Polarization dependence of single-photon sensitivity in superconducting nanowires at extended infrared wavelengths

Polarization dependent performance of superconducting nanowire single-photon detectors (SNSPDs) remains one of the obstacles to free space applications of SNSPDs. Here, we study the wavelength dependence of polarization anisotropy in two most widely used SNSPD geometries: meander and fractal nanowires based on niobium nitride thin films. In particular, we observe that polarization sensitivity becomes more pronounced at longer wavelengths, accompanied by an oscillatory behavior of photon count rate (PCR) on polarization angle. Furthermore, we report a wavelength-dependent shift in the polarization angle corresponding to maximum and minimum PCR and also find that experimental data consistently exhibit higher polarization anisotropy than predicted by simulations—likely due to fabrication-induced substrate effects. We deduce that the polarization anisotropy is a direct result of significant difference between the refractive indices of substrate and nanowire to that of the air, which surrounds them. We also outline potential directions for future work in improving our understanding of polarization anisotropy in SNSPDs. Our results provide new insight into the geometric factors governing polarization sensitivity in SNSPDs and underscore the potential of fractal nanowires in enabling broadband, polarization-independent single photon detection.