Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

Abstract

We report an experimental test of the photodetection mechanism in a nanowire superconducting single photon detector. Detector tomography allows us to explore the 0.8-8 eV energy range via multiphoton excitations. High accuracy results enable a detailed comparison of the experimental data with theories for the mechanism of photon detection. We show that the temperature dependence of the efficiency of the superconducting single photon detector is determined not by the critical current but by the current associated with vortex unbinding. We find that both quasiparticle diffusion and vortices play a role in the detection event. DOI: 10.1103/PhysRevLett.112.117604 PACS numbers: 79.20.Ws, 03.65.Wj, 85.25.Oj Nanowire superconducting single photon detectors (SSPDs or SNSPDs) [1,2] are currently the most promising detection systems in the infrared, achieving detection efficiencies of up to 93% at 1550 nm [3]. Despite these technological advances, the fundamentals of the working principle of these detectors are poorly understood and under active investigation, both theoretically [4][5][6][7][8][9][10][11] and experimentally [12][13][14][15][16][17][18][19][20][21][22].A typical SSPD consists of a few nm thin film of a superconducting material such as NbN or WSi, nanofabricated into a meandering wire geometry. When biased sufficiently close to the critical current of the superconductor, the energy of one or several photons can be enough to trigger a local transition to the resistive state, resulting in a detection event. The energy of the absorbed photon is distributed through an avalanchelike process, creating a nonequlibrium population of quasiparticles. This quasiparticle population then disrupts the supercurrent flow, resulting eventually in a detection event.In this Letter, we address the nature of this disruption, which lies at the heart of the photodetection mechanism in SSPDs. At present, there are three important open questions. First, it is unknown whether the detection event occurs when the energy of the incident photon causes a cylindrical volume inside the wire to transition to the normal state [see Fig. 1(a)] [1], or whether it is enough for the superconductivity to be weakened but not destroyed by the depletion of Cooper pairs over a more extended region [see Fig. 1The second open question is whether magnetic vortices play any role in the detection mechanism. There are two varieties of vortex-based models. The first is an extension of the normal-core model, where, a vortex-antivortex pair forms at the point where the photon is absorbed [ Fig. 1(c)] [5]. In the second, the weakening of superconductivity lowers the energy barrier for either a vortex crossing [6,23] or a vortex-antivortex pair crossing [ Fig. 1(d)].The last open question pertains to the temperature dependence of the photoresponse of SSPDs. Intuitively, one would expect the SSPD to be less efficient at lower temperatures, as the detector works by breaking superconductivity and the energy gap of a superconductor decreases with increasing temp...

Optically probing the detection mechanism in a molybdenum silicide superconducting nanowire single-photon detector

“…For 220 nm-wide NbN SNSPDs made from nanobridges, and also with nanodetectors and meanders, the energy-current relation was found to be linear in the range of 0.75-8.26 eV using the quantum detector tomography (QDT). 24 A result consistent with this was found for TaN detectors 37 and for a series of NbN meanders of varying widths. 38 Nevertheless, a nonlinear behaviour for NbN meanders probed with a filtered black body light source was later observed in the 0.5-2.75 eV range 28 by using the two probability thresholds g ¼ 50% and 90% of the normalised PCR.…”

“…For NbN, the energy-current relation was found to be linear 24 over a large range of energies using quantum detector tomography 25 (QDT), which is evident for the role of a diffuse cloud of quasiparticles in the detection process. 26 Moreover, the position-dependent measurements 27 and external magnetic field-based study 28 highlight the role of vortices in the detection mechanism.…”

Experimental investigation of the detection mechanism in WSi nanowire superconducting single photon detectors

“…The gradual decrease in efficiency at low bias currents could be due to the finite probability of overlap between the excitations along the length of the detector [40,43]. The small jumps in efficiency which occur at I b = 4µA and I b = 5.5µA are related to the different model (i.e.…”

“…For NbN SSPDs, we recently demonstrated [18,19] that the detection event is due to a vortex crossing induced by a cloud of quasiparticles which reduces the barrier potential for vortex entry. The energy dissipated by the vortex crossing the nanowire leads to a transition to the normal state [20][21][22].…”

“…This functional dependence is a key signature of the detection mechanism. By the use of multiphoton excitations, the current-energy relation can be measured with sufficient accuracy over a large range of energies to exclude certain classes of models [18].…”