Off-axis metasurfaces for folded flat optics

• Published in: Nature communications 2023-09, Vol.14 (1), p.5602-8, Article 5602
• Main Authors: Born, Brandon, Lee, Sung-Hoon, Song, Jung-Hwan, Lee, Jeong Yub, Ko, Woong, Brongersma, Mark L.
• Format: Article
• Language: English
• Subjects: 639/624/1075/146; 639/624/400/1021; Design; Fabrication; High aspect ratio; Humanities and Social Sciences; Light; Lithography; Metasurfaces; Miniaturization; multidisciplinary; Nanofabrication; Nanolithography; Numerical aperture; Optical components; Optics; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-023-41123-x

The overall size of an optical system is limited by the volume of the components and the internal optical path length. To reach the limits of miniaturization, it is possible to reduce both component volume and path length by combining the concepts of metasurface flat optics and folded optics. In addition to their subwavelength component thickness, metasurfaces enable bending conventional folded geometries off axis beyond the law of reflection. However, designing metasurfaces for highly off-axis illumination with visible light in combination with a high numerical aperture is non-trivial. In this case, traditional designs with gradient metasurfaces exhibit low diffraction efficiencies and require the use of deep-subwavelength, high-index, and high-aspect-ratio semiconductor nanostructures that preclude inexpensive, large-area nanofabrication. Here, we describe a design approach that enables the use of low-index (n ≈ 1.5), low-aspect ratio structures for off-axis metagratings that can redirect and focus visible light ( λ  = 532 nm) with near-unity efficiency. We show that fabricated optical elements offer a very large angle-of-view (110°) and lend themselves to scalable fabrication by nano-imprint lithography. The authors show how concepts from metasurface flat optics and folded optics can be combined to push the limits in miniaturization of optical systems. Such optical systems can be fabricated using low-index materials, opening up a path for large-area fabrication using nanoimprint lithography.