5 research outputs found
Photon-number dependent afterpulsing in superconducting nanostrip single-photon detectors
Superconducting nanostrip single-photon detectors (SNSPD) are wide-spread
tools in photonic quantum technologies. Here, we study afterpulsing in
commercial SNSPD made of amorphous superconducting material. We find that 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 one. Our
observations exclude the electrical circuit as the primary cause of
afterpulsing. We propose a phenomenological model which qualitatively explains
our findings via the introduction of slowly relaxing "afterpulsing centers". We
argue that two-level systems in amorphous materials are the most plausible
physical candidates for the role of such afterpulsing centers
Phonon heat capacity and self-heating normal domains in NbTiN nanostrips
Self-heating normal domains in thin superconducting NbTiN nanostrips with the granular structure were characterized via steady-state hysteretic current–voltage characteristics measured at different substrate temperatures. The temperature dependence and the magnitude of the current, which sustains a domain in equilibrium at different voltages, can only be explained with a phonon heat capacity noticeably less than expected for 3D Debye phonons. This reduced heat capacity coincides with the value obtained earlier from magnetoconductance and photoresponse studies of the same films. The rate of heat flow from electrons at a temperature
Te
to phonons in the substrate at a temperature
TB
is proportional to
(Tep−TBp)
with the exponent p ≈ 3, which differs from the exponents for heat flows mediated by the electron–phonon interaction or by escaping of 3D Debye phonons via the film/substrate interface. We attribute both findings to the effect of grains on the phonon spectrum of thin NbTiN films. Our findings are significant for understanding the thermal transport in superconducting devices exploiting thin granular films.Peer Reviewe
Local thermal fluctuations in current-carrying superconducting nanowires
We analyze the effect of different types of fluctuations in internal electron
energy on the rates of dark and photon counts in straight current-carrying
superconducting nanowires. Dark counts appear due to thermal fluctuations in
statistically independent cells with the effective size of the order of the
coherence length; each count corresponds to an escape from the equilibrium
state through an appropriate saddle point. For photon counts, spectral
broadening of the deterministic cut off in the spectra of the detection
efficiency can be phenomenologically explained by local thermal fluctuations in
the electron energy within cells with the same effective volume as for dark
counts
Enhancing the performance of superconducting nanowire-based detectors with high-filling factor by using variable thickness
© 2021 IOP Publishing Ltd. Current crowding at bends of superconducting nanowire single-photon detector (SNSPD) is one of the main factors limiting the performance of meander-style detectors with large filling factors. In this paper, we propose a new concept to reduce the influence of the current crowding effect, a so-called variable thickness SNSPD, which is composed of two regions with different thicknesses. A larger thickness of bends in comparison to the thickness of straight nanowire sections locally reduces the current density and reduces the suppression of the critical current caused by current crowding. This allows variable thickness SNSPD to have a higher critical current, an improved detection efficiency, and decreased dark count rate in comparison with a standard uniform thickness SNSPD with an identical geometry and film quality