# Polarization-dependent beam shifts upon metallic reflection in high-contrast imagers and telescopes

**Authors:** R. G. van Holstein, C. U. Keller, F. Snik, S. P. Bos  
**arXiv:** [2308.10940](https://arxiv.org/abs/2308.10940)  
**Date:** 2023-08-21  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.10940v1/

## Key points

- Future space- and ground-based high-contrast imagers and telescopes aim to achieve extreme contrasts at close separations from the star
- Achievable contrast will be limited by reflection-induced polarization aberrations
- Numerical modeling provides some insight into these aberrations but does not fully capture all effects, origin, characteristics, or mitigation strategies
- Study conducted to understand polarization aberrations produced by reflection off flat metallic mirrors
- Polarization ray tracing used to numerically compute and interpret aberrations
- Four beam shifts (spatial Goos-Hänchen shift, angular Goos-Hänchen shift, spatial Imbert-Federov shift, angular Imbert-Federov shift) fully reproduced by polarization ray tracing
- Only spatial Goos-Hänchen and Imbert-Federov shifts relevant for high-contrast imagers and telescopes as they affect performance of coronagraphs and polarimetric speckle suppression close to the star
- Mitigation strategies suggested: keeping f-numbers large, angles of incidence small; mirror coatings designed with retardance close to 180 degrees instead of maximum reflectivity
- Insights can improve current high-contrast imaging techniques and those used on Extremely Large Telescopes (ELTs)
- Understanding reflection-induced polarization aberrations enhances ability to directly image rocky exoplanets with greater accuracy

## Summary

In order to directly image rocky exoplanets in reflected (polarized) light, future space- and ground-based high-contrast imagers and telescopes aim to achieve extreme contrasts at close separations from the star. However, the achievable contrast will be limited by reflection-induced polarization aberrations. While numerical modeling can provide some insight into these aberrations, it does not fully capture the range of effects, their origin and characteristics, or possible ways to mitigate them. To address this issue, a study was conducted to understand polarization aberrations produced by reflection off flat metallic mirrors at a fundamental level. The researchers used polarization ray tracing to numerically compute these aberrations and interpret the results in terms of the polarization-dependent spatial and angular Goos-Hänchen and Imbert-Federov shifts of the beam of light. The study found that all four beam shifts were fully reproduced by polarization ray tracing. The researchers investigated the origin, characteristics, sizes, and directions of these shifts. Of the four beam shifts, only the spatial Goos-Hänchen and Imbert-Federov shifts were found to be relevant for high-contrast imagers and telescopes because they are visible in the focal plane and create a polarization structure in the point spread function (PSF) that reduces the performance of coronagraphs and polarimetric speckle suppression close to the star. Based on their findings, the researchers suggest several strategies for mitigating beam shifts in optical systems. They recommend keeping f-numbers large and angles of incidence small. Additionally, mirror coatings should not be optimized for maximum reflectivity but should instead be designed to have a retardance close to 180 degrees. The insights gained from this study can help improve current high-contrast imaging techniques as well as those used on Extremely Large Telescopes (ELTs). By understanding how reflection produces polarization aberrations and implementing mitigation strategies accordingly, researchers can enhance our ability to directly image rocky exoplanets from afar with greater accuracy than ever before - furthering our knowledge about these distant worlds.