A study on the performance of half-wave plates designed for sub-mm studies

A number of millimetre (mm) and sub-millimetre (sub-mm) instruments use half wave plates (HWP) as polarisation modulators, but a thorough account of the non-idealities arising in the HWP is crucial for the analysis of systematic errors. For Cosmic Microwave Background (CMB) experiments, studies propagating the measured spectral performance of a HWP through to scientific results, such as the tensor-to-scalar ratio, r, have not been conducted. Presented in this work is the measured spectral performance of an embedded metal mesh HWP designed for the NIKA2 experiment. The results were propagated through to calculate the bias on r introduced from it's non-idealities and a comparison between metal mesh and Pancharatnam type HWPs is conducted.