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Korneeva Y, Florya I, Vdovichev S, Moshkova M, Simonov N, Kaurova N, et al. Comparison of hot spot formation in nbn and mon thin superconducting films after photon absorption. IEEE Trans Appl Supercond. 2017;27(4):1–4.
Abstract: In superconducting single-photon detectors (SSPD), the efficiency of local suppression of superconductivity and hotspot formation is controlled by diffusivity and electron-phonon interaction time. Here, we selected a material, 3.6-nm-thick MoNx film, which features diffusivity close to those of NbN traditionally used for SSPD fabrication, but with electron-phonon interaction time an order of magnitude larger. In MoN ∞ detectors, we study the dependence of detection efficiency on bias current, photon energy, and strip width, and compare it with NbN SSPD. We observe nonlinear current-energy dependence in MoNx SSPD and more pronounced plateaus in dependences of detection efficiency on bias current, which we attribute to longer electron-phonon interaction time.
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Trifonov A, Tong C-YE, Lobanov Y, Kaurova N, Blundell R, Goltsman G. Photon absorption near the gap frequency in a hot electron bolometer. IEEE Trans Appl Supercond. 2017;27(4):1–4.
Abstract: The superconducting energy gap is a fundamental characteristic of a superconducting film, which, together with the applied pump power and the biasing setup, defines the instantaneous resistive state of the Hot Electron Bolometer (HEB) mixer at any given bias point on the I-V curve. In this paper we report on a series of experiments, in which we subjected the HEB to radiation over a wide frequency range along with parallel microwave injection. We have observed three distinct regimes of operation of the HEB, depending on whether the radiation is above the gap frequency, far below it or close to it. These regimes are driven by the different patterns of photon absorption. The experiments have allowed us to derive the approximate gap frequency of the device under test as about 585 GHz. Microwave injection was used to probe the HEB impedance. Spontaneous switching between the superconducting (low resistive) state and a quasi-normal (high resistive) state was observed. The switching pattern depends on the particular regime of HEB operation and can assume a random pattern at pump frequencies below the gap to a regular relaxation oscillation running at a few MHz when pumped above the gap.
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Ryabchun S, Tong C-YE, Paine S, Lobanov Y, Blundell R, Goltsman G. Temperature resolution of an HEB receiver at 810 GHz. IEEE Trans Appl Supercond. 2009;19(3):293–6.
Abstract: We present the results of direct measurements of the temperature resolution of an HEB receiver operating at 810 GHz, in both continuum and spectroscopic modes. In the continuum mode, the input of the receiver was switched between black bodies with different physical temperatures. With a system noise temperature of around 1100 K, the receiver was able to resolve loads which differed in temperature by about 1 K over an integration time of 5 seconds. This resolution is significantly worse than the value of 0.07 K given by the radiometer equation. In the spectroscopic mode, a gas cell filled with carbonyl sulphide (OCS) gas was used and the emission line at 813.3537060 GHz was measured using the receiver in conjunction with a digital spectrometer. From the observed spectra, we determined that the measurement uncertainty of the equivalent emission temperature was 2.8 K for an integration time of 0.25 seconds and a spectral resolution of 12 MHz, compared to a 1.4 K temperature resolution given by the radiometer equation. This relative improvement is due to the fact that at short integration times the contribution from 1/f noise and drift are less dominant. In both modes, the temperature resolution was improved by about 40% with the use of a feedback loop which adjusted the level of an injected microwave radiation to maintain a constant operating current of the HEB mixer. This stabilization scheme has proved to be very effective to keep the temperature resolution of the HEB receiver to close to the theoretical value given by the radiometer equation.
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Korneeva Y, Florya I, Semenov A, Korneev A, Goltsman G. New generation of nanowire NbN superconducting single-photon detector for mid-infrared. IEEE Trans Appl Supercond. 2011;21(3):323–6.
Abstract: We present a break-through approach to mid-infrared single-photon detection based on nanowire NbN superconducting single-photon detectors (SSPD). Although SSPD became a mature technology for telecom wavelengths (1.3-1.55 μm) its further expansion to mid-infrared wavelength was hampered by low sensitivity above 2 μm. We managed to overcome this limit by reducing the nanowire width to 50 nm, while retaining high superconducting properties and connecting the wires in parallel to produce a voltage response of sufficient magnitude. The new device exhibits 10 times better quantum efficiency at 3.5 μm wavelength than the “standard” SSPD.
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Xiaolong Hu, Holzwarth CW, Masciarelli D, Dauler EA, Berggren KK. Efficiently coupling light to superconducting nanowire single-photon detectors. IEEE Trans. Appl. Supercond.. 2009;19(3):336–40.
Abstract: We designed superconducting nanowire single-photon detectors (SNSPDs) integrated with silver optical antennae for free-space coupling and a dielectric waveguide for fiber coupling. According to our finite-element simulation, (1) for the free-space coupling, the absorptance of the NbN nanowire for TM-polarized photons at the wavelength of 1550 nm can be as high as 96% by adding silver optical antennae; (2) for the fiber coupling, the absorptance of the NbN nanowire for TE-like-polarized photons can reach 76% including coupling efficiency at the wavelength of 1550 nm by adding a silicon nitride waveguide and an inverse-taper coupler.
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Yamashita T, Miki S, Qiu W, Fujiwara M, Sasaki M, Wang Z. Temperature dependent performances of superconducting nanowire single-photon detectors in an ultralow-temperature region. IEEE Trans. Appl. Supercond.. 2010;21(3):336–9.
Abstract: We report on the performance of a fiber-coupled superconducting nanowire single-photon detector (SNSPD) from 4 K down to the ultralow temperature of 16 mK for a 1550 nm wave length. The system detection efficiency (DE) increased with de creasing the temperature and reached the considerably high value of 15% with a dark count rate less than 100 cps below 1.5 K, even without an optical cavity structure. We also observed saturation of the system DE in its bias current dependency at 16 mK, which indicates that the device DE of our SNSPD nearly reached intrinsic DE despite the device having a large active area of 20 μm × 20 μm. The dark count was finite even at 16 mK and the black body radiation becomes its dominant origin in the low temperatures for fiber-coupled devices.
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Yang JKW, Kerman AJ, Dauler EA, Cord B, Anant V, Molnar RJ, et al. Suppressed critical current in superconducting nanowire single-photon detectors with high fill-factors. IEEE Trans. Appl. Supercond.. 2009;19(3):318–22.
Abstract: In this work we present a new fabrication process that enabled the fabrication of superconducting nanowire single photon detectors SNSPD with fill-factors as high as 88% with gaps between nanowires as small as 12 nm. This fabrication process combined high-resolution electron-beam lithography with photolithography. Although this work was motivated by the potential of increased detection efficiency with higher fill-factor devices, test results showed an unexpected systematic suppression in device critical currents with increasing fill-factor.
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Tretyakov I, Ryabchun S, Finkel M, Maslennikov S, Maslennikova A, Kaurova N, et al. Ultrawide noise bandwidth of NbN hot-electron bolometer mixers with in situ gold contacts. IEEE Trans. Appl. Supercond.. 2011;21(3):620–3.
Abstract: We report a noise bandwidth of 7 GHz in the new generation of NbN hot-electron bolometer (HEB) mixers that are being developed for the space observatory Millimetron. The HEB receiver driven by a 2.5-THz local oscillator offered a noise temperature of 600 K in a 50-MHz final detection bandwidth. As the filter center frequency was swept this value remained nearly constant up to the cutoff frequency of the cryogenic amplifier at 7 GHz. We believe that such a low value of the noise temperature is due to reduced radio frequency (RF) loss at the interface between the superconducting film and the gold contacts. We have also performed gain bandwidth measurements at the superconducting transition on HEB mixers with various lengths and found them to be in excellent agreement with the results of the analytical and numerical models developed for the HEB mixer with both diffusion and phonon cooling of hot electrons.
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Lobanov Y, Tong E, Blundell R, Hedden A, Voronov B, Gol'tsman G. Large-signal frequency response of an HEB mixer: from 300 MHz to terahertz. IEEE Trans. Appl. Supercond.. 2011;21(3):628–31.
Abstract: We present a study of the large signal frequency response of an HEB mixer over a wide frequency range. In our experiments, we have subjected the HEB mixer to incident electromagnetic radiation from 0.3 GHz to 1 THz. The mixer element is an NbN film deposited on crystalline quartz. The mixer chip is mounted in a waveguide cavity, coupled to free space with a diagonal horn. At microwave frequencies, electromagnetic radiation is applied through the coaxial bias port of the mixer block. At higher frequencies the input signal passes via the diagonal horn feed. At each frequency, the incident power is varied and a family of I-V curves is recorded. From the curves we identify 3 distinct regimes of operation of the mixer separated by the phonon relaxation frequency and the superconducting energy gap frequency observed at about 3 GHz and 660 GHz respectively. In this paper, we will present observed curves and discuss the results of our experiment.
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Lobanov YV, Tong C-YE, Hedden AS, Blundell R, Voronov BM, Gol'tsman GN. Direct measurement of the gain and noise bandwidths of HEB mixers. IEEE Trans Appl Supercond. 2011;21(3):645–8.
Abstract: The intermediate frequency (IF) bandwidth of a hot electron bolometer (HEB) mixer is an important parameter of the mixer, in that it helps to determine its suitability for a given application. With the availability of wideband low noise amplifiers, it is simple to measure the performance of an HEB mixer over a wide range of IF at a fixed LO frequency using the standard Y-factor method. This in-situ method allows us to measure both the gain and noise bandwidths simultaneously. We have also measured mixer output impedance with a vector network analyser. Intrinsic time constant has been extracted from the impedance data and compared to the mixer's bandwidths determined from receiver Y-factor measurement.
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