RF characterization of a large-diameter embedded reflective half-wave plate

Many of the current and future polarization experiments require the quasi-optical modulation of linear polarization at millimeter and submillimeter wavelengths. The need for large-aperture telescopes (diameters larger than 500 mm) combined with broad frequency coverage has made half-wave plate technologies based on birefringent crystals often not compliant with other mission requirements, such as weight allocation. The embedded reflective half-wave plate (ER-HWP) is a new device developed to achieve the required phase-shifts across a wide range of frequencies, in particular a multioctave bandwidth of approximately 150%(Pisano et al., 2016). A device with a diameter of 650, mm and a thickness of 750 μm has been successfully manufactured at Cardiff University. In this work, we present the room-temperature experimental characterization of this ER-HWP. A coherent test system, capable of transmitting and receiving S- and P- polarizations, was employed to measure the differential phase-shift, the axes' reflection coefficients, the modulation efficiency, and the beam pattern distortion introduced by the device. We measured a modulation efficiency and axes' reflection coefficient greater than 90% within the band of interest, together with an acceptable level of the beam pattern deformation. A subset of these measurements is planned to be repeated at approximately 10, K using a large-diameter cryogenic testbed, currently under development at Sapienza University, capable of testing devices with an effective diameter of up to 1,m mounted at the coldest stage.