A research team headed up at Seoul National University has pioneered an innovative metasurface-based folded lens system, paving the way for a new generation of slimline cameras for use in smartphones and augmented/virtual reality devices.
Traditional lens modules, built from vertically stacked refractive lenses, have fundamental thickness limitations, mainly due to the need for space between lenses and the intrinsic volume of each individual lens. In an effort to overcome these restrictions, the researchers – also at Stanford University and the Korea Institute of Science and Technology – have developed a lens system using metasurface folded optics. The approach enables unprecedented manipulation of light with exceptional control of intensity, phase and polarization – all while maintaining thicknesses of less than a millimetre.
Folding the light path
As part of the research – detailed in Science Advances – the team placed metasurface optics horizontally on a glass wafer. These metasurfaces direct light through multiple folded diagonal paths within the substrate, optimizing space usage and demonstrating the feasibility of a 0.7 mm-thick lens module for ultrathin cameras.
“Most prior research has focused on understanding and developing single metasurface elements. I saw the next step as integrating and co-designing multiple metasurfaces to create entirely new optical systems, leveraging each metasurface’s unique capabilities. This was the main motivation for our paper,” says co-author Youngjin Kim, a PhD candidate in the Optical Engineering and Quantum Electronics Laboratory at Seoul National University.
According to Kim, creation of a metasurface folded lens system requires a wide range of interdisciplinary expertise, including a fundamental understanding of conventional imaging systems such as ray-optic-based lens module design, knowledge of point spread function and modulation transfer function analysis and imaging simulations – both used in imaging and optics to describe the performance of imaging systems – plus a deep awareness of the physical principles behind designing metasurfaces and the nano-fabrication techniques for constructing metasurface systems.
“In this work, we adapted traditional imaging system design techniques, using the commercial tool Zemax, for metasurface systems,” Kim adds. “We then used nanoscale simulations to design the metasurface nanostructures and, finally, we employed lithography-based nanofabrication to create a prototype sample.”
Smoothing the “camera bump”
The researchers evaluated their proposed lens system by illuminating it with an 852 nm laser, observing that it could achieve near-diffraction-limited imaging quality. The folding of the optical path length reduced the lens module thickness to half of the effective focal length (1.4 mm), overcoming inherent limitations of conventional optical systems.
“Potential applications include fully integrated, miniaturized, lightweight camera systems for augmented reality glasses, as well as solutions to the ‘camera bump’ issue in smartphones and miniaturized microscopes for in vivo imaging of live animals,” Kim explains.
Kim also highlights some more general advantages of using novel folded lens systems in devices like compact cameras, smartphones and augmented/virtual reality devices – especially when compared with existing approaches – including include the ultraslim and lightweight form factor, and the potential for mass production using standard semiconductor fabrication processes.
When it comes to further research and practical applications in this area over the next few years, Kim points out that metasurface folded optics “offer a powerful platform for light modulation” within an ultrathin form factor, particularly since the system’s thickness remains constant regardless of the number of metasurfaces used.
“Recently, there has been growing interest in co-designing hardware-based optical elements with software-based AI-based image processing for end-to-end optimization, which maximizes device functionality for specific applications,” he says. “Future research may focus on combining metasurface folded optics with end-to-end optimization to harness the strengths of both advanced hardware and AI.”
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