Saturated single-photon detection up to 5 μm in the mid-infrared using NbN superconducting microstrip detectors

We demonstrate a mid-infrared superconducting microstrip single-photon detector (SMSPD) based on a 5 nm thick, 0.92-μm wide, and 50 μm long NbN microbridge. The detection sensitivity is enhanced via He + ion irradiation with a high fluence of 1×10 17 ions/cm 2 . The SMSPD exhibits saturated internal detection efficiency over a broad spectral range from 400 nm to 5 μm at 0.32 K, with a minimum timing jitter of 28 ps (at 1064 nm). The detection current scales with photon energy as α ≈ –0.5, indicating vortex-assisted hotspot formation as the underlying detection mechanism.