Interplay Between Kinetic Inductance, Nonlinearity, and Quasiparticle Dynamics in Granular Aluminum Microwave Kinetic Inductance Detectors

Microwave kinetic inductance detectors (MKIDs) are central equipment in radioastronomy, thanks to their ease of fabrication and potential for multiplexing, which allow for millimeter-wavelength cameras with thousands of pixels. However, increasing their kinetic inductance to obtain maximum responsivity degrades their performance, due to the onset of nonlinearities. This work determines the optimum working point of MKIDs by taking into account the interplay between these two competing factors. It is also found that a lower-gapped ground plane can act as a phonon trap, providing lower noise-equivalent power in MKIDs limited by quasiparticle noise.