Modeling and Testing Kinetic Inductance Detectors for Astrophysics (KIDs)

Kinetic inductance detectors (KIDs) are used in a growing number of applications for sensing electromagnetic radiation, from mm-wavelengths to X-rays. They are relatively easy to fabricate and to read out. Background-limited performance in arrays of thousands of devices has been demonstrated from the ground, even at very low power levels, as in high-resolution spectroscopy. KIDs are made from superconducting thin films in which absorbed radiation splits Cooper pairs into quasiparticles, a process that changes the film's inductance. The films are patterned into microresonators whose resonance properties respond to the inductance change and can be monitored by measuring the complex transmission of a microwave signal in an adjacent, coupled readout line. Multiple sensors can be formed with distinct resonant frequencies and read out simultaneously using a frequency comb of microwave probe tones. Models for KIDs have been developed that agree with many measurements. Nevertheless, these models have significant limitations and are difficult to use for optimizing designs for applications. In particular, the heating of quasiparticles by the readout signal and the diffusion of quasiparticles in the microresonator are two important pieces of physics that are not yet well understood and strongly affect device performance. We propose to develop a model for KIDs that includes these effects and test the model over a targeted parameter space that includes a range of detector volumes, optical power, and readout power. A particularly important goal of the modeling is to understand and demonstrate the large dynamic range expected from MKIDs when using tone tracking and power tuning in the readout system. Test devices will be made from superconducting films, TiN and Al, at the University of Wisconsin, and cryogenic tests will occur there and at NASA/Goddard. Our investigation will advance understanding of the fundamental operational aspects of KIDs. The basic detector physics results from our tested model will be made available in published software that can be used by other groups as they design KIDs for specific applications. This code will allow instrument designers to anticipate KID performance in the focal planes of future missions from mm to visible wavelengths.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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