Design and performance of hafnium optical and near-IR kinetic inductance detectors

We report on the design and performance of Microwave Kinetic Inductance Detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 $\mu \Omega$ cm with an RRR = 1.6. Resonators on the device displayed internal quality factors of around 200,000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process which resulted in pixels with decay times of about 40 $\mu$s and resolving powers of ~9 at 800 nm.