Microstructured vortex and azimuthal cosine phase mask design for high-contrast imaging

At the extreme contrast levels required to image Earth-like planets around Sun-like stars, the polarization dependence of the vector vortex coronagraph becomes a limiting factor, making wavefront control difficult to perform in both polarizations simultaneously. An alternative is to use a polarization-independent scalar vortex phase mask, but achromatizing scalar masks remains challenging. We investigate using metasurfaces to increase the bandwidth of scalar vortex phase masks. Our design shows an improvement of up to 2 orders of magnitude compared to a scalar vortex made of a helical-shaped dielectric substrate. However, the characteristic phase discontinuities of scalar vortex phase masks introduce phase artifacts and remain challenging to manufacture accurately. The cosine phase mask is an alternative approach to implementing a coronagraphic phase mask with a continuously varying azimuthal phase profile but without the phase jump of the scalar vortex. In addition, it requires smaller phase coverage. We therefore also investigate a metasurface implementation of the cosine mask. We present results obtained using rigorous coupled-wave analysis and finite-difference time-domain simulations and find that the phase jumps of a scalar vortex result in significant stellar leakage, which does not appear in the case of the cosine mask. We then present the simulated coronagraphic performance and residual chromaticity of both designs and discuss their advantages and drawbacks. We conclude that metasurface scalar vortex and cosine phase masks are promising coronagraphic phase masks in light of upcoming ground and space telescope missions combining deep contrast and insensitivity to low-order aberrations.