Electrothermal feedback in superconducting nanowire single-photon detectors

Abstract

We investigate the role of electrothermal feedback in the operation of superconducting nanowire singlephoton detectors ͑SNSPDs͒. It is found that the desired mode of operation for SNSPDs is only achieved if this feedback is unstable, which happens naturally through the slow electrical response associated with their relatively large kinetic inductance. If this response is sped up in an effort to increase the device count rate, the electrothermal feedback becomes stable and results in an effect known as latching, where the device is locked in a resistive state and can no longer detect photons. We present a set of experiments which elucidate this effect and a simple model which quantitatively explains the results. DOI: 10.1103/PhysRevB.79.100509 PACS number͑s͒: 85.25.Oj, 74.78.Ϫw, 85.60.Gz Superconducting nanowire single-photon detectors ͑SNSPDs͒ combine high speed, high detection efficiency ͑DE͒ over a wide range of wavelengths, and low dark counts. [1][2][3][4] Of particular importance is their high singlephoton timing resolution of ϳ30 ps, 4 which permits extremely high data rates in photon-counting communications applications. 5,6 Full use of this electrical bandwidth is limited, however, by the fact that the maximum count rates of these devices are much smaller ͑a few hundred MHz for 10 m 2 active area and decreasing as the area is increased 2 ͒, limited by their large kinetic inductance and the input impedance of the readout circuit. 2,7 To increase the count rate, therefore, one must either reduce the kinetic inductance ͑by using a smaller active area or different materials or substrates͒ or increase the load impedance. 7 However, either of these approaches causes the wire to "latch" into a stable resistive state where it no longer detects photons. 8 This effect arises when negative electrothermal feedback, which in normal operation allows the device to reset itself, is made fast enough that it becomes stable. We present experiments which probe the stability of this feedback, and we develop a model which quantitatively explains our observations. The operation of an SNSPD is illustrated in Fig. 1͑a͒. A nanowire ͑typically ϳ100 nm wide and 5nm thick͒ is biased with a dc current I 0 near its critical current I c . The nanowire has kinetic inductance L and is read out using a load impedance R L ͑typically a 50⍀ transmission line͒. When a photon is absorbed, a short ͑Ͻ100 nm long͒ normal domain is nucleated, giving the wire a resistance R n ͑t͒. This results in Joule heating which causes the normal domain ͑and consequently, R n ͒ to expand in time exponentially. The expansion is counteracted by negative electrothermal feedback from the load R L , which forms a current divider with R n , and diverts a current I L into the load ͑so that the current in the nanowire is reduced to I d ϵ I 0 − I L ͒, reducing the heating. However, in a correctly functioning device, this feedback is unstable: the inductive time constant is long enough so that before I L becomes appreciable, Joule heating has already increased R n , ...

Rise time of voltage pulses in NbN superconducting single photon detectors

“…Because in our model the resistance of the initial domain depends on its initial temperature Tspot (via temperature dependent resistivity (Te,Id)), it leads to a dependence of rise on Tspot, but only for small lengths; at large lengths, the size of the resistive domain is determined mainly by Joule dissipation and rise weakly depends on Tspot. Note that for the chosen parameters and chosen model for (Te,Id), we did not find in theory 'latching' [16] of the detector at any L. Experimentally, 'latching' was also not observed even for the detector with L=20 m.…”

Numerical analysis of detection-mechanism models of superconducting nanowire single-photon detector

“…As a consequence, part of the bias current is redirected into the readout line, which effectively acts as a 50 X parallel impedance, or an additionally installed parallel ohmic resistance, thus reducing the Joule-heating. Depending on the details of this electro-thermal feedback, 13 the detector can be operated in the desired self-recovering mode or it latches into a resistive state, when the normal-conducting domain is stabilized by self-heating. This electro-thermal feedback imposes limits on the minimum achievable recovery time and the maximum count rate.…”

Current distribution in a parallel configuration superconducting strip-line detector

“…4 In these large area SSLDs, a relatively low ratio of bias current to critical current (I B /I C < 55%) is required to prevent device latching. 8 This bias current value is less than the threshold current required to induce the cascade switching of multiple strip-lines; 6 the device operates in the "single-strip switch regime." [9][10][11] In this regime, only the strip-line which has been struck switches partially into the normal state; the other parallel strips in the block remain completely superconducting.…”