Strong critical current density enhancement in NiCu/NbN superconducting nanostripes for optical detection

We present measurements of ferromagnet/superconductor _NiCu/NbN_ and plain superconducting _NbN_ nanostripes with the linewidth ranging from 150 to 300 nm. The NiCu _3 nm_/NbN _8 nm_bilayers, as compared to NbN _8 nm_, showed a up to six times increase in their critical current density, reaching at 4.2 K the values of 5.5 MAcm2 for a 150 nm wide nanostripe meander and 12.1 MAcm2 for a 300 nm one. We also observed six-time sensitivity enhancement when the 150 nm wide NiCu/NbN nanostripe was used as an optical detector. The strong critical current enhancement is explained by the vortex pinning strength and density increase in NiCu/NbN bilayers and confirmed by approximately tenfold increase in the vortex polarizability factor