Abstract: Thin-film superconducting microwave resonators have applications as ultrasensitive detectors, mixers, amplifiers, and frequency multiplexers in high-energy physics, astronomy, and quantum information processing. Though simple in principle and straightforward to fabricate, there is a wealth of interesting underlying physics in their behavior.

We have developed the theory and experimental evidence behind the surprising ability of sub-gap frequency microwave readout power to create large numbers of excess quasi-particles, leading to increased noise, electrothermal feedback, and a nonlinear device response.

Analytical and numerical models of how absorbed power drives a superconductor into an athermal state are based on quantitative calculations of the nonequilibrium quasi-particle energy distribution interacting with photons and high-energy phonons.

The models suggest how aspects of device design, such as the acoustic match with the substrate and the thermal isolation of the resonator, affect performance and how the resonator operating point and readout strategy may be chosen to maximize responsivity.

New results on the frequency dependence of the detector response (particularly in the sub-mm/THz band) are also obtained, via the quasi-particle generation efficiency.

This work has consequences for a number of superconducting quantum devices, most significantly kinetic inductance detectors.