# Demonstration of the broadband half-wave plate using the nine-layer sapphire for the CMB polarization experiment

## TL;DR

This paper demonstrates a nine-layer sapphire achromatic half-wave plate capable of covering a broad frequency range for CMB polarization experiments, with measurements aligning well with predictions.

## Contribution

The development and experimental validation of the broadest broadband achromatic half-wave plate using nine sapphire layers for millimeter-wave applications.

## Key findings

- Measured modulation efficiency across 33-260 GHz
- Good agreement between measurements and theoretical predictions
- First demonstration of such a broad AHWP at millimeter wavelengths

## Abstract

We report the development of the achromatic half-wave plate (AHWP) at millimeter wave for cosmic microwave background polarization experiments. We fabricate an AHWP consisting of nine a-cut sapphire plates based on the Pancharatnam recipe to cover a wide frequency range. The modulation efficiency and the phase are measured in a frequency range of 33 to 260 GHz with incident angles up to 10 degrees. We find the measurements at room temperature are in good agreement with the predictions. This is the broadest demonstration of the AHWP at the millimeter wave.