Any changes in the surface or dimension in nanoscale devices alter their thermal transport. Hence, controlling thermal transport is critical in these devices.
Study: Self-heating hotspots in superconducting nanowires cooled by phonon black-body radiation. Image Credit: remotevfx.com/Shutterstock.com
Within this framework, the performance metrics of a single-photon detector based on a superconducting nanowire are influenced by the thermal boundary conductance between the substrate and the nanowire.
Due to the lack of a straightforward characterization method, understanding thermal boundary conductance in superconducting nanowire devices remains unclear. An article published in the journal Nature Communications presented an easy method for measuring the thermal boundary conductance between nanowires and substrates quantitatively.
These measurements agree with acoustic mismatch theory for a broad range of substrates. Despite performing numerical simulations, the open question on the mechanism underlying thermal boundary conductance remained unanswered. The present work could serve as guidance for the thermal engineering of next-generation superconducting nanowire single-photon detectors.
Superconducting Nanowire Single-Photon Detectors
Single photons are quantum creatures that are attractive candidates for serving as a medium in quantum technology. Thus, single-photon detectors are a pivot technology for realizing the potential of quantum photonic systems.
A superconducting nanowire is an interesting mesoscopic 1-dimensional (1D) object, a fundamental requirement for various quantum technologies. Despite the potential of superconducting nanowires to entirely alter the heat transfer in a nanoscale system, their thermal properties are often studied incidentally.
Superconducting nanowire single-photon detectors have a unique combination of speed in terms of high-count rates, low timing jitter, high detection efficiencies, and low dark count rates, making them desirable detectors for a wide variety of applications.
Phase slip in a thin superconducting wire occurs on the scale of the superconducting coherence length, and phase-slip coherent tunneling is affected by heating in phase-slip junctions. Superconducting nanowire single-photon detectors rely on localized hotspots to detect infrared photons. Here, the energy deposited into the superconducting nanowire single-photon detectors is gradually released into the substrate as phonons.
The thermal boundary conductance between the dielectric substrates and superconducting nanowire single-photon detectors was the determinant of the device's performance. During the early stage of photodetection, the photon energy absorbed by superconducting nanowire single-photon detectors is divided into phonon excitations and quasiparticles.
Consequently, the energy available to distort the superconducting state is reduced. Pair-breaking phonons that escape into the substrate lower the thermal boundary conductance and increase the detection efficiency in a device.
Self-Heating Hotspots in Superconducting Nanowire
In the present work, the thermal boundary conductance between superconducting nanowires and substrates was quantified by measuring the self-heating hotspot current (Ihs), which is the current required to sustain a hotspot inside the nanowire.
Although this type of quantification was previously reported, it was restricted to the micrometer scale, one substrate type, and did not match the theoretical expectations. Furthermore, some values for the thermal boundary conductance reported in the literature were larger than the theoretical values. In contrast, the reinterpretation of others through the present scheme agreed with the theoretical values.
Additionally, previous studies on superconducting nanowire single-photon and related detectors often used a linearized heat transfer model, which was demonstrated to be incompatible with the obtained data.
To measure the thermal boundary conductance between superconducting nanowires and substrates and to attribute the self-heating hotspots in nanowires, the measurements of Ihs (bath temperature, Tb) for 17 NbN nanowires were compared across six different substrate materials using experimental and finite element electrothermal simulations.
The results revealed that the present method works well to extract the thermal boundary conductance. However, the extraction of exponent n that describes the power law cooling to the substrate does not fetch reliable results.
The present method was primarily applied to nanowire devices constructed from the same materials and designs to process state-of-the-art superconducting nanowire single-photon detectors. Moreover, the present method lacked special requirements, such as device design or experimental setup, which are typical requirements in previous superconducting nanowire single-photon detector measurements.
In conclusion, the present method of extracting the thermal boundary conductance was simple, and the extracted values matched those expected via acoustic modeling to a great degree. Moreover, electrothermal simulations illustrate the conditions required to obtain better accuracy.
While previous reports on similar measurements lacked the understanding of the mechanism of thermal boundary conductance due to the lack of comparison with theoretical expectations, reanalyzing the data with the proposed scheme showed an excellent agreement with the current model.
Thus, the present study demonstrated that superconducting nanowires prepared for high-efficiency single-photon detection could serve as a promising platform to probe heat transfer phenomena at the nanoscale, facilitating investigations to yield improved detectors.
Dane, A. et al. (2022). Self-heating hotspots in superconducting nanowires cooled by phonon black-body radiation. Nature Communications. https://doi.org/10.1038/s41467-022-32719-w
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