Low gap superconducting single photon detectors for infrared sensitivity

The quantum efficiency of NbN and NbTiN superconducting single photon detectors drops with decreasing photon energy. A lower gap material would enable single photon detection deeper in the infrared. We have fabricated a NbSi detector and compare its characteristics with a NbTiN device. NbSi ( T C ≃ 2 K ) has a smaller superconducting gap than NbTiN or NbN ( T C ≃ 15 K ) . We measure the detection efficiency for a wavelength range from 1100 to 1900 nm. In this range the NbSi detector shows a 10-fold increase in relative efficiency with respect to the NbTiN detector.