Topology-optimized freeform broadband optical metagrating for high-efficiency large-angle deflection

A metagrating made of periodic arrays of subwavelength inclusions has great capability for efficient manipulationof the impinging wavefront at subwavelength scale. However, it suffers from a significant practical issue in that thediffraction efficiency declines rapidly along with the increase of deflection angle, especially for broadband appli-cation scenarios. Here, we comprehensively investigated different topology optimization approaches to design thebroadband optical metagrating for high-efficiency large-angle deflection, including single-wavelength optimiza-tion (SO), max-min optimization (MO), and average optimization (AO). It is demonstrated that the AO approachis more appropriate to optimize the broadband high-efficiency metagrating. Specifically, a freeform metagratingof up to about a 70 degrees deflection angle at a central wavelength of 10.6 mu m with diffraction efficiency exceeding 80%over a broad bandwidth of 4.36 mu m in the infrared range is demonstrated based on the AO approach. It is alsoshown that, for all three optimization approaches, the quasi-catenary metagrating optimized from initial catenarystructures has the evident advantage on bandwidth compared to that optimized from initial discrete structures. Ourresults provide insights into the inverse design of metagratings and may find potential applications in broadbandachromatic metalenses and other broadband meta-devices. (c) 2023 Optica Publishing Group under the terms of the Potica Open Access Publishing Agreement.