Model of the instrumental polarization induced by a rotating HWP with anti-reflective sub-wavelength structures for CMB polarimetry
Abstract
Future CMB polarization experiments require precise control of instrumental systematics in the search for primordial B-mode polarization. A rotating half-wave plate (HWP) provides effective polarization modulation, but its non-ideal electromagnetic response can generate HWP-synchronous instrumental polarization. In this work, we characterize the response of a multi-layer sapphire HWP with anti-reflective sub-wavelength structures (SWS) using rigorous coupled-wave analysis. For three asymmetric SWS models, we find no substantial differences in the transmittance, polarization efficiency, or instrumental-polarization response that can be attributed to the type of SWS asymmetry. The fourth-harmonic instrumental polarization relevant to CMB polarization reconstruction is typically of order 10−4 in the CMB bands, which is expected to be mitigated. We further evaluate an actually fabricated SWS geometry, whose asymmetry enhances the retardance, and compare its response with that of an idealized model having the same type of asymmetry. Comparison of the realistic SWS with the idealized model shows that higher transmittance tends to be associated with lower instrumental polarization. These results suggest that improving the total transmission, including the effect of the fabricated SWS profile, may be more important for reducing HWP-induced instrumental polarization than tuning the SWS asymmetry itself.
Related
- Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS
- Occultation polarimetry as a probe of unresolved polarization
- A Pipeline for the AI-Assisted Analysis of Imaging X-ray Polarimetry Explorer Observations of Black Hole X-ray Binaries and Limits on Their Polarization Variability
- Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars
- Energy-independent X-Ray Polarization in 4U 1630-47 in the Intermediate State from Spectroscopy and Polarimetry
BibTeX
@misc{takaku2026model,
title = {{Model of the instrumental polarization induced by a rotating HWP with anti-reflective sub-wavelength structures for CMB polarimetry}},
author = {Ryota Takaku and Hiroaki Imada and Guillaume Patanchon and Yuki Sakurai},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.38165},
eprint = {2610.38165},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.38165},
}
RIS
TY - UNPB
TI - Model of the instrumental polarization induced by a rotating HWP with anti-reflective sub-wavelength structures for CMB polarimetry
AU - Ryota Takaku
AU - Hiroaki Imada
AU - Guillaume Patanchon
AU - Yuki Sakurai
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2610.38165
UR - https://arxiv.org/abs/2610.38165
AB - Future CMB polarization experiments require precise control of instrumental systematics in the search for primordial B-mode polarization. A rotating half-wave plate (HWP) provides effective polarization modulation, but its non-ideal electromagnetic response can generate HWP-synchronous instrumental polarization. In this work, we characterize the response of a multi-layer sapphire HWP with anti-reflective sub-wavelength structures (SWS) using rigorous coupled-wave analysis. For three asymmetric SWS models, we find no substantial differences in the transmittance, polarization efficiency, or instrumental-polarization response that can be attributed to the type of SWS asymmetry. The fourth-harmonic instrumental polarization relevant to CMB polarization reconstruction is typically of order 10^−4 in the CMB bands, which is expected to be mitigated. We further evaluate an actually fabricated SWS geometry, whose asymmetry enhances the retardance, and compare its response with that of an idealized model having the same type of asymmetry. Comparison of the realistic SWS with the idealized model shows that higher transmittance tends to be associated with lower instrumental polarization. These results suggest that improving the total transmission, including the effect of the fabricated SWS profile, may be more important for reducing HWP-induced instrumental polarization than tuning the SWS asymmetry itself.
ER -