We first investigate the microscopic working principle of the nanowire superconducting single-photon detectors (SSPDs), and we find that the edge of the nanowire is much more sensitive than the... Show moreWe first investigate the microscopic working principle of the nanowire superconducting single-photon detectors (SSPDs), and we find that the edge of the nanowire is much more sensitive than the central part. The experimental results agree quantitatively with the theory based on a photon-assisted vertex- entry model. We then explore its potential to be used as a probe for near-field detection. Our calculations show that the SSPDs have a much higher signal collection efficiency than the conventional near-field optical microscopy, and that their spacial resolution can reach 20 nm. With photon number resolving ability, a SSPD fabricated on a micro-pillar can be used to investigate the quantum information of the optical near-field at subwavelength scales. Show less