A Silicon Membrane Optical Cavity for Long-Wave Infrared Superconducting Nanowire Single-Photon Detectors

Superconducting nanowire single-photon detectors (SNSPDs) have demonstrated saturated quantum efficiency in the long-wave infrared spectrum by reducing the thickness and the critical temperature of the film. However, the ultrathin film has finite light absorption, limiting the detection efficiency of the detector without an optical cavity. At long-wave infrared wavelengths, most oxide dielectrics exhibit significant optical absorption, and the required cavity length also increases, posing additional design and fabrication challenges. In this work, a silicon membrane-based optical cavity fabricated on a silicon-on-insulator (SOI) substrate is proposed to address these issues. Meanwhile, a free-space coupling system has been built for characterizing the device response spectrum. The superconducting nanowire, made from 5 nm thick WSi_x film with a reduced critical temperature of 2.5 K, exhibited saturated quantum efficiency from 7.8 to 12.5 μm. The detector's detection efficiency reached a resonance peak at 11.19 μm and a maximum value of 39.7%, which was in agreement with the design. Background counts were found mainly from the room-temperature blackbody radiation photons, which were reduced to 4 cps after shielding the detector in a vacuum. Correspondingly, the minimum noise equivalent power of 1.06 × 10^-19 W·Hz^-0.5 was obtained. These results demonstrate that the silicon membrane cavity not only preserves the superconductivity of the nanowire but also functions as an effective optical resonator. Therefore, it offers a promising platform for enhancing the sensitivity of SNSPDs in the long-wave infrared range.