A Review of Halfwave Plate Technology for CMB Observations circa 2008

The birefringent halfwave plate (HWP) polarization modulator, while new to the CMB field, has been used for decades at shorter wavelengths. The basic idea is simple; a polarized plane wave passing through a HWP will have its polarization orientation rotated by ∆ψ = 2θ, where θ is the angle between the incident polarization and one of the optical crystal axes. Rotating a HWP in front of a linearly polarized detector rotates the polarization sensitivity of that detector on the sky, nominally without affecting its beam pattern. This virtue, rotating polarization sensitivity without rotating a potentially asymmetric beam, is the main reason many upcoming CMB polarimeters (including ACT, Alpaca, Bicep2/Spud, Clover, EBEX, Polarbear, and Spider) will use birefringent sapphire HWP polarization modulators.