arXiv:1606.09406 · Journal of Physics Conference Series
Superconducting detector of IR single-photons based on thin WSi films
V A Seleznev, A V Divochiy, Yu B Vakhtomin, Pavel Morozov, Philipp I. Zolotov, Д.Д. Васильев, Konstantin M. Moiseev, Elizaveta I. Malevannaya, K V Smirnov
We have developed the deposition technology of WSi thin films 4 to 9 nm thick with high temperature values of superconducting transition (T c ~4 K). Based on deposed films there were produced nanostructures with indicative planar sizes ~100 nm, and the research revealed that even on nanoscale the films possess of high critical temperature values of the superconducting transition (T c ~3.3-3.7 K) which certifies high quality and homogeneity of the films created. The first experiments on creating superconducting single-photon detectors showed that the detectors' SDE (system detection efficiency) with increasing bias current (I b ) reaches a constant value of ~30% (for X=1.55 micron) defined by infrared radiation absorption by the superconducting structure. To enhance radiation absorption by the superconductor there were created detectors with cavity structures which demonstrated a practically constant value of quantum efficiency >65% for bias currents I b >0.6-I c . The minimal dark counts level (DC) made 1 s -1 limited with background noise. Hence WSi is the most promising material for creating single-photon detectors with record SDE/DC ratio and noise equivalent power (NEP).
We have developed the deposition technology of WSi thin films 4 to 9 nm thick with high temperature values of superconducting transition (T c ~4 K). Based on deposed films there were produced nanostructures with indicative planar sizes ~100 nm, and the research revealed that even on nanoscale the films possess of high critical temperature values of the superconducting transition (T c ~3.3-3.7 K) which certifies high quality and homogeneity of the films created. The first experiments on creating superconducting single-photon detectors showed that the detectors' SDE (system detection efficiency) with increasing bias current (I b ) reaches a constant value of ~30% (for X=1.55 micron) defined by infrared radiation absorption by the superconducting structure. To enhance radiation absorption by the superconductor there were created detectors with cavity structures which demonstrated a practically constant value of quantum efficiency >65% for bias currents I b >0.6-I c . The minimal dark counts level (DC) made 1 s -1 limited with background noise. Hence WSi is the most promising material for creating single-photon detectors with record SDE/DC ratio and noise equivalent power (NEP).
Superconducting single-photon detectors (SNSPD) are receivers in optical systems for space communications and exhibit high quantum efficiency and count rate. The receiver in the optical system of atmospheric communication should provide highly efficient detection of radiation with a wavelength of 1.55 µm. The cut-off wavelength of SNSPD is the maximum wavelength of radiation at which high detection efficiency is maintained and depends on the geometrical parameters of the sensitive element of the detector and the thickness and structure of the thin film of superconducting material parameters. In this work, we simulate the cut-off wavelength depending on the thickness of the superconducting film WxSi1-x and the stoichiometric composition with a nanowire width of 100 nm and a ratio of the bias current to the critical 0.7. It is shown that the largest value of the cut-off wavelength λc = 3.06 µm corresponds to a film thickness of 5 nm and a Si content of 25%. With a film thickness of 7 nm, detection with the required wavelength of 1.55 µm is provided over the entire investigated range of the stoichiometric composition of the film: when the Si content is from 15 to 25%.
Superconducting nanowire single-photon detectors (SNSPD) are used in quantum optics when record-breaking time resolution, high speed, and exceptionally low levels of dark counts (false readings) are required. Their detection efficiency is limited, however, by the absorption coefficient of the ultrathin superconducting film for the detected radiation. One possible way of increasing the detector absorption without limiting its broadband response is to make a detector in the form of several vertically stacked layers and connect them in parallel. For the first time we have studied single-photon detection in a multilayer structure consisting of three superconducting layers of amorphous tungsten silicide (WSi) separated by thin layers of amorphous silicon. Two operating modes of the detector are illustrated: an avalanche regime and an arm-trigger regime. A shift in these modes occurs at currents of ∼0.5–0.6 times the critical current of the detector.
We report on our progress in research and development of ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs were made of the 4-nm-thick NbN films with Tc ~11 K, patterned as meander-shaped, 100-nm-wide strips, and covering an area of 10×10 μm2. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, a hotspot of excited electrons and redistribution of the biasing supercurrent, jointly produce a picosecond voltage transient signal across the superconducting nanostripe. The SSPDs are typically operated at 4.2 K, but their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by the optical absorption of our 4-nm-thick NbN film. With the wavelength increase of the incident photons,the QE of SSPDs decreases significantly, but even at the wavelength of 6 μm, the detector is able to count single photons and exhibits QE of about 10-2 %. The dark (false) count rate at 2 K is as low as 2x10-4 s,-1 which makes our detector essentially a background-limited sensor. The very low dark-count rate results in a noise equivalent power (NEP) below 10-18 WHz-1/2 for the mid-infrared range (6 μm). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for another, lower-Tc materials with a narrow superconducting gap and low quasiparticles diffusivity. The use of such superconductors should shift the cutoff wavelength below 10 μm.
Superconducting single photons detectors (SSPDs) have emerged in recent years as a promising alternative for
fast and sensitive infrared detectors working in the photon counting mode. In particular, those detectors combine
very low dark count rates (below 1 Hz), high speed (above 1 GHz), photon number resolution and reasonable
quantum efficiency (10% at telecom wavelengths). They already found applications in quantum cryptography
systems and integrated circuit failure analysis, but could also be used as ultimate sensors in matrix configurations.
We show here the optimization of SSPD fabrication and their optical metrology at CEA. SSPD are fabricated
by patterning a 80 nm wide nanowire in a very thin (4 nm) NbN film on sapphire, forming a pixel of several
microns size. A cryogenic all-fibered optoelectronic system has been developed and allows precise metrology of
the optical performances of SSPD. When biasing near the critical current of the nanowire, we demonstrate a
detection quantum efficiency of 8% at 1.55 µm, which is also found to be strongly polarization dependent. This
quantum efficiency being limited by optical absorption, we propose a prism coupling based absorption enhancing
structure that allows reaching 100% quantum efficiency.
We present our studies on ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs are patterned by electron beam lithography from 4-nm thick NbN film into meander-shaped strips covering square area of 10/spl times/10 /spl mu/m/sup 2/. The advances in the fabrication technology allowed us to produce highly uniform 100-120-nm-wide strips with meander filling factor close to 0.6. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, an avalanche of excited hot electrons and the biasing supercurrent, jointly produce a picosecond voltage transient response across the superconducting nanostrip. The SSPDs are typically operated at 4.2 K, but they have shown that their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by optical absorption of our 4-nm-thick NbN film. For 1.55 /spl mu/m photons, QE was /spl sim/20% and decreases exponentially with the increase of the optical wavelength, but even at the wavelength of 6 /spl mu/m the detector remains sensitive to single photons and exhibits QE of about 10/sup -2/%. The dark (false) count rate at 2 K is as low as 2 /spl times/ 10/sup -4/ s/sup -1/, what makes our detector essentially a background-limited sensor. The very low dark-count rate results in the noise equivalent power (NEP) as low as 10/sup -18/ WHz/sup -1/2/ for the mid-infrared range (6 /spl mu/m). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for the other, lower-T/sub c/ superconductors with the narrow superconducting gap and low quasiparticle diffusivity. The use of such materials will shift the cutoff wavelength towards the values even longer than 6 /spl mu/m.
Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)