Time Delay of the Resistive-State Formation in Superconducting Stripes Excited by Single Optical Photons

186 LLE Review, Volume 92 Recently proposed superconducting single-photon detectors (SSPD’s), based on ultrathin, submicrometer-width NbN superconducting stripes, are characterized by picosecond response times, high quantum efficiency, broadband singlephoton sensitivity, and extremely low dark counts.1,2 The devices have immediately found a variety of applications ranging from noninvasive testing of very-large-scale integrated (VLSI) circuits3 to quantum cryptography.4,5 Their single-photon-counting ability has been interpreted within a phenomenological hot-electron photoresponse model proposed in Ref. 1 and elaborated in Ref. 6. The model describes the formation of a hotspot,7 right after the single-photon absorption event, followed by in-plane growth of a resistive hotspot area due to the high efficiency of the excited quasiparticle multiplication process in NbN films.8 During this stage, however, the resistive state does not appear across the superconducting stripe because the size of a single normal hotspot, created by an optical photon, is significantly smaller than our stripe width.2 The resistive state appears due to a supplementary action of the device’s bias current density j, which should be close to the stripe’s critical current density jc. After the supercurrent is expelled from the normal hotspot region, the bias current density in the stripe’s “sidewalks” jsw exceeds jc [see Fig. 92.32(c)], resulting in the penetration of the electric field in the sidewalk areas of the stripe.6 As a result, we observe a voltage pulse that reflects the initial act of photon capture.