(a) Scanning electron micrograph of a meandered Nb SNSPD with a detection area of . The dark regions are Nb, while the sapphire substrate is lighter. (b) Equivalent circuit for the nanowire and readout with a photon-induced hotspot resistance . Prior to photon absorption, and flows fully through . The nanowire is outlined by the dotted box; the arrows indicate the flow of current. (c) Equivalent circuit as the bias current is returning to the device after the hotspot has cooled back to the zero-resistance state. For this stage, the inductive time constant of this circuit, , sets the time scale for reset of the detector, which characterizes the decay of the load current toward zero.
(a) Equivalent circuit; , . (b) Measured (solid lines) and simulated (dashed lines) output pulses, , for device (A) with the self-resetting case labeled by the three regimes of operation: (i) the device is in equilibrium with ; (ii) initial heating: a photon has been absorbed, the hotspot is growing and the current is transferring into the load; (iii) the hotspot resistance has returned to zero, and the current is returning to the device with a time constant . (c) Numerical results for (dashed lines) and (solid lines) for (upper curve) and (lower curve); the latter shows the latching case. We find from these simulations. We note that as defined here can be finite even with because is calculated according to Eq. (7) and, therefore, only depends on the electron temperature, and not on . is calculated from the simulation according to Eq. (6).
Normalized latching current predicted by the model (open symbols) and measured (solid symbols) vs , obtained by varying in the simulations and experiments. For the meander, we plot predictions for two values of the diffusion constant: , as used in all other simulations, and , which is used only here.
: Simulation results for Nb devices (A), (B), and (C) showing the dependence of and on and ; ; and is labeled for device (A) with and is equal to 5.0 ns. For device (C), (not labeled).
Parameters of the devices studied in this work. The sheet resistance is measured approximately 5 K above . The reported values of the kinetic inductance for each device in the table include approximately 10 nH of inductance from the measurement leads (Ref. 29).
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