Spatially dependent sensitivity of superconducting meanders as single-photon detectors

Abstract

The photo-response of a thin current-carrying superconducting stripe with a 90-degree turn is studied within the time-dependent Ginzburg-Landau theory. We show that the photon acting near the inner corner (where the current density is maximal due to the current crowding [J. R. Clem and K. K. Berggren, Phys. Rev. B 84, 174510 (2011)]) triggers the nucleation of superconducting vortices at currents much smaller than the expected critical one, but does not bring the system to a higher resistive state and thus remains undetected. The transition to the resistive state occurs only when the photon hits the stripe away from the corner due to there uniform current distribution across the sample, and dissipation is due to the nucleation of a kinematic vortex-antivortex pair near the photon incidence. We propose strategies to account for this problem in the measurements.PACS numbers: 74.78. Na, 74.25.Fy Superconducting current-carrying thin-film stripes have recently received a revival of interest due to their promising application for single-photon detection [1] with a high maximum count rate, broadband sensitivity, fast response time and low dark counts [2][3][4]. The singlephoton absorption event leads to the formation of a nonequilibrium "hotspot" with suppressed superconductivity, the area of which grows in time, forming a normal belt across the strip [5]. The latter leads to redistribution of the current between the now resistive superconductor and a parallel shunt resistor, where a voltage pulse is detected, before a hotspot region cools down (on a timescale of few hundred picoseconds [6]) and the system returns to its initial superconducting state. Although the hotspot mechanism nicely explains the photon detection in the visible and near UV range, the detection mechanism in the near-infrared range is still debated (see e.g. Ref. [7] and references therein). Superconducting fluctuations, e.g., excitation of superconducting vortices [8][9][10][11][12], have been put forward as an explanation. Dissipative crossing of such vortices, which hop over the edge barrier, or are created due to the unbinding of thermally activated vortex-antivortex pairs, provides a good description of the experiment [10]. This vortex-based mechanism was also shown to be the dominating origin for dark counts [11,12,14], which leads to decoherence in the photon detection process.To increase the efficiency of the photon counting, detectors are usually fabricated as a meandering superconducting wire [3]. However, these structures are vulnerable to edge imperfections [3], which significantly reduce the photon counting rate. Moreover, the critical current in these systems is mostly determined by sharp inner corners where the supercurrent density is maximal due to current crowding [15,16]. While the appearance of edge imperfections can be reduced by present day technology [3], the effect of current crowding in such meandering geometries still demands further investigations.In this work we therefore study the effect of the turns of a meandering s...

Dependence of count rate on magnetic field in superconducting thin-film TaN single-photon detectors

“…The estimated detection efficiencies (DE) range from ∼ 0.1 at maximum I b and 400 nm photons, to ≈ 10 −4 at the lowest measured I b and 985 nm photons. From the DE up to 0.1 we can conclude that most photon-detection events occur along the straight sections of the meander 18 , which is also supported by recent simulations 23 . Except for the scattering in some datasets, R ph appear to be constant with a field-independent photon count-rate.…”

Effect of magnetic field on the photon detection in thin superconducting meander structures

“…This also leads to an increase of the vortex-crossing rate in the bends [18]. Simulations suggest that a meander has spatially dependent sensitivity [22]. According to Ref.…”

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

“…13,39,40 We conclude that there is a difference in the nature of photon and dark counts in SSPDs: photon counts scale with a field scale B C inherent to the material, whereas dark counts scale with the change in critical current under the influence of magnetic field, which depends on geometry. This difference between photon and dark counts is as of yet unexplained and carries implications for the design of SSPDs: it means that the only way in which an SSPD can be made more efficient by an applied magnetic field is by choosing a geometry where the critical current is not adversely affected by the applied field such as a spiral.…”