of Optics have developed new technology to deliver these attributes to maximum effect. In an article in Scientific progress, they describe the impression of free-form optics with a nanophotonic optical element called “a metasurface”.
The metasurface is a veritable forest of tiny, nanoscale silver structures on a thin metallic film that conforms, in this breakthrough, to the freeform of optics – realizing a new optical component that researchers call a metaform.
The metaform is able to defy conventional laws of reflection, collecting visible light rays entering an AR / VR eyepiece from all directions and redirecting them directly into the human eye.
Nick Vamivakas, professor of quantum optics and quantum physics, compared nanoscale structures to small-scale radio antennas. “When we turn on the device and illuminate it with the right wavelength, all of these antennas start to oscillate, radiating new light that delivers the image we want downstream.”
“Metasurfaces are also called ‘flat optics’, so writing metasurfaces on freeform optics creates a whole new kind of optical component,” says Jannick Rolland, Brian J. Thompson professor of optical engineering and director of the Center for Freeform Optics.
“This type of optical component can be applied to any mirror or lens, adds Rolland,” so we are already finding applications in other types of components “such as sensors and mobile cameras.
WHY THE FREEFORM OPTICAL IS NOT ENOUGH
The first demonstration took many years.
The objective is to direct visible light entering the AR / VR glasses towards the eye. The new device uses a freespace optical combiner to achieve this. However, when the combiner is part of a free-form optic that curves around the head to conform to a spectacle format, not all of the light is directed to the eye. Free-form optics alone cannot solve this specific challenge.
This is why the researchers had to take advantage of a metasurface to build a new optical component.
“Integrating these two technologies, free form and metasurfaces, understanding how both interact with light and leveraging it to get a good image was a major challenge,” says lead author Daniel Nikolov, optical engineer at the group. of Rolland’s research.
THE CHALLENGE OF MANUFACTURING
Another obstacle was to go “from the macroscopic scale to the nanometric scale”, explains Rolland. The actual focusing device is approximately 2.5 millimeters in diameter. But even that is 10,000 times larger than the smallest of the nanostructures imprinted on the free-form optics.
“From a design standpoint, this involved changing the shape of the freeform lens and distributing the nanostructures on the lens in such a way that the two work synergistically, so that you get an optical device with great. good optical performance ”, explains Nikolov.
This forced Aaron Bauer, an optical engineer with the Rolland Group, to find a way around the inability to directly specify metasurfaces in optical design software. In fact, different software has been used to realize an integrated metaform device.
The making was intimidating, Nikolov says. Electron beam lithography had to be used, in which electron beams were used to cut sections of the thin-film metasurface where the silver nanostructures were to be deposited. Writing with electron beams on curved free-form surfaces is atypical and requires the development of new manufacturing processes.
Researchers used a JEOL electron beam lithography (EBL) machine at the University of Michigan’s Lurie Nanofabrication Facility. To write the metasurfaces on curved freeform optics, they first created a 3D map of the freeform surface using a laser probe measurement system. The 3D card was then programmed into the JEOL machine to specify at what height each of the nanostructures had to be fabricated.
“We were pushing the capabilities of the machine,” Nikolov says. Fei Cheng, postdoctoral associate in the Vamivakas group; Hitoshi Kato, a representative of JEOL from Japan, and the Michigan staff of the nanofabrication lab, worked with Nikolov to achieve manufacturing success “after multiple iterations of the process.”
“It’s a dream come true,” says Rolland. “It required integrated teamwork where every contribution was essential to the success of this project.”