Metasurface-based scalar vortex phase mask in pursuit of 1e-10 contrast

Abstract

Imaging Earth-like planets around sun-like stars has become one of the main science drivers for future space telescope missions. High-contrast imaging using a vortex coronagraph has proven to be a promising approach for achieving this goal. However, at the huge contrast levels required for future space-based telescopes the vectorial nature of the well-established vector vortex phase mask becomes a limiting factor, since it imprints phase ramps of opposite signs on the two circular polarizations. An alternative polarization-independent approach is using a scalar vortex phase mask, which affects both polarizations in the same way. The achromatic performance of scalar vortex phase masks for space-based applications has still to be improved, though. Metasurfaces provide a promising approach to implement a scalar vortex phase mask with relatively simple fabrication techniques. Their demonstrated ability to implement broadband phase and amplitude masks makes them a prime candidate for achieving achromatic performance in pursuit of the 10−10 contrast limit required by NASA’s Habitable Worlds Observatory. We present a metasurface-based design of a scalar vortex phase mask providing a helical phase ramp across a large bandwidth. We first use rigorous coupled-wave analysis to create a library of square metasurface building blocks (nanoblocks) and choose an optimal set of nanoblock sizes providing broadband 2π phase coverage at a given nanoblock height. We then arrange the nanoblocks in a design providing a helical phase ramp and propagate the phase and transmission provided by the mask through a wavefront propagation software to obtain contrast curves at several wavelengths. Finally we apply electric field conjugation to dig a half-sided dark hole from 3-10 λ/D reaching 3.7 × 10−9 contrast in 20% bandwidth.

Benefits of adding radial phase dimples on scalar coronagraph phase masks

“…Nevertheless, the performance is certainly more than sufficient for use on the next generation of very large ground-based telescopes. To reach the HWO target contrast value of 1e − 10 across a 20% bandwidth, additional achromatization techniques, such as using phase dimples together with achromat plate combinations 13,16 or metasurfaces, 17,18 will thus likely be necessary.…”