05 May 2021
University of Rochester combines metasurfaces and freeform optics for compact high-resolution systems.
Customizable optics using these components in small form-factor platforms is also attractive for several high-resolution applications beyond AR/VR, including 3D-vision, mobile cameras and remote sensing.
A project at the University of Rochester Institute of Optics has now developed a fabrication procedure for combining metasurfaces and freeform optics into one optical component, offering designers a route to simplifying the architectures of miniaturized optical systems. The work was published in Science Advances.
"There has not been any realization of a generalizable design-to-fabrication process for creating metasurfaces on a freeform substrate where both the metasurface phase response and the shape of the freeform optic are leveraged for imaging and aberration correction," noted the team in its published paper.
Metasurfaces achieve their novel wavefront control through the presence of subwavelength structures manufactured on a substrate, influencing the reflection and refraction properties of that surface.
Freeform optical surfaces, technically defined as surfaces with no axis of rotational symmetry around a normal axis, are not limited to the classical descriptors of optical shape, and do not fall neatly into categories such as spheres, conics, or aspherics.
"Freeform optics has been around in some disguised forms, but modern advances in both fabrication processes and optical materials are making it possible to conceive and design freeform optical surfaces of much greater complexity," commented Rochester's Jannick Rolland to Optics.org in 2013, on the creation of the Centre for Freeform Optics (CeFO) by Rochester and UNC Charlotte.
"In the past, fabrication methods had to try and keep up with optical design. But now fabrication is ahead, and we are trying to catch up with the design mathematics and metrology. Just one weak link in the line from design to assembly will ensure that an optical product does not work, so we need all these different areas of expertise to work together."
The new project from Rolland's research group at CeFO represents a further evolution of this fabrication philosophy, allowing metasurface fabrication on a freeform substrate through the use of electron beam lithography (EBL) so that both can work in tandem. The team calls the resulting component a "metaform."
A dream come true
“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.
A proof-of-concept metaform mirror component measuring 1.5 x 2 millimeters, designed with AR/VR and near-eye applications in mind, was manufactured using modified EBL to create a controlled distribution of silver nanostructures on a curved freeform surface - a sufficiently complicated procedure that the first demonstration required years to bring to fruition, according to the team.
The component successfully behaved as reflective metaform on an opaque backplate, although modifications to the design will be needed to create a fully see-through metaform component able to take its place in a display architecture. Once optimized, however, such components could be incorporated into AR glasses as a near-eye metaform combiner, to deliver light from a microdisplay to a wearer.
"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," commented CeFo in its announcement. "Freeform optics alone cannot solve this specific challenge; hence metasurfaces had to leveraged too."
With the market for AR/VR glasses alone being potentially worth $20 billion according to Rochester - and one focus of the Optics.org AR/VR/MR Focus publication - the metaform concept and the design degrees of freedom it enables could support a new generation of optical systems.
"This required integrated teamwork where every contribution was critical to the success of the project," noted Rolland. "This is a dream come true."
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