Design and prototyping of broadband metasurface scalar phase masks for high-contrast imaging

NASA's future Habitable Worlds Observatory (HWO) will enable the direct detection and characterization of Earth-like planets around Sun-like stars using high-contrast imaging. One of the most promising approaches to achieve this goal is to use a coronagraph. A good candidate for implementation in HWO is the vortex coronagraph, which is featured in both earlier mission concepts HabEx and LUVOIR. However, HWO would benefit from a scalar vortex coronagraph instead of the well-established vector vortex coronagraphs in order to increase throughput and reach its ambitious goal of characterizing Earth-like exoplanets. Metasurfaces present a promising technology for achromatizing scalar vortex coronagraphs, because they allow for more design freedom in a single layer of constant thickness compared to scalar vortex phase masks based on variable thickness of a dielectric substrate. Here, we present our progress in developing metasurface scalar vortex phase masks. We present updated broadband designs of scalar metasurface phase masks of different topographies (vortex and phase knife), and simulate their performance with appropriate simulation tools. We also discuss first manufacturing attempts at such masks, and outline the next steps needed to push their performance towards the levels required for HWO.