Superconducting fast detector based on the nonequilibrium inductance response of a film of niobium nitride

Abstract

A new type of fast detector is proposed, whose operation is based on the variation of the kinetic inductance of a superconducting film caused by nonequilibrium quasiparticles created by the electromagnetic radiation. The speed of the detector is determined by the rate of multiplication of photo-excited quasiparticles, and is nearly independent of the temperature, being less than 1 ps for NbN. Models based on the Owen-Scalapino scheme give a good description of the experimentally determined dependence of the power-voltage sensitivity of the detector on the modulation frequency. The lifetime of the quasiparticles is determined, and it is shown that the reabsorption of nonequilibrium phonons by the condensate has a substantial effect even in ultrathin NbN films 5 nm thick, and results in the maximum possible quantum yield. A low concentration of equilibrium quasiparticles and a high quantum yield result in a detectivity D*=10¹² W⁻¹·Hz^1/2 at a temperature T=4.2 K and D*=10¹⁶ W⁻¹·cm· Hz^1/2 at T=1.6 K.