Electromagnetic Properties of Aluminum-Based Bilayers for Kinetic Inductance Detectors

The complex conductivity of a superconducting thin film is related to the quasiparticle density, which depends on physical temperature and can also be modified by external pair breaking with photons phonons. This relationship forms underlying operating principle Kinetic Inductance Detectors (KIDs), where detection threshold governed energy gap. We investigate electromagnetic properties thin-film aluminum that proximitized either normal metal layer copper or lower $T_C$, such as iridium, in order extend range KIDs. Using Usadel equations along Nam expressions for conductivity, we calculate density states resulting bilayers understand dependence threshold, surface impedance, intrinsic quality factor relative thicknesses. calculations analyses provide theoretical insights designing aluminum-based bilayer kinetic inductance detectors microwave athermal phonons at frequencies well below pure film.