A near-eye display is a device that brings a visual display as close to you as headphones bring sound. A well-known recent example is Google’s smart glasses, Google Glass. A small, lightweight near-eye display would be of great use for these mobile devices or in industries such as medicine and aerospace, where there exist stringent size and weight constraints. The potential applications of compact optical systems have driven interest in freeform optics.
Traditional optics, such as a lens, relies on the shape of its surface and its volume to bend light. However, it is difficult to manufacture the sharp curvatures and complex forms utilized in freeform optics using existing technologies. To solve this problem, UW Electrical Engineering (EE) and Physics Assistant Professor Arka Majumdar and his group have developed visible frequency freeform optical elements by leveraging nano-patterned surfaces, known as metasurfaces. These planar metasurfaces mimic the curved surfaces in traditional optics and induce spatially varying changes in phase with an ultrathin and flat form factor, enabling cheap and simple fabrication of freeform elements.
The work comes from Majumdar’s paper “Metasurface Freeform Nanophotonics,” which was recently published in Scientific Reports. The researchers demonstrated metasurfaces with a cubic phase profile, demonstrating extended depth of focus and focal-tunable lenses. This work clearly demonstrates the effectiveness of metasurface technology to build ultra-thin freeform optical elements.
The technology developed by Majumdar’s group is inspired by earlier work in diffractive optics, where spatial phase shifts were achieved by varying the thickness of the device. Majumdar’s group took a slightly different approach — they use sub-wavelength structures, which allow for multiple phase-shifts by only changing the lateral geometry. This enables a flat component, which is easier to manufacture.
“Not only do these structures eliminate the need for multiple stage lithography, the sub-wavelength nature allows us to fabricate spatial profiles with large phase gradients,” lead author and UW physics graduate student Alan Zhan said. “Realizing large phase gradients opens the possibility of building monolithic optical systems, like electronic integrated circuits.”
These metasurface freeform elements being developed by Majumdar and his group could enable complex optical systems, encompassing uses from vision-correcting eyewear to multi-focal augmented reality visors to implantable microscopes, all while maintaining an ultra-compact form-factor.
“The two fields metasurface and freeform optics are led by two disconnected groups of scientists, and our approach will open up more opportunities for dialogue and cooperation between these two fields,” Majumdar said. “Our current focus is on integrating these elements into existing systems. By incorporating them with optical systems, they’ll be able to demonstrate imaging. A next step would be integrating them with solid-state mirrors, so they’ll be able to develop monolithic optical systems.”
Co-authors on the paper are EE graduate student Shane Colburn and postdoctoral scholar Dr. Chris Dodson. The research is funded in part by an Intel Early Career Award and Amazon Catalyst.