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