Instrumentation & Measurement Magazine 24-5 - 72

Fig. 4. Counting rate measured at 3.3 K as a function of the bias current.
The optical power P at 1550 nm is tuned from P = 1 nW to P = 500 nW. Black
squares represent the dark counts (DK).
numbers of incident photons. We changed the number of
photons by tuning the laser input power and changing the
optical attenuation value. In case of single photon detection,
the counting rate versus input power graph is expected to be
a straight line with slope 1 (in logarithmic scale) because the
number of counted pulses is proportional to the incident photons.
If there are non-linear effects in the photo response, the
slope in the curve is larger than one and the device cannot be
considered a single photon detector.
As illustrated in Fig. 5, the counts versus optical power
curve is a line whose slope is 1.003 ± 0.011 (in logarithmic scale)
which guarantees the single photon detection. Hence, we conclude
that the detection is not compromised by the presence of
the Al layer.
With a statistical analysis of pulses, we could also estimate
a dead time of 5 ns (see inset in Fig. 5). As a final step, we also
measured the photo response at 650 nm (Fig. 6) and noticed a
saturation in the counting rate versus bias current curve.
Fig. 6. MoSi/Al counting rate as a function of the bias current at λ= 650 nm.
The curve tends to a saturation, reached between Ib
= 9,5 μA and Ib
= 10 μA.
That measurement shows that, in the operating regime,
the counting rate at 650 nm does not depend on the bias current,
which guarantees the reliability of the measured value.
The absence of saturation in the measurement at 1550 nm does
not indicate that the device cannot operate at that wavelength;
it simply indicates that the fabrication process was not optimized
to detect that specific wavelength.
Conclusions
After an introduction to SNSPD properties and applications,
we present the results of the fabrication and characterization
of MoSi/Al SNSPDs. In our device, we covered the superconductor
with a thin Al layer, and as a result, we measured
enhanced critical current and critical temperature. Such properties
remain practically preserved over time. Moreover, we
demonstrated that the Al film does not affect single photon detection
by measuring the single photon counting at 1550 nm at
T = 3.3 K. These results demonstrate that MoSi/Al SNSPDs are
good candidates for infrared photon detection also at T > 1K.
References
[1] G. N. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A.
Semenov, K. Smirnov, B. Voronov, and A. Dzardanov, C. Williams
and R. Sobolewski, " Picosecond superconducting single-photon
optical detector, " Applied Physics Letters, vol. 79, no. 6, 2001.
[2] P. Hu, H. Li, L. You, H. Wang, Y. Xiao, J. Huang, X. Yang, W.
Zhang, Z. Wang, and X. Xie, " Detecting single infrared photons
toward optimal system detection efficiency, " Optics Express, vol.
28, pp. 36884-36891, 2020.
[3] B. Korzh, Q. Zhao, J. P. Allmaras et al., " Demonstration of sub-3 ps
temporal resolution with a superconducting nanowire singlephoton
detector, " Nature Photonics, vol. 14, no. 4, pp. 250-255,
2020.
Fig. 5. Count rate at 1550 nm with different laser input power. The inset
presents the measured voltage pulse.
72
[4] A. Semenov, A. Engel, H. W. Hu¨ bers, K. Il'in, and M. Siegel,
" Spectral cut-off in the efficiency of the resistive state formation
caused by absorption of a single-photon in current-carrying
superconducting nano-strips, " Eur. Physical J. B, vol. 47, p. 495,
2005.
IEEE Instrumentation & Measurement Magazine
August 2021
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