Design of NbN superconducting nanowire single photon detectors with enhanced infrared photon detection efficiency*

We optimize the design of nanowire superconducting single photon detectors (SNSPD) by accounting for a position dependence in the internal detection efficiency (IDE): wide wires biased at relatively lower currents (~80% of the depairing current) have higher detection efficiency for photons absorbed at the edges compared to the central part. This can be explained by a higher optical absorption for photons at the edges combined with a lower barrier for vortex entry and vortex crossing is the dominant effect in a detection event.

In order to understand the role of the position-dependent IDE we calculate the average absorption efficiency as well as the IDE for a set of SNSPD geometries. At a wavelength of 1550nm, the IDE can be enhanced significantly by placing a dielectric nanowire on top of the superconducting nanowire. We demonstrate a coarsely optimized structure using a 50 nm wide and 30 nm thick silicon wire on top of a 150 nm wide nanowire. For the optimal design both the absorption efficiency and the IDE are increased by 70% and 32%, respectively.