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 NiCuNb 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 NiCuNb bilayer microbridge. The sensitive picosecond photoresponse makes our NiCuNb 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 effect