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Men’shchikov, E. M., Gogidze, I. G., Sergeev, A. V., Elant’ev, A. I., Kuminov, P. B., Gol’tsman, G. N., et al. (1997). Superconducting fast detector based on the nonequilibrium inductance response of a film of niobium nitride. Tech. Phys. Lett., 23(6), 486–488.
Abstract: A new type of fast detector is proposed, whose operation is based on the variation of the kinetic inductance of a superconducting film caused by nonequilibrium quasiparticles created by the electromagnetic radiation. The speed of the detector is determined by the rate of multiplication of photo-excited quasiparticles, and is nearly independent of the temperature, being less than 1 ps for NbN. Models based on the Owen-Scalapino scheme give a good description of the experimentally determined dependence of the power-voltage sensitivity of the detector on the modulation frequency. The lifetime of the quasiparticles is determined, and it is shown that the reabsorption of nonequilibrium phonons by the condensate has a substantial effect even in ultrathin NbN films 5 nm thick, and results in the maximum possible quantum yield. A low concentration of equilibrium quasiparticles and a high quantum yield result in a detectivity D*=1012 W−1·Hz1/2 at a temperature T=4.2 K and D*=1016 W−1·cm· Hz1/2 at T=1.6 K.
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Tong, C. - Y. E., Kawamura, J., Todd, R. H., Papa, D. C., Blundell, R., Smith, M., et al. (2000). Successful operation of a 1 THz NbN hot-electron bolometer receiver. In Proc. 11th Int. Symp. Space Terahertz Technol. (pp. 49–59).
Abstract: A phonon-cooled NbN superconductive hot-electron bolometer receiver covering the frequency range 0.8-1.04 THz has successfully been used for astronomical observation at the Sub-Millimeter Telescope Observatory on Mount Graham, Arizona. This waveguide heterodyne receiver is a modified version of our fixed-tuned 800 GHz HEB receiver to allow for operation beyond 1 THz. The measured noise temperature of this receiver is about 1250 K at 0.81 THz, 560 K at 0.84 THz, and 1600 K at 1.035 THz. It has a 1 GHz wide IF bandwidth, centered at 1.8 GHz. This receiver has recently been used to detect the CO (9-8) molecular line emission at 1.037 THz in the Orion nebula. This is the first time a ground-based heterodyne receiver has been used to detect a celestial source above 1 THz.
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Meledin, D., Tong, C. Y. - E., Blundell, R., Kaurova, N., Smirnov, K., Voronov, B., et al. (2003). Study of the IF bandwidth of NbN HEB mixers based on crystalline quartz substrate with an MgO buffer layer. IEEE Trans. Appl. Supercond., 13(2), 164–167.
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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Ryabchun, S., Tong, C. - Y. E., Blundell, R., Kimberk, R., & Gol'tsman, G. (2007). Study of the effect of microwave radiation on the operation of HEB mixers in the terahertz frequency range. IEEE Trans. Appl. Supercond., 17(2), 391–394.
Abstract: We have investigated the effect of injecting microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the hot-electron bolometer mixer incorporated into a THz heterodyne receiver. More specifically, we show that exposing the mixer to microwave radiation does not cause a significant rise of the receiver noise temperature and fall of the mixer conversion gain so long as the microwave power is a small fraction of local oscillator power. The injection of a small, but controlled amount of microwave power therefore enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the gain stability of hot electron bolometer mixer receivers.
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Krause, S., Mityashkin, V., Antipov, S., Gol'tsman, G., Meledin, D., Desmaris, V., et al. (2016). Study of IF bandwidth of NbN hot electron bolometers on GaN buffer layer using a direct measurement method. In Proc. 27th Int. Symp. Space Terahertz Technol. (pp. 30–32).
Abstract: In this paper, we present a reliable measurement method to study the influence of the GaN buffer layer on phonon-escape time in comparison with commonly used Si substrates and, in consequence, on the IF bandwidth of HEBs. One of the key aspects is to operate the HEB mixer at elevated bath temperatures close to the critical temperature of the NbN ultra-thin film, where contributions from electron-phonon processes and self-heating effects are relatively small, therefore IF roll-off will be governed by the phonon-escape.Two independent experiments were performed at GARD and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. The entire IF chain was characterized by S-parameter measurements. We compared the measurement results of epitaxial NbN grown onto GaN buffer-layer with Tc of 12.5 K (4.5nm) with high quality polycrystalline NbN films on Si substrate with Tc of 10.5K (5nm) and observed a strong indication of an enhancement of phonon escape to the substrate by a factor of two for the NbN/GaN material combination.
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