Superconducting single photon detectors for ultrasensitive visible and IR detection

Superconducting single photons detectors (SSPDs) have emerged in recent years as a promising alternative for fast and sensitive infrared detectors working in the photon counting mode. In particular, those detectors combine very low dark count rates (below 1 Hz), high speed (above 1 GHz), photon number resolution and reasonable quantum efficiency (10% at telecom wavelengths). They already found applications in quantum cryptography systems and integrated circuit failure analysis, but could also be used as ultimate sensors in matrix configurations. We show here the optimization of SSPD fabrication and their optical metrology at CEA. SSPD are fabricated by patterning a 80 nm wide nanowire in a very thin (4 nm) NbN film on sapphire, forming a pixel of several microns size. A cryogenic all-fibered optoelectronic system has been developed and allows precise metrology of the optical performances of SSPD. When biasing near the critical current of the nanowire, we demonstrate a detection quantum efficiency of 8% at 1.55 µm, which is also found to be strongly polarization dependent. This quantum efficiency being limited by optical absorption, we propose a prism coupling based absorption enhancing structure that allows reaching 100% quantum efficiency.