Source : University of Rochester illustration /Michael Osadciw
“Image” is everything in $20 billion market place for AR/VR glasses. Consumers are trying to find glasses that are compact and easy to wear, delivering high-quality imagery with socially acceptable optics that don’t appear like “bug eyes.”
University of Rochester researchers at the Institute of Optics have come up with a completely unique technology to deliver those attributes with maximum effect. In a paper in Science Advances, they describe imprinting freeform optics with a nanophotonic optical element called “a metasurface.”
The metasurface may be a veritable forest of small , silver, nanoscale structures on a thin metallic film that conforms, in this advance, to the freeform shape of the optics—realizing a latest optical component the researchers call a metaform.
The metaform is able to defy the laws of reflection, gathering the visible light rays entering an AR/VR glasses from all directions, and redirecting them directly into human eye.
Nick Vamivakas, a professor of quantum optics and physics , likened the nanoscale structures to small-scale radio antennas. “When we actuate the device and illuminate it with the right wavelength, all of those antennas start oscillating, radiating new light that delivers the image we would like downstream.”
“Metasurfaces also are called ‘flat optics’ so writing metasurfaces on freeform optics is creating a completely new sort of optical component,” says Jannick Rolland, the Brian J. Thompson Professor of Optical Engineering and director of the Centre for Freeform Optics.
Adds Rolland, “This type of optical component are often applied to any mirrors or lenses, so we are already finding applications in other sorts of components” like sensors and mobile cameras.
Why freeform optics weren’t enough
The first demonstration required many years to finish .
The goal is to direct the light entering the AR/VR glasses to the eye. The new device uses a freespace optical combiner to help do this . However, when the combiner is a component of freeform optics that curve around the head to evolve 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 create 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 clear image was a serious challenge,” says lead author Daniel Nikolov, an optical engineer in Rolland’s research group.
The challenge of fabrication
Another obstacle was bridging “from macroscale to nanoscale,” Rolland says. the particular focusing device measures about 2.5 millimeters across. But even that’s 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 freeform lens and distributing the nanostructures on the lens in a way that the 2 of them work in synergy, so you get an optical device with good optical performance,” Nikolov says.
This required Aaron Bauer, an optical engineer in Rolland’s group, to seek out how to bypass the lack 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 down the metasurfaces on a curved freeform optic they first created a 3D map of the freeform surface using laser-probe measuring instrument . 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 within 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 process.”
“This may be a dream come true,” Rolland says. “This required integrated teamwork where every contribution was critical to the success of this project.”
What is freeform optics?
Freeform optics is an emerging technology that uses lenses and mirrors with surfaces that lack an axis of symmetry within or outside the optics diameter to make optical devices that are lighter, more compact, and simpler than ever before.
Applications include 3D imaging and visualization, augmented and Virtual Reality, infrared and military optical systems, efficient automotive and LED lighting, energy research, remote sensing, semiconductor manufacturing and inspection, and medical and assistive technologies.
Rolland, Bauer, and collaborators at Centre for Freeform Optics recently published a paper in Optica providing an summary of this technology, including early development of lenses without rotational symmetry; the design, fabrication, testing, and assembly of freeform optics; underlying theory, and outlook for future.
The findings were reported in University of Rochester
