Development of antenna-coupled superconducting nanowire single photon detectors (SNSPDs) for long wavelength detection

Lowering the energy threshold of single photon detectors enables sensitivity to longer mid-to far-infrared wavelengths, offering performance benefits for dark matter detection, molecular spectroscopy, and quantum communications. Superconducting nanowire single-photon detectors (SNSPDs) have been developed in recent years as a promising technology for time-correlated single-photon counting for the infrared wavelength range beyond 10 μm due to their low dark-count rates, zero readout noise and compatibility with different optical schemes.  Conventionally, SNSPDs are embedded in optical cavities made of transparent dielectrics to improve absorption in the thin nanowire layer. However, at longer wavelengths, it becomes increasingly difficult to find low loss high- and low-index dielectrics. In this work, we use an alternative approach of antenna-assisted free space coupling of SNSPDs optimized for 25 μm wavelength operation and fabricated from high-resistivity superconducting thin films of Tungsten Silicide (WSi). Antenna coupling increases the free-space absorption while minimizing the active nanowire area. Additionally, we integrate a superconducting nanowire avalanche photodetector (SNAP) architecture and impedance matched tapers for higher signal-to noise performance and better impedance matching with the antennas while maintaining sensitivity at these longer wavelengths. This design achieves competitive noise-equivalent power and high detection efficiency, demonstrating a new class of photodetector for the far-infrared applications.