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Meledin D, Tong CY-E, Blundell R, Kaurova N, Smirnov K, Voronov B, et al. Study of the IF bandwidth of NbN HEB mixers based on crystalline quartz substrate with an MgO buffer layer. IEEE Trans Appl Supercond. 2003;13(2):164–7.
Abstract: In this paper, we present the results of IF bandwidth measurements on 3-4 nm thick NbN hot electron bolometer waveguide mixers, which have been fabricated on a 200-nm thick MgO buffer layer deposited on a crystalline quartz substrate. The 3-dB IF bandwidth, measured at an LO frequency of 0.81 THz, is 3.7 GHz at the optimal bias point for low noise receiver operation. We have also made measurements of the IF dynamic impedance, which allow us to evaluate the intrinsic electron temperature relaxation time and self-heating parameters at different bias conditions.
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Tretyakov I, Ryabchun S, Finkel M, Maslennikova A, Kaurova N, Lobastova A, et al. Low noise and wide bandwidth of NbN hot-electron bolometer mixers. Appl Phys Lett. 2011;98:033507 (1 to 3).
Abstract: We report a record double sideband noise temperature of 600 K (5hν/kB) offered by a NbN hot-electron bolometer receiver at 2.5 THz. Allowing for standing wave effects, this value was found to be constant in the intermediate frequency range 1–7 GHz, which indicates that the mixer has an unprecedentedly large noise bandwidth in excess of 7 GHz. The insight into this is provided by gain bandwidth measurements performed at the superconducting transition. They show that the dependence of the bandwidth on the mixer length follows the model for an HEB mixer with diffusion and phonon cooling of the hot electrons.
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Tret’yakov IV, Ryabchun SA, Kaurova NS, Larionov PA, Lobastova AA, Voronov BM, et al. Optimum absorbed heterodyne power for superconducting NbN hot-electron bolometer mixer. Tech Phys Lett. 2010;36(12):1103–5.
Abstract: Absorbed heterodyne power has been measured in a low-noise broadband hot-electron bolometer (HEB) mixer for the terahertz range, operating on the effect of electron heating in the resistive state of an ultrathin superconducting NbN film. It is established that the optimum absorbed heterodyne power for the HEB mixer operating at 2.5 THz is about 100 nW.
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Zhang W, Miao W, Yao QJ, Lin ZH, Shi SC, Gao JR, et al. Spectral response and noise temperature of a 2.5 THz spiral antenna coupled NbN HEB mixer. Phys Procedia. 2012;36:334–7.
Abstract: We report on a 2.5 THz spiral antenna coupled NbN hot electron bolometer (HEB) mixers, fabricated with in-situ process. The receiver noise temperature with lowest value of 1180 K is in good agreement with calculated quantum efficiency factor as a function of bias voltage. In addition, the measured spectral response of the spiral antenna coupled NbN HEB mixer shows broad frequency coverage of 0.8-3 THz, and corrected response for optical losses, FTS, and coupling efficiency between antenna and bolometer falls with frequency due to diffraction-limited beam of lens/antenna combination.
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Semenov A, Richter H, Hübers H-W, Petrenko D, Tretyakov I, Ryabchun S, et al. Optimization of the intermediate frequency bandwidth in the THz HEB mixers [abstract]. In: Proc. 25th Int. Symp. Space Terahertz Technol.; 2014. 54.
Abstract: We report on the studies of the intermediate frequency (IF) bandwidth of quasi-optically coupled NbN hot-electron bolometer (HEB) mixers which are aimed at the optimization of the mixer performance at terahertz frequencies. Extension of the IF bandwidth due to the contribution of electron diffusion to the heat removal from NbN microbolometers has been already demonstrated for NbN HEBs at subterahertz frequencies. However, reducing the size of the microbolometer causes degradation of the noise temperature. Using in-situ multilayer manufacturing process we succeeded to improve the transparency of the contacts for electrons which go away from microbolometer to the metallic antenna. The improved transparency and hence coupling efficiency counterbalances the noise temperature degradation. HEB mixers were tested in a laboratory heterodyne receiver with a narrow-band cold filter which allowed us to eliminate direct detection. We used a local oscillator with a quantum cascade laser (QCL) at a frequency of 4.745 THz [1] which was developed for the H-Channel of the German Receiver for Astronomy at Terahertz frequencies (GREAT). Both the noise and gain bandwidth were measured in the IF range from 0.5 to 8 GHz using the hot-cold technique and preliminary calibrated IF analyzer with a tunable microwave filter. For optimized HEB geometry we found the noise bandwidth as large as 7 GHz. We compare our results with the conventional and the hot-spot mixer models and show that further extension of the IF bandwidth should be possible via improving the sharpness of the superconducting transition. The cross characterization of the HEB mixer was performed in the test bed of GREAT at the Max-Planck-Institut für Radioastronomie with the same QCL LO and delivered results which were consistent with the laboratory studies.
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Seliverstov SV, Rusova AA, Kaurova NS, Voronov BM, Goltsman GN. AC-biased superconducting NbN hot-electron bolometer for frequency-domain multiplexing. In: Proc. 28th Int. Symp. Space Terahertz Technol.; 2017. p. 120–2.
Abstract: We present the results of characterization of fast and sensitive superconducting antenna-coupled THz direct detector based on NbN hot-electron bolometer (HEB) with AC-bias. We discuss the possibility of implementation of the AC-bias for design the readout system from the multi-element arrays of HEBs using standard technique of frequency-domain multiplexing. We demonstrate experimentally that this approach does not lead to significant deterioration of the HEB sensitivity compared with the value obtained for the same detector with DC- bias. Results of a numerical calculations of the HEB responsivity at AC-bias are in a good agreement with the experiment.
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Tretyakov IV, Anfertyev VA, Revin LS, Kaurova NS, Voronov BM, Vaks VL, et al. Sensitivity and resolution of a heterodyne receiver based on the NbN HEB mixer with a quantum-cascade laser as a local oscillator. Radiophys Quant Electron. 2018;60(12):988–92.
Abstract: We present the results of experimental studies of the basic characteristics and operation features of a terahertz heterodyne detector based on the superconducting NbN HEB mixer and a quantum cascade laser as a local oscillator operating at a frequency of 2.02 THz. The measured noise temperature of such a mixer amounted to 1500 K. The spectral resolution of the detector is determined by the width of the local-oscillator spectral line whose measured value does not exceed 1 MHz. The quantum-cascade laser could be linearly tuned with respect to frequency with the coefficient 7.2 MHz/mA within the limits of the current oscillation bandwidth.
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Tretyakov I, Kaurova N, Voronov BM, Goltsman GN. About effect of the temperature operating conditions on the noise temperature and noise bandwidth of the terahertz range NbN hot-electron bolometers [abstract]. In: Proc. 29th Int. Symp. Space Terahertz Technol.; 2018. 113.
Abstract: Results of an experimental study of the noise temperature (Tn) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (Tb) and NbN bridge length are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (Tc) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature Tb close to Tc was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at Tb ≪ Tc for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.
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Krause S, Mityashkin V, Antipov S, Gol’tsman G, Meledin D, Desmaris V, et al. Reduction of phonon escape time for nbn hot electron bolometers by using gan buffer layers. IEEE Trans Terahertz Sci Technol. 2017;7(1):53–9.
Abstract: In this paper, we investigated the influence of the GaN buffer layer on the phonon escape time of phonon-cooled hot electron bolometers (HEBs) based on NbN material and compared our findings to conventionally employed Si substrate. The presented experimental setup and operation of the HEB close to the critical temperature of the NbN film allowed for the extraction of phonon escape time in a simplified manner. Two independent experiments were performed at GARD/Chalmers and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. By fitting the normalized IF measurement data to the heat balance equations, the escape time as a fitting parameter has been deduced and amounts to 45 ps for the HEB based on Si substrate as in contrast to a significantly reduced escape time of 18 ps for the HEB utilizing the GaN buffer layer under the assumption that no additional electron diffusion has taken place. This study indicates a high phonon transmissivity of the NbN-to-GaN interface and a prospective increase of IF bandwidth for HEB made of NbN on GaN buffer layers, which is desirable for future THz HEB heterodyne receivers.
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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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