Black Silicon BRDF and Polarization for Coronagraphic Pupil Masks

TL;DR

This paper develops an FDTD model to analyze the polarization-dependent optical properties of black silicon masks, aiming to improve coronagraphic performance for space telescopes observing Earth-like exoplanets.

Contribution

It introduces a novel FDTD modeling approach for black silicon masks, addressing the limitations of scalar diffraction models at sub-5 micron feature sizes.

Findings

FDTD model characterizes polarization-dependent BRDF of black silicon.

Identifies shortcomings of current modeling approaches.

Provides insights for designing better pupil masks for high-contrast imaging.

Abstract

Future space observatories will likely have segmented primaries, causing diffraction effects that reduce coronagraph performance. Reflective binary pupil apodizer masks can mitigate these, with the metamaterial black silicon (BSi) showing promise as a strong absorber. To bring contrast ratios to the $10^-{10}$ level as needed to observe Earth-like exoplanets, feature sizes on these BSi masks will need to be less than $5$ microns when paired with MEMS (micro-electromechanical systems) deformable mirrors. As scalar diffraction cannot reliably model this feature size, we developed a Finite-Difference Time-Domain (FDTD) model of BSi masks using Meep software. We characterize the FDTD-derived polarization-dependent bidirectional reflectance distribution function of BSi and discuss the model's shortcomings.