Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a leading technology for single-photon detection owing to their high sensitivity, low dark-count rate, and excellent temporal resolution. Extending SNSPDs from single-pixel devices to scalable detector arrays is essential for spatially resolved single-photon detection and imaging applications. However, the development of large-scale mid-wave infrared (MWIR) SNSPD arrays remains challenging because of the reduced photon energy at longer wavelengths, the limited intrinsic detection efficiency of superconducting nanowires, and the increasing complexity of cryogenic multi-channel readout. In this work, we design and fabricate a 64-pixel MWIR SNSPD array based on an amorphous tungsten silicide (WSix) superconducting thin film. The array adopts a compact architecture that integrates two-wire superconducting nanowire avalanche photodetector (2-SNAP) units with superconducting delay-line-based spatiotemporal encoding, enabling the spatial localization and temporal identification of photon detection events with only two high-speed readout channels. The detector and delay-line structures are separately optimized, while a microstrip-based impedance-matching structure is introduced to improve microwave signal transmission and suppress signal reflection between the detector units and delay lines. The characteristic impedance and propagation characteristics of nanowires with different widths are considered to ensure reliable temporal encoding and signal readout.
At an operating temperature of 2.25 K, the fabricated array is experimentally characterized using pulsed optical excitation, and distinct temporal peaks corresponding to the 64 pixels are obtained. The average temporal separation between adjacent pixels is 47.2 ps, and the average full width at half maximum (FWHM) of the temporal peaks is 25.6 ps, demonstrating clear pixel-to-pixel temporal discrimination. The measured single-photon timing jitter is 162.5 ps, while the reconstructed photon-count distribution shows good pixel-to-pixel response uniformity. These results verify the feasibility of the proposed delay-line-based spatiotemporal encoding scheme for multi-pixel photon-event localization with a substantially reduced number of readout channels.
A free-space MWIR optical measurement system is further established to characterize the photon-response characteristics of the WSix SNSPD array over the wavelength range of 1.55-3.75 μm. The internal quantum efficiency (IQE) reaches saturation over 1.55-2.25 μm and is estimated to be 74%, 36%, and 21% at wavelengths of 3.00, 3.50, and 3.75 μm, respectively. The wavelength-dependent response indicates that WSi_x superconducting nanowires retain appreciable single-photon response well into the MWIR spectral range. In addition, the results demonstrate that the combination of superconducting nanowires with optimized optical and microwave structures provides a practical approach for extending SNSPD operation toward longer infrared wavelengths.
To further demonstrate the capability of the detector for weak thermal-radiation sensing, passive single-photon thermal imaging is performed at a wavelength of 3.75 μm using a 480 ℃ soldering iron as the thermal radiation source. The thermal radiation emitted by the target is collected and spatially scanned in free space, and the corresponding photon-count-rate distribution is recorded to reconstruct the target image. The resulting photon-count image resolves the geometric features of the target, demonstrating the feasibility of passive MWIR thermal imaging based on single-photon detection.
Overall, this work demonstrates a 64-pixel WSix SNSPD array that integrates multi-pixel spatiotemporal photon-event encoding, MWIR single-photon detection, and passive thermal imaging within a compact two-channel readout architecture. The combination of delay-line-based spatial encoding and long-wavelength single-photon sensitivity provides a promising device architecture for reducing the cryogenic readout complexity of scalable MWIR SNSPD arrays, and establishes a foundation for applications in single-photon imaging, weak thermal-radiation detection, and high-sensitivity infrared sensing.