Pulse Response of a Kinetic Inductance Detector in the Nonlinear Regime

Over the last few years, kinetic inductance detectors (KIDs) became the object of increasing interest as photon and phonon detectors. From this perspective, the pulse response of such detectors deserves an in-depth study. In most applications, the sensitivity of the KID is ultimately limited by the white noise from the cryogenic amplifier, which is reduced by increasing the power supplied to the device. On the other hand, a high readout power leads to a nonlinear response of the microresonator, originating from the dependence on the current acquired by the kinetic inductance. This paper describes a model for the response to optical pulses of a KID driven to the nonlinear regime, taking into account not only the electrical effects but also the thermal ones induced by power absorption. The model has been validated on data collected using an aluminium resonator developed within the CALDER project (http://www.roma1.infn.it/exp/calder/).