Freeform Metagratings Based on Complex Light Scattering Dynamics for Extreme, High Efficiency Beam Steering

Abstract Conventional phased‐array metasurfaces utilize subwavelength‐scale nanoparticles or nanowaveguides to specify spatially‐dependent amplitude and phase responses to light. An alternative design strategy is based on freeform inverse optimization, in which wavelength‐scale elements are designed to produce devices that possess exceptionally high efficiencies. In this report, we theoretically analyze the physical mechanisms enabling high efficiency in freeform‐based periodic metasurfaces, i.e., metagratings. An in‐depth coupled mode analysis of ultra‐wide‐angle beam deflectors and wavelength splitters shows that the extraordinary performance of these designs originates from the large number of propagating modes supported by the metagrating, in combination with complex multiple scattering dynamics exhibited by these modes. We also apply our coupled mode analysis to conventional nanowaveguide‐based metagratings to understand and quantify the factors limiting the efficiencies of these devices. We envision that freeform metasurface design methods will open new avenues towards high‐performance, multi‐functional optics by utilizing strongly coupled nanophotonic modes and elements.