Broadband multi-layer anti-reflection coatings with mullite and duroid for half-wave plates and alumina filters for CMB polarimetry

TL;DR

This paper presents a broadband two-layer anti-reflection coating using mullite and Duroid for sapphire HWPs and alumina filters, significantly reducing reflection and enhancing CMB polarization measurements.

Contribution

The development and successful implementation of a broadband two-layer AR coating with specific cryogenic materials for improved CMB polarimetry instrumentation.

Findings

Reflectivity reduced to about 2% at 90/150 GHz

Reflectivity below 1% at 220/280 GHz

Coatings used in the Simons Array experiment

Abstract

A broadband two-layer anti-reflection (AR) coating was developed for use on a sapphire half-wave plate (HWP) and an alumina infrared (IR) filter for cosmic microwave background (CMB) polarimetry. Measuring tiny CMB B-mode signals requires maximizing the number of photons reaching the detectors and minimizing spurious polarization due to reflection with an off-axis incident angle. However, a sapphire HWP and an alumina IR filter have high refractive indices of about 3.1, and an AR coating must be applied to them. Thermally sprayed mullite and Duroid 5880LZ were selected in terms of index and coefficient of thermal expansion for use at cryogenic temperatures. With these materials, the reflectivity was reduced to about 2% at 90/150 GHz and <1% at 220/280 GHz. The design, fabrication, and optical performance evaluation of the AR coatings are described. The coatings were used in a current ground-based CMB experiment called the Simons Array. They could also be applied to next-generation CMB experiments, such as the Simons Observatory.