Layered ferromagnet/superconductor heterostructures: Nonequilibrium quasiparticle dynamics and photodetector applications

Time-resolved optical pump-probe and photoimpedance studies of proximized ferromagnet/superconductor nanobilayers are presented. The weak ferromagnetic nature of an ultrathin Ni0.48Cu0.52 film makes it possible to observe the dynamics of the nonequilibrium superconductivity in NiCuNb hybrids through time-resolved measurements of a near-surface optical reflectivity change, which is generated by femtosecond optical pump pulses and discussed within a nonequilibrium two-temperature electron-heating model. We observed that the NiCu overlay significantly reduced the slow bolometric contribution present in the photoresponse of a pure Nb film, resulting in a strong enhancement of the nonequilibrium kinetic-inductive component of the transient photoimpedance, measured as an _700-ps-wide voltage waveform generated across an optically excited current-biased NiCuNb bilayer microbridge. The sensitive picosecond photoresponse makes our NiCuNb heterostructures suitable for “engineered” ultrafast superconducting photodetectors since the photoimpedance signals observed in plain Nb bridges were at least 10 ns long and were due to the light-induced simple-heating effec