Large area single photon detectors based on parallel configuration NbN nanowires
(Color online) Schematic view of the cascade switch mechanism in a single block of five wires in parallel, as described in the text. Only one block is shown together with the external circuit. (a) Each nanowire is biased close to its critical current. (b) One photon is absorbed in a wire. (c) A hotspot is created and the current of that nanowire is redistributed among the other wires where the current is now higher than the critical current. (d) A current assisted superconducting-to-normal transition takes place so that all the wires of the block become normal: the bias current is deviated into the external load giving rise to a signal.
(Color online) Schematic view of the fabrication process. (a) Direct current magnetron sputtering of an NbN film on an MgO (100) substrate. (b) Electron beam lithography, Ti/Au deposition, and lift-off to define pads in coplanar waveguide geometry and alignment markers. (c) HSQ electron beam lithography of the parallel SSPD geometry. (d) Reactive ion etching of the NbN film not covered either by the gold layer or by the HSQ mask.
Scanning electron micrograph of four blocks (out of 32) made of five strips in parallel in 40 × 40 μm2 SSPDs. The HSQ mask is still present at the end of the fabrication process.
(Color online) Current–voltage characteristic at 5 K of a 40 × 40 μm2 area parallel SSPD with a series inductance of 470 nH to avoid latching.
(Color online) Typical nonaveraged signal pulse taken at 4.77 K. The pulse is obtained with a bias current of 255 μA. In the inset the signal pulse height is plotted as a function of the bias current Ib. The straight line is a guide for the eye.
(Color online) Count rates as a function of the bias current for different relative light intensities (1550 nm wavelength) as described in the text. The numbers denote the relative intensity of the incident light: from top to bottom 1 (black squares), 0.5 (red pentagons), 0.25 (blue diamonds), 0.1 (purple triangle-up), 0.01 (yellow triangle-down), and the dark counts (black dots). The dark counts curve is also displayed and is well below the counts level.
(Color online) Count rates as a function of light intensity (1550 nm wavelength), for two different bias currents: 285 μA (blue squares) and 251 μA (red diamonds). As described in the text, the blue solid line corresponds to single photon response and the red dashed line to two-photons response.
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