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arXiv:1008.0895 · Journal of Applied Physics

Reset dynamics and latching in niobium superconducting nanowire single-photon detectors

Anthony J. Annunziata, Orlando Quaranta, Daniel F. Santavicca, Alessandro Casaburi, Luigi Frunzio, Mikkel Ejrnaes, Michael J. Rooks, Roberto Cristiano, S. Pagano, A. Frydman, Daniel E. Prober

Published Oct 15, 2010• 125 citations
arXivHTMLPDF

Abstract

We study the reset dynamics of niobium (Nb) superconducting nanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.

DOI
10.1063/1.3498809

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We study the reset dynamics of niobium (Nb) superconducting nanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.
We modeled the response of superconducting nanowire single-photon detectors during a photodetection event, taking into consideration only the thermal and electrical properties of a superconducting NbN nanowire on a sapphire substrate. Our calculations suggest that heating which occurs after the formation of a photo-induced resistive barrier is responsible for the generation of a measurable voltage pulse. We compared this numerical result with experimental data of a voltage pulse from a slow device, i.e. large kinetic inductance, and obtained a good fit. Using this electro-thermal model, we estimated the temperature rise and the resistance buildup in the nanowire, and the return current at which the nanowire becomes superconducting again. We also show that the reset time of these photodetectors can be decreased by the addition of a series resistance and provide supporting experimental data. Finally we present preliminary results on a detector latching behavior that can also be explained using the electro-thermal model.
The hotspot relaxation time (τth) was systematically studied on superconducting nanowire single-photon detectors (SNSPDs) made from ultrathin NbN films with different substrates and chemical compositions. The τth values were measured to be between 9.2 ps and 41.1 ps at the bath temperature of 2.15 K. The temperature dependence of τth ∝ TC−1.5 was observed, which indicated a dominant electron-phonon scattering relaxation mechanism in disordered NbN films. In addition, the increase in τth results in a higher latching probability in NbN-based SNSPDs with the same electrical response time (τe). The experimental results confirmed that the devices suffered latching when the ratio of the electric and thermal relaxation time was less than 133. This observation is an interesting reference for the design and process optimization of SNSPDs.
We investigate the performance of superconducting nanowire photon detectors fabricated from ultra-thin Nb. A direct comparison is made between these detectors and similar nanowire detectors fabricated from NbN. We find that Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. We show that the devices can be stabilized by reducing the input resistance of the readout. Nb detectors optimized in this way are shown to have approximately 2/3 the reset time of similar large-active-area NbN detectors of the same geometry, with approximately 6% detection efficiency for single photons at 470 nm.
We realize fast, efficient and small footprint niobium nitride superconducting nanowire single photon detectors atop of photonic waveguides. By reducing the bias current of the nanowire, in order to break the superconductivity and trigger a detection event, more than one photon needs to be absorbed in a localized section of the wire within a very short time delay (hot-spot relaxation time), making such devices promising also for multiphoton sensing applications. We adopt a near-infrared pump-probe technique in a cryogenic environment to investigate the bias current dependence of the hot-spot relaxation time. A minimum relaxation time of (22 ± 1) ps is obtained when applying a bias current of 50% of the switching current at a bath temperature of 1.7K. Our study reveals a strong increase of the picosecond relaxation time with increasing bias current. We further adopt the same technique for determining the multi-photon detection regimes of the detector, which are in agreement with standard quantum detector tomography. In this context, we introduce a practical model and reconstruction method for determining the detector sensitivity regimes. Our work provides a complete description of the detector working operation in both number photon threshold sensitivity and time-delay sensitivity. The results allow for implementing on-chip measurement architectures for the characterization of weak classical light emitters and fast single photon sources with only one detector, driven at different biasing currents, with a drastic reduction of the time uncertainty limitations of typical correlation measurement systems.
By changing the bias scheme of a superconducting nanowire single photon detector (SNSPD), the current-voltage (I-V) characteristics of SNSPDs are extensively studied. Using a quasi-constant-voltage bias, the SNSPD nonlatching property is observed in the low voltage range. When the bias current surpasses the critical current, a thermal relaxation oscillation occurs because of the hotspot's periodic appearance, growth, and disappearance. This produces a smooth transition in theI-Vcurve instead of latching. The oscillation frequency is proportional to the voltage of the SNSPD. A linear dependence of the oscillation frequency on the SNSPD voltage is derived from the circuit model of SNSPD electronics, which fits well with the experimental results. The kinetic inductance of the SNSPD is extracted from the fitting as the sole parameter. In the high-voltage range, theI-Vcurve is deduced from Skocpol, Beasley, and Tinkham's self-heating hotspot model, which is consistent with the experimental result. By measuring the return current, we obtained a heat transfer coefficient (α) of 0.7-2 × 105W · m2· K1between the nanowire and the substrate.
We investigate how the bias current affects the hot-spot relaxation dynamics in niobium nitride. We use for this purpose a near-infrared pump-probe technique on a waveguide-integrated superconducting nanowire single-photon detector driven in the two-photon regime. We observe a strong increase in the picosecond relaxation time for higher bias currents. A minimum relaxation time of (22 ± 1) ps is obtained when applying a bias current of 50% of the switching current at 1.7 K bath temperature. We also propose a practical approach to accurately estimate the photon detection regimes based on the reconstruction of the measured detector tomography at different bias currents and for different illumination conditions.