Consumers are looking for augmented reality/virtual reality (AR/VR) glasses that are compact and easy to wear, delivering high-quality imagery with socially acceptable optics that don't look like “bug eyes.” Researchers have imprinted freeform optics with a nanophotonic optical element called a metasurface.
The metasurface is a forest of tiny, silver nanoscale structures on a thin metallic film that conforms to the freeform shape of the optics, resulting in a new optical component called a metaform. The metaform is able to defy 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 that can be applied to any mirrors or lenses; as a result, applications exist in sensors and mobile cameras. The goal is to direct the visible light entering the AR/VR glasses to the eye. The new device uses a freespace optical combiner to help do that; however, 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. Freeform optics alone cannot solve this specific challenge.
That's why the researchers had to leverage a metasurface to build a new optical component. Another obstacle was bridging from macroscale to nanoscale. The actual focusing device measures about 2.5 millimeters across. But even that is 10,000 times larger than the smallest of the nano-structures imprinted on the freeform optic. 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, producing an optical device with good optical performance.
Fabrication 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.
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 at what height each of the nanostructures needed to be fabricated.