Demonstration of Ultrasensitive KIDs for Future THz Space Borne Polarimeters

We present measurements and simulations of the polarization purity of leaky lens-antenna coupled microwave kinetic inductance detectors (KIDs) at 1.5 THz. From polarized phase and amplitude beam pattern measurements we find the integrated cross-polarization ratio to be at –21.5 dB for 1 f#$\lambda$ spatial sampling. The measurements agree well with the theoretical description which is based on a combination of in-transmission simulation of the antenna feed, and an in-reception analysis of the antenna-KID system. A neutral density filter limited the power per detector to around 500 fW, enabling these measurements to be taken on detectors that in a low background have a measured noise equivalent power of 5–7$\times 10^{-20}$ W$/\sqrt{\text{Hz}}$. These combined measurements show that these detectors are excellent candidates for large scale and high-performance imaging polarimetric instruments.