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

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 deg. We find the measurements at room temperature are in good agreement with the predictions. This is the most broadband demonstration of an AHWP at millimeter wave.