We probe the local detection efficiency in a nanowire superconducting single-photon detector along the cross-section of the wire with a spatial resolution of 10 nm. We experimentally find a strong variation in the local detection efficiency of the device. We demonstrate that this effect explains previously observed variations in NbN detector efficiency as function of device geometry.Nanowire superconducting single-photon detectors (SSPDs) consist of a superconducting wire of nanoscale cross-section [1], typically 4 nm by 100 nm. Photon detection occurs when a single quantum of light is absorbed and triggers a transition from the superconducting to the normal state. SSPDs have high efficiency, low jitter, low dark count rate and fast reset time [2], and are therefore a key technology for, among others, quantum key distribution Although progress has been made recently, the underlying physical mechanism responsible for photon detection on the nanoscale is still under active investigation. A combination of theory [6,7], experiments [8][9][10] and simulations [11,12] on NbN SSPDs indicates that the absorption of a photon destroys Cooper pairs in the superconductor and creates a localized cloud of quasiparticles that diverts current across the wire. This makes the wire susceptible to the entry of a superconducting vortex from the edge of the wire. Energy dissipation by this moving vortex drives the system to the normal state.An important and unexpected implication of this detection model is a nanoscale position variation in the photodetection properties of the device. The conditions at the entry point of the vortex determine the energy required for it to cross the wire. This causes photons absorbed close to the edge to have a local detection efficiency (LDE) [13] compared to photons absorbed in the center of the wire [12]. This effect has practical implications for the operation of SSPDs, since it represents a potential limitation of the detection efficiency. In addition, SSPDs have been proposed for nanoscale sensing, either in a near-field optical microscope configuration [14] or as a subwavelength multiphoton probe [15], where this effect would be of major importance for the properties of such a microscope. While this effect has been predicted theoretically, clear experimental evidence is missing. * renema@physics.leidenuniv.nlIn this work, we experimentally explore the nanoscale variations in the intrinsic response of the detector. We spatially resolve the LDE with a resolution of approximately 10 nm, better than λ/50, using far-field illumination only. We find that our results are qualitatively consistent with numerical simulations [11,12]. Our results provide excellent quantitative agreement with experiments that indicate a polarization dependence in the LDE that was hitherto not understood [16].The key technique used in this work is a differential polarization measurement that probes the IDE of the detector (see Figure 1). The technique is based on the fact that polarized light is absorbed preferentially in dif...

Superconducting properties of three series of amorphous W x Si 1x films with different thickness and stoichiometry were investigated by dc transport measurements in a magnetic field up to 9 T. These amorphous W x Si 1x films were deposited by magnetron cosputtering of the elemental source targets onto silicon substrates at room temperature and patterned in the form of bridges by optical lithography and reactive ion etching. Analysis of the data on magnetoconductivity allowed us to extract the critical temperatures, superconducting coherence lengths, magnetic penetration depths, and diffusion constants of electrons in the normal state as functions of film thickness for each stoichiometry. Two basic time constants were derived from transport and time-resolving measurements. A dynamic process of the formation of a hotspot was analyzed in the framework of a diffusion-based vortex-entry model. We used a two-stage diffusion approach and defined a hotspot size by assuming that the quasiparticles and normal-state electrons have the same diffusion constant. With this definition and these measured material parameters, the hotspot in the 5-nm-thick W 0.85 Si 0.15 film had a diameter of 107 nm at the peak of the number of nonequilibrium quasiparticles. Superconducting properties of three series of amorphous W x Si 1−x films with different thickness and stoichiometry were investigated by dc transport measurements in a magnetic field up to 9 T. These amorphous W x Si 1−x films were deposited by magnetron cosputtering of the elemental source targets onto silicon substrates at room temperature and patterned in the form of bridges by optical lithography and reactive ion etching. Analysis of the data on magnetoconductivity allowed us to extract the critical temperatures, superconducting coherence lengths, magnetic penetration depths, and diffusion constants of electrons in the normal state as functions of film thickness for each stoichiometry. Two basic time constants were derived from transport and time-resolving measurements. A dynamic process of the formation of a hotspot was analyzed in the framework of a diffusion-based vortex-entry model. We used a two-stage diffusion approach and defined a hotspot size by assuming that the quasiparticles and normal-state electrons have the same diffusion constant. With this definition and these measured material parameters, the hotspot in the 5-nm-thick W 0.85 Si 0.15 film had a diameter of 107 nm at the peak of the number of nonequilibrium quasiparticles.

We experimentally investigate the effect of a magnetic field on photon detection in superconducting single-photon detectors. At low fields, the effect of a magnetic field is through the direct modification of the quasiparticle density of states of the superconductor, and magnetic field and bias current are interchangable, as is expected for homogeneous dirty-limit superconductors. At the field where a first vortex enters the detector, the effect of the magnetic field is reduced, up until the point where the critical current of the detector starts to be determined by flux flow. From this field on, increasing the magnetic field does not alter the detection of photons anymore, whereas it does still change the rate of dark counts. This result points at an intrinsic difference in dark and light counts, and also shows that no enhancement of the intrinsic detection efficiency of a straight SSPD wire is achievable in a magnetic field.

We measure the maximal distance at which two absorbed photons can jointly trigger a detection event in NbN nanowire superconducting single photon detector (SSPD) microbridges by comparing the one-photon and two-photon efficiency of bridges of different overall lengths, from 0 to 400 nm. We find a length of 23 ± 2 nm. This value is in good agreement with to size of the quasiparticle cloud at the time of the detection event.Nanowire superconducting single photon detectors (SSPDs) [1] are a crucial technology for a variety of applications [2]. These devices consist of a thin superconducting film which detects photons when biased to a significant fraction of its critical current. Although details of the microscopic mechanism are still in dispute [3], the present understanding of this process in NbN SSPDs is as follows [4][5][6][7][8][9][10][11][12][13][14]: after the absorption of a photon, a cloud of quasiparticles is created, which is known as a hotspot. This cloud diffuses, spreading out over some area of the wire. This causes the redistribution of bias current, which triggers a vortex unbinding from the edge of the wire, if the applied bias current is such that the current for vortex entry is exceeded. The transition of a vortex across the wire creates a normal-state region, which grows under the influence of Joule heating from the bias current, leading to a measureable voltage pulse and a detection event [15].Recently, applications of these detectors have been demonstrated or proposed which rely the ability of such devices to operate as multiphoton detectors, such as multiphoton subwavelength imaging [16], ultrasensitive higher order autocorrelation [17] and near-field multiphoton sensing [18]. These applications make use of the fact that when biased at lower currents than required for single-photon detection, the detector responds only when several photons are absorbed simultaneously. This multiphoton response has moreover proven to be of great significance in investigating the question of the working mechanism of such devices.For these multiphoton applications to work, the two photons must be absorbed within some given distance of each other, which we will refer to as the hotspot interaction length s. This length determines the efficiency of an SSPD in the multiphoton regime: photons which are absorbed far away from each other along the wire will not be able to jointly cause a detection event, resulting in a reduction of the two-photon detection probability.In this work, use this effect to measure the hotspot interaction length. Our experiment is based on comparing Figure 1. a) Sketch of the experiment. Top pannel: a nanowire of length L is illuminated uniformly, and the current the nanowire is set to be in the single-photon regime. Photon absorption at any point in the wire is sufficient to cause a detection event. In the bottom pannel, the detector is in the two-photon regime, and a detection is observed only if the second photon is absorbed in the region (red spot) where an excess quasiparticle concentrati...