Optimizing metasurfaces to achieve deeper direct imaging contrasts: current performance and lessons learned from fabrication

As exoplanet direct imaging progresses, the demand for high performance coronagraphic phase masks is ever increasing. Metasurfaces, or arrays of subwavelength structures with highly tailorable geometry and material composition on thin substrate, have the potential to revolutionize starlight-blocking optics. We report on the design, fabrication, and characterization of one such device for the H-band. The optic, a charge-2, all-Si metasurface scalar vortex (MSV), is designed using a novel variable spacing framework which enables it to exhibit achromatic behavior over a 230 nm bandwidth (1.45-1.70µm). We fabricate the MSV using a photolithography based process, which is fast and relatively cheap. We characterize the phase and amplitude apodization of the mask using a digital holographic microscope. We test the coronagraphic performance of the MSV and report contrast results. We report on paths for future improvement and progress on a charge-6, Si3N4 on SiO2 visible light metasurface design.