Proceedings Article | 5 October 2023
KEYWORDS: Design and modelling, Polarization, Phase shifts, Coronagraphy, Nanostructures, K band, Astronomy, Imaging systems, Finite-difference time-domain method, Simulations, Metalenses, Exoplanetary science, Astronomical instrumentation engineering, Astronomical imaging, Optical vortices
As exoplanet direct imaging progresses towards lower planet-star contrasts and smaller, less separated planets, the need for technological improvement in imaging systems remains ever present. Metasurface optics, or arrays of subwavelength structures with highly tailorable geometry and composition on a thin substrate, have the potential to greatly advance coronagraph systems at various stages of the optical pipeline by correcting aberrations induced by other optical components and improving upon the performance of the conventional optics that are currently used. Metasurfaces can provide achromatic phase, amplitude, and/or polarization control in a compact package. Polarization insensitive phase control devices are of particular interest, because such scalar devices are less sensitive to the polarization aberrations that can negatively impact vector optics, which are currently more prevalent in coronagraph systems. Our work provides a general overview of metasurface optics and addresses the specific application of scalar-vortex (MSV) phase masks for vortex coronagraphy and vortex fiber nulling (VFN). We detail a multi-shape, variable period design process which we use to develop MSVs of various topological charge. The MSVs we developed include a J and V band charge-6, an H-band charge-2, and a K-band charge-1 MSV. The J, H, K, and V devices exhibit achromatic behavior over 15%, 12%, a 11%, and a 24% bandwidth, respectively. We also develop a multiplexed vector vortex-phase dimple metasurface for the H-band as a showcase of another way in which metasurfaces can advance direct imaging systems. We demonstrate simulated K-band MSV performance in the Keck Observatory VFN instrument with on-axis coupling below 10−3 . We detail the path to a MSV that can achieve contrasts that will enable the imaging of terrestrial planets.
