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arXiv:2303.17861 · Physical review. B./Physical review. B

Photon number dependent afterpulsing in superconducting nanostrip single-photon detectors

S. M. F. Raupach, Mariia Sidorova, Alexej D. Semenov

Published Aug 10, 2023• 8 citations
arXivHTMLPDF

Abstract

Superconducting nanostrip single-photon detectors (SNSPDs) are widespread tools in photonic quantum technologies. Here, we study the afterpulsing phenomenon in commercial SNSPDs exhibiting significant levels of afterpulses even at low counting rates. We find different contributions, where the probability of an afterpulse is not a constant but depends on the mean number of photons per light pulse including mean numbers much less than 1. Our observations exclude the electrical circuit as the primary cause of the main contribution to the observed afterpulsing probability, which exhibits a strong dependence on mean photon number. We propose a phenomenological model that qualitatively explains our findings via the introduction of slowly relaxing ``afterpulsing centers,'' storing the absorbed photons' energy for several tens of nanoseconds. We conjecture that two-level systems in amorphous materials are physical candidates for the role of such afterpulsing centers.

DOI
10.1103/physrevb.108.054507

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Superconducting nanostrip single-photon detectors (SNSPDs) are widespread tools in photonic quantum technologies. Here, we study the afterpulsing phenomenon in commercial SNSPDs exhibiting significant levels of afterpulses even at low counting rates. We find different contributions, where the probability of an afterpulse is not a constant but depends on the mean number of photons per light pulse including mean numbers much less than 1. Our observations exclude the electrical circuit as the primary cause of the main contribution to the observed afterpulsing probability, which exhibits a strong dependence on mean photon number. We propose a phenomenological model that qualitatively explains our findings via the introduction of slowly relaxing ``afterpulsing centers,'' storing the absorbed photons' energy for several tens of nanoseconds. We conjecture that two-level systems in amorphous materials are physical candidates for the role of such afterpulsing centers.
We report on the observation of a non-uniform dark count rate in Superconducting Nanowire Single Photon Detectors (SNSPDs), specifically focusing on an afterpulsing effect present when the SNSPD is operated at a high bias current regime. The afterpulsing exists for real detection events (triggered by input photons) as well as for dark counts (no laser input). In our standard set-up, the afterpulsing is most likely to occur at around 180 ns following a detection event, for both real counts and dark counts. We characterize the afterpulsing behavior and speculate that it is not due to the SNSPD itself but rather the amplifiers used to boost the electrical output signal from the SNSPD. We show that the afterpulsing indeed disappears when we use a different amplifier with a better low frequency response. We also examine the short-lived enhancement of detection efficiency during the recovery of the SNSPD due to temporary perturbation of the bias and grounding conditions.
There is increasing interest in using superconducting optical photon detectors in a variety of applications. These applications require detectors that have extremely low dark count rates, high count rates, and high quantum efficiency. I will describe our work on two types of superconducting detectors, the Superconducting Nanowire Single Photon Detector (SNSPD or nSSPD) and superconducting Transition-Edge Sensor (TES). An SNSPD is an ultra-thin, ultra-narrow (nm scale) superconducting meander that is current biased just below its critical current density. When one or more photon is absorbed, a hot spot is formed that causes the superconductor to develop a resistance and consequently a voltage pulse. At NIST and JPL, we have been developing nanowire detectors using an amorphous alloy of tungsten-silicide. An example of this detector is shown in Fig 1(a). In this case, there are two tungsten silicide layers separated by a dielectric layer to optimize detection of light for any polarization[1]. Fig 1(b) is an example of a TES detector with gold cooling fins to enhance timing of the detector[2]. By exploiting the sharp superconducting-to-normal resistive transition of tungsten at 100mK, TES detectors give an output signal that is proportional to the cumulative energy in an absorption event. This proportional pulse-height enables the determination of the energy absorbed by the TES and the direct conversion of sensor pulse-height into photon number. I will discuss our progress towards developing both types of detectors with quantum efficiencies approaching 100%.
We demonstrate laser-induced relaxation oscillations in superconducting nanowire single-photon detectors (SNSPDs). These oscillations appear when a voltage-biased Nb Ti N nanobridge detector is illuminated with intense pulsed-laser light at a repetition rate of approximately 19 MHz . They differ from the well-known relaxation oscillations by a stepwise increase in frequency and phase locking of the oscillations to the laser pulses. We create a model that incorporates electrical feedback and excludes thermal effects to simulate and explain the origin of the observed laser-induced relaxation oscillations. Qualitative agreement to the experiment is achieved using realistic values for the parameters in the model.
Superconducting nanowire single-photon detectors (SNSPDs) are an attractive technology for applications which benefit from high-count rate photon detection such as quantum communication protocols, photon-starved classical communications, metrology of single-emitters, and photonic quantum computing. However, at high-count rates, a significant portion of photons may be incident during recovery intervals following prior detection events. After detection, the device efficiency recovers over nanoseconds to tens of nanoseconds before reaching a steadystate value. Therefore, for a given SNSPD bias current setpoint, a single efficiency and recovery time specification may not be sufficient to characterize the performance of a device at high count rates. Here we present and experimentally validate a model of the recovery time as a function of bias current for any given system efficiency. These results may be used to improve analysis of measurements performed using SNSPDs operating in the high count rate regime.