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IEEE Transactions on Applied Superconductivity

A Stochastic SPICE Model for Superconducting Nanowire Single Photon Detectors and Other Nanowire Devices

Adam N. McCaughan, Dylan M. Oh, Sae Woo Nam

Published Jan 24, 2019• 15 citations
Publisher

Abstract

Superconducting nanowire devices, such as the superconducting nanowire single photon detector (SNSPD) or nanocryotron, have a time-dependent stochasticity that depends on the current flowing through them. When modeling complex circuits made of several such devices (for instance, an array of SNSPDs), the ability to include this randomness can be important for predicting unwanted effects and interactions within the circuit. We present a modification of the model described by Berggren et al. that allows for the inclusion of this stochasticity into the nanowire device model. We then verify the model against experiment using a tungsten silicide SNSPD, and show that the modified model replicates the stochasticity of the physical device.

DOI
10.1109/tasc.2019.2892049

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Superconducting nanowire devices, such as the superconducting nanowire single photon detector (SNSPD) or nanocryotron, have a time-dependent stochasticity that depends on the current flowing through them. When modeling complex circuits made of several such devices (for instance, an array of SNSPDs), the ability to include this randomness can be important for predicting unwanted effects and interactions within the circuit. We present a modification of the model described by Berggren et al. that allows for the inclusion of this stochasticity into the nanowire device model. We then verify the model against experiment using a tungsten silicide SNSPD, and show that the modified model replicates the stochasticity of the physical device.
Superconducting nanowire single-photon detectors (SNSPDs) exhibit complex switching behaviors due to electro thermal feedback during the detection process. Modeling and understanding these behaviors is integral for designing superconducting devices; however, many models often prioritize accuracy over computational speed and intuitive integration for circuit designers. Here, we build upon a growing architecture of SPICE tools for superconducting nanowire devices by capturing complex residual heating effects in a compact thermal model of an SNSPD. We demonstrate that our model is comparable to more complicated thermal models of superconducting nanowire devices, including finite-element simulations, and is applicable for the fast development of SNSPD circuits.
© 2018 Author(s). To analyze the switching dynamics and output performance of a superconducting nanowire single photon detector (SNSPD), the nanowire is usually modelled as an inductor in series with a time-varying resistor induced by the absorption of a photon. Our recent experimental results show that, due to the effect of kinetic inductance, for a SNSPD made of a nanowire of sufficient length, its geometrical length can be comparable to or even longer than the effective wavelength of frequencies contained in the output pulse. In other words, a superconducting nanowire can behave as a distributed transmission line so that the readout pulse depends on the photon detection location and the transmission line properties of the nanowire. Here, we develop a distributed model for a superconducting nanowire and apply it to simulate the output performance of a long nanowire designed into a coplanar waveguide. We compare this coplanar waveguide geometry to a conventional meander nanowire geometry. The simulation results agree well with our experimental observations. With this distributed model, we discuss the importance of microwave design of a nanowire and how impedance matching can affect the output pulse shape. We also discuss how the distributed model affects the growth and decay of the photon-triggered resistive hotspot.
To analyze the switching dynamics and output performance of a superconducting nanowire single photon detector (SNSPD), the nanowire is usually modelled as an inductor in series with a time-varying resistor induced by the absorption of a photon. Our recent experimental results show that, due to the effect of kinetic inductance, for a SNSPD made of a nanowire of sufficient length, its geometrical length can be comparable to or even longer than the effective wavelength of frequencies contained in the output pulse. In other words, a superconducting nanowire can behave as a distributed transmission line so that the readout pulse depends on the photon detection location and the transmission line properties of the nanowire. Here, we develop a distributed model for a superconducting nanowire and apply it to simulate the output performance of a long nanowire designed into a coplanar waveguide. We compare this coplanar waveguide geometry to a conventional meander nanowire geometry. The simulation results agree well with our experimental observations. With this distributed model, we discuss the importance of microwave design of a nanowire and how impedance matching can affect the output pulse shape. We also discuss how the distributed model affects the growth and decay of the photon-triggered resistive hotspot.
Superconducting nanowire single-photon detectors (SNSPDs) have demonstrated remarkable efficiency, timing resolution, and intrinsic dark count rate properties, but the SNSPD community currently lacks a comprehensive model of the single-photon detection process. In this work, we conduct a detailed examination of the current detection mechanism models and compare their predictions to new experimental measurements of the intrinsic timing properties and polarization dependence of specialized NbN test devices. First, we consider the energy downconversion cascade using the kinetic equations to describe the non-equilibrium electron and phonon systems immediately following photon absorption. These calculations provide estimates for the energy loss and fluctuations during this process, and provide qualitative information about the way energy is partitioned between the electron and phonon systems. To study the suppression of superconductivity following downconversion, we apply the most advanced existing model, that of Vodolazov (2017), but find it inadequate to quantitatively describe the timing properties of these detectors. By extending the model to use the generalized time-dependent Ginzburg-Landau equations, we achieve better quantitative agreement with experiment. However, the generalized model still provides only a qualitative picture of the detection process. We also conduct an experimental examination of the heat transfer process in WSi nanowires by examining the nanowire reset dynamics, steady-state dissipation, and crosstalk between elements of an array. The results are compared to existing electrothermal models, but these models fail to adequately describe the dynamics of the system. A generalized form of the electrothermal model provides better fitting to experiment, but incorporation of non-equilibrium effects is likely needed to provide a fully quantitative description of the system. These results are directly connected to some of the thermal challenges of SNSPD array development. Informed by the crosstalk results, we demonstrate a new multiplexing technique based on thermal coupling between two active nanowire layers, known as the thermal row-column. This method promises to enable kilopixel to megapixel scale imaging arrays for low photon-flux applications. Finally, we discuss the design and characterization of the ground detector for the Deep Space Optical Communication (DSOC) demonstration mission.
Abstract Superconducting nanowire-based devices are being hailed as promising single-photon detectors that exhibit excellent combined properties. In particular, their unrivalled time-resolution ability has made these devices potentially revolutionary for the commercial ultrafast single-photon measurement and analysis fields. In this review, we present a detailed summary of the influencing factors and the intrinsic physical mechanism of the temporal resolution in superconducting nanowire single-photon detectors (SNSPDs). We elaborate on the two internal components of temporal resolution, the time response and time jitter, as well as on many measurement methods and device structure modifications attempted to exclude external interference, thus approaching the ultimate limits of time-resolved single photon detection. The investigation of the temporal resolution of SNSPDs not only sheds light on the intrinsic physical mechanism but also contributes to time-related practical engineering applications.