Timing performance of 30-nm-wide superconducting nanowire avalanche photodetectors
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(a) Colorized scanning electron microscope (SEM) image of a 4-SNAP resist (hydrogen silsesquioxane) mask on NbN with each section colored differently. (b) Equivalent electrical circuit of a 4-SNAP. The arrows pointing at the secondary sections represent the current redistributed from the initiating section to the secondary sections after the initiating section switches to the normal state.
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(a) Schematic representation of instances during the photodetection process. A photon from an optical pulse emitted at t 0 is absorbed in the initiating section (t HSN), generating a resistive slab along the width of the nanowire (t ξ). After the avalanche, the SNAP bias current is diverted into the load, and an output voltage pulse forms across the load resistor. The arrival of this pulse can be detected once the rising edge of the SNAP pulse crosses the trigger level of the oscilloscope (t SNAP). We measured the time delay between t SNAP and a reference t FPD, the instant at which the rising edge of the photodetection signal from a fast photodiode crossed the trigger level of the oscilloscope. The voltage (V) vs time (t) curves represent the oscilloscope traces of the fast photodiode (left hand side) and SNAP (right hand side) pulses. The dashed lines represent the 50% and 95% thresholds. (b) IRF (normalized by the maximum of each trace) of a 30-nm-wide 2-SNAP at bias currents: I B/I SW = 1, 0.93, 0.85, 0.78, 0.73, 0.69, and 0.64. The curved arrow indicates the direction of increasing I B. The double-pointed arrow indicates the MLD at I B/I SW = 0.73. The MLD of the IRF was set to 0 s at I B/I SW = 1. (c) Jitter of a 2-, 3-, and 4-SNAP based on 30-nm-wide nanowires as a function of the normalized bias current (I B/I SW). The switching currents of the 2-, 3-, and 4-SNAP were 13.2 μA, 17.9 μA, and 27.8 μA, respectively. The vertical dashed lines indicate the avalanche currents of the SNAPs.2 The data for the jitter of 3- and 4-SNAPs biased below I AV are not shown (see Ref. 12) as the devices were not operating as single-photon detectors (they were instead operating in arm-trigger regime as described in Ref. 2). (d) IRF asymmetry vs. I B/I SW for the same devices shown in panel (c).
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(a) Simulated time evolution of the current diverted to the read out resistor (R load = 50 Ω) by a 2-SNAP after a resistive ξ-slab is formed in the initiating section (at time t ξ = 0 s) for I B/I SW = 0.96, 0.87, 0.81, 0.77, 0.73, 0.70, and 0.66. The kinetic inductance of each section of the 2-SNAP was L 0 = 13 nH and the series inductor was L S = 130 nH, corresponding to the device of Figure 2(b). Arrows indicate the time at which the resistive ξ-slab is formed (t ξ); the time at which I out reaches 95% of its maximum (t 95%) for I B/I SW = 0.96; the detector peak time for I B/I SW = 0.96 (t P); and the direction of increasing I B. (b) Experimental MLD vs I B (squares) and simulated t P vs I B (stars) for the 2-SNAP of Figure 2(b). The error on the MLD values was assumed to be twice the bin size of the IRF histograms. The value of the MLD for the highest I B was set to 0 s.
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