Nonbolometric infrared detection in thin superconducting films via photoproduction of fluxon pairs

Conventional models have been unable to account for observations of enhanced infrared photodetection in granular superconducting films. The authors propose a mechanism for quantum-limited photodetection in thin films of either conventional or high-temperature superconductors. A photon with hf≫2Δ is absorbed at a spot in the film, creating a pair of highly excited quasi-particles which very rapidly break additional Cooper pairs and distribute the excess energy among a large number of quasi-particles. This mechanism is essentially a photon-assisted phase slip, in which the photon supplies the additional energy needed to permit the current to nucleate the vortex pair. Assuming unity quantum efficiency for this process, the time-average voltage responsivity is Φ₀/hf=1/(2ef), which yields 10 ⁴ V/W for 1-eV photons. This picture of photofluxonic detection in a superconductor is directly analogous to photoconductive detection in an intrinsic semiconductor via photoproduction of electron-hole pairs. Experimental evidence of nonbolometric photodetection in an NbN thin film is presented and critically examined in light of this mechanism. The application of devices based on this principle for fast, sensitive infrared detectors is discussed