INFLUENCE OF PHONON TRAPPING ON THE PERFORMANCE OF NBN KINETIC INDUCTANCE DETECTORS

Voltage and microwave photoresponse of NbN thin films to modulated and pulsed optical radiation reveals, far below the superconducting trans ition, a response time consistent with the lifetime of nonequilibrium quasiparticles. We show that even in 5 nm thick films at 4.2 K the pho non trapping is significant resulting in a quasiparticle lifetime of f ew nanosecond that is an order of magnitude larger than the recombinat ion time. Values and temperature dependence of the quasiparticle lifet ime obey the Bardeen-Cooper-Schrieffer theory and are in quantitative agreement with the electron-phonon relaxation rate determined from the resistive response near the superconducting transition. We discuss a positive effect of the phonon trapping on the performance of kinetic i nductance detectors.