Characterization of MKIDs for CMB observation at 220 GHz with the South Pole telescope

We present an updated design of the 220 GHz microwave kinetic inductance detector (MKID) pixel for SPT-3G+, the next-generation camera for the South Pole Telescope. We show results of the dark testing of a 63-pixel array with mean inductor quality factor Qi=4.8×105 , aluminum inductor transition temperature Tc=1.19 K, and kinetic inductance fraction αk=0.32 . We optically characterize both the microstrip-coupled and CPW-coupled resonators, and find both have a spectral response close to prediction with an optical efficiency of η∼70% . However, we find slightly lower optical response on the lower edge of the band than predicted, with neighboring dark detectors showing more response in this region, though at level consistent with less than 5% frequency shift relative to the optical detectors. The detectors show polarized response consistent with expectations, with a cross-polar response of ∼10% for both detector orientations