Superconducting nanowire single photon detectors (SNSPDs) are fast becoming the detectors of choice for photon counting applications due to their increasingly remarkable characteristics. Recent efforts to achieve >90% detection efficiencies have focussed on using optical cavity stacks to increase absorption probability, and the use of amorphous materials to increase the probability of an absorbed photon leading to an output pulse. However, cavities have narrow optimised bandwidth and amorphous materials typically require sub 1 K temperatures. In this paper, the authors demonstrate that an alternative technique - layering two SNSPDs on top of one another with only a thin insulator layer between them - can be realised in NbN with high production yield. In this design, a photon absorption in one wire leads to a hotspot in the other through transmission of athermal phonons through the insulator, combining both the high absorption efficiency of thicker wires with the high triggering efficiency of thin wires. Not only are system detection efficiencies up to 98% (at 1590 nm wavelength and 0.8 K) demonstrated, but also a high fabrication yield, with an impressive 16 (33) out of 45 devices from a single batch achieving efficiencies >90% (80%) at 1550 nm wavelength and at a favourable temperature of 2.1 K.
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