Decay Times of Optical Pulses for Aluminum CPW KIDs

The recombination rate of quasiparticle excitations and metal film thickness are both important factors in determining the sensitivity of kinetic inductance detectors (KIDs). To maximize KID sensitivity, we aim to quantify the interdependence of these two detector attributes. We have measured the decay times of optical pulses produced by illuminating aluminum CPW resonators with an infrared LED. Measurements were made using both 1/4-wavelength and 1/2-wavelength resonators for film thicknesses between 20 and 50 nm for a range of temperatures and microwave readout powers. We observed several millisecond decay times for all thicknesses, with an elevated decay time ( ∼ 5 ms) and critical temperature ( ∼ 1.5 K) for the 20-nm-thick film.