“Image” is everything in the $20 billion market for AR/VR glasses. Consumers are looking for glasses that are compact and easy to use, providing high quality images with socially acceptable optics that don’t look like “googly eyes.”

Researchers at the University of Rochester’s Institute of Optics have devised novel technology to deliver those attributes to maximum effect. on a paper in Progress of sciencedescribe printing free-form optics with a nanophotonic optical element called a “metasurface”.

The metasurface is a veritable forest of tiny nanoscale silvery structures in a thin metallic film that conforms, in this advance, to the free form of the optic, accounting for a new optical component that the researchers call metaform.

The metaform is able to defy conventional laws of reflection, gathering 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, compared the nanoscale structures to small-scale radio antennas. “When we turn on the device and shine it at the correct wavelength, all these antennas start to oscillate, radiating new light that gives the image we want downstream.”

“Metasurfaces are also called ‘planar optics,’ so writing metasurfaces in freeform optics is creating an entirely new type of optical component,” says Jannick Rolland, the Brian J. Thompson Professor of Optical Engineering and director of the Center for Optics. Free Form.

Adds Rolland: “This type of optical component can be applied to any mirror or lens, so we are already finding applications in other types of components” such as sensors and mobile cameras.

Why Freeform Optics Wasn’t Enough

The first demonstration required many years to complete.

The goal is to direct the visible light entering the AR/VR glasses towards the eye. The new device uses a free space optical combiner to help do that. However, when the combiner is part of a free-form optic that curves around the head to fit the format of a pair of glasses, not all of the light is directed at the eye. Freeform optics alone cannot solve this specific challenge.

That’s why the researchers had to take advantage of a metasurface to build a new optical component.

“Integrating these two technologies, freeform and metasurfaces, understanding how they both interact with light, and harnessing that to get a good image was a big challenge,” says lead author Daniel Nikolov, an optical engineer in Rolland’s research group.

The manufacturing challenge.

Another hurdle was moving “from the macroscale to the nanoscale,” says Rolland. The actual focus device is about 2.5 millimeters wide. But even that is 10,000 times larger than the smallest of the nanostructures printed in free-form optics.

“From a design point of view, that meant changing the shape of the lens freely and distributing the nanostructures in the lens in a way that both work in synergy, so that you get an optical device with good optical performance,” Nikolav says.

This required Aaron Bauer, an optical engineer in Rolland’s group, to find a way around the inability to specify metasurfaces directly in optical design software. In fact, different software programs were used to achieve an integrated metaform device.

Manufacturing was daunting, says Nikolov. He required the use of electron beam lithography, 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 free-form curved surfaces is unusual and requires the development of new manufacturing processes.

The researchers used a JEOL electron beam lithography (EBL) machine at the University of Michigan’s Lurie Nanofabrication Facility. To write the metasurfaces in a curved freeform optic, they first created a 3D map of the freeform surface using a laser probe measurement 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,” says Nikolov. Fei Cheng, postdoctoral associate in the Vamivakas group; Hitoshi Kato, a representative of JEOL Japan, and staff from the Michigan nanofabrication lab collaborated with Nikolov to achieve successful fabrication “after multiple iterations of the process.”

“This is a dream come true,” says Rolland. “This required integrated teamwork where each contribution was critical to the success of this project.”

What is free form optics?

Free-form optics is an emerging technology that uses lenses and mirrors with surfaces that lack an axis of symmetry inside or outside the diameter of the optic to create optical devices that are lighter, more compact, and more effective than ever.

Applications include 3D imaging and visualization, augmented and virtual reality, military and infrared optical systems, efficient automotive and LED lighting, energy research, remote sensing, semiconductor manufacturing and inspection, and medical and assistive technologies.

Rolland, Bauer, and colleagues at the Center for Freeform Optics recently published an article in Optics provide an overview of this technology, including the early development of lenses without rotational symmetry; the design, manufacture, testing and assembly of freeform optics; underlying theory and outlook for the future.