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 Ni(0.48)Cu(0.52) 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 similar to 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.