Ultrafast photoresponse of superconductor/ferromagnet Nb/NiCu heterostructures

We report on femtosecond optical pump-probe studies of proximized ferromagnet/superconductor (F/S) hybrids, consisting of Ni0.5Cu0.5 layers deposited on top of Nb films. The weak ferromagnetic nature of the completely proximised Ni0.5Cu0.5 film makes possible to observe the dynamics of the nonequilibrium superconductivity through the near-surface optical reflectivity change measurements. The time-resolved photoresponse transient of the NiCu(21 nm)/Nb bilayer in the superconducting state shows strongly suppressed slow bolometric component. The fast relaxation time is also discussed accordingly to current theories on S/F heterostructures. The proposed S/F nanobilayers represent a new, artificially designed superconductor with the features (sub-picosecond photoresponse with suppressed bolometric component) very desirable for superconducting photodetectors and photon counters. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)