Vachtomin, Y. B., Antipov, S. V., Maslennikov, S. N., Smirnov, K. V., Polyakov, S. L., Kaurova, N. S., et al. (2004). Noise temperature measurements of NbN phonon-cooled hot electron bolometer mixer at 2.5 and 3.8 THz. In Proc. 15th Int. Symp. Space Terahertz Technol. (pp. 236–241). Northampton, Massachusetts, USA.
Abstract: We present the results of noise temperature measurements of NbN phonon-cooled HEB mixers based on a 3.5 nm NbN film deposited on a high-resistivity Si substrate with a 200 nm – thick MgO buffer layer. The mixer element was integrated with a log-periodic spiral antenna. The noise temperature measurements were performed at 2.5 THz and at 3.8 THz local oscillator frequencies for the 3 µm x 0.2 µm active area devices. The best uncorrected receiver noise temperatures found for these frequencies are 1300 K and 3100 K, respectively. A water vapour discharge laser was used as the LO source. We also present the results of direct detection contribution to the measured Y-factor and of a possible error of noise temperature calculation. This error was more than 8% for the mixer with in-plane dimensions of 2.4 x 0.16 µm 2 at the optimal noise temperature point. The use of a mesh filter enabled us to avoid the effect of direct detection and decrease optical losses by 0.5 dB. The paper is concluded by the investigation results of the mixer polarization response. It was shown that the polarization can differ from the circular one at 3.8 THz by more than 2 dB.
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Antipov, S. V., Vachtomin, Y. B., Maslennikov, S. N., Smirnov, K. V., Kaurova, N. S., Grishina, E. V., et al. (2004). Noise performance of quasioptical ultrathin NbN hot electron bolometer mixer at 2.5 and 3.8 THz. In Proc. 5-th MSMW (Vol. 2, pp. 592–594). Kharkov, Ukraine.
Abstract: To put space-based and airborne heterodyne instruments into operation at frequencies above 1 THz the superconducting NbN hot-electron bolometer (HEB) will be incorporated into heterodyne receiver as a mixer. At frequencies above 1.3 THz the sensitivity of the NbN HEB mixers outperform the one of the Schottky diodes and SIS-mixers, and the receiver noise temperature of the NbN HEB mixers increase with frequency. In this paper we present the results of the noise temperature measurements within one batch of NbN HEB mixers based on 3.5 mn thick superconducting NbN film grown on Si substrate with MgO buffer layer at the LO frequencies 2.5 THz and 3.8 THz.
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Baselmans, J. J. A., Hajenius, M., Gao, J. R., Klapwijk, T. M., de Korte, P. A. J., Voronov, B., et al. (2004). Doubling of sensitivity and bandwidth in phonon cooled hot electron bolometer mixers. Appl. Phys. Lett., 84(11), 1958–1960.
Abstract: We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. We show experimentally that both the receiver noise temperature and the gain bandwidth can be improved by more than a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature TN,DSB=950 K
at 2.5 THz and 4.3 K, uncorrected for losses in the optics. At the same bias point, we obtain an IF gain bandwidth of 6 GHz.
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Kaurova, N. S., Finkel, M. I., Maslennikov, S. N., Vahtomin, Y. B., Antipov, S. V., Smirnov, K. V., et al. (2004). Submillimeter mixer based on YBa2Cu3O7-x thin film. In Proc. 1-st conf. Fundamental problems of high temperature superconductivity (291). Moscow-Zvenigorod.
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Bruderer, S., Benz, A. O., van Dishoeck, E. F., Melchior, M., Doty, S. D., van der Tak, F., et al. (2010). Herschel/HIFI detections of hydrides towards AFGL 2591. Envelope emission versus tenuous cloud absorption. Astron. Astrophys., 521, L44 (1 to 7).
Abstract: The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory allows the first observations of light diatomic molecules at high spectral resolution and in multiple transitions. Here, we report deep integrations using HIFI in different lines of hydrides towards the high-mass star forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+ and SH+ have not been detected. All molecules except for CH and CH+ are seen in absorption with low excitation temperatures and at velocities different from the systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P = 3/22,- – 1/21,+ ) and CH+(J = 1–0, J = 2–1) lines are detected in emission at the systemic velocity. We can assign the absorption features to a foreground cloud and an outflow lobe, while the CH and CH+ emission stems from the envelope. The observed abundance and excitation of CH and CH+ can be explained in the scenario of FUV irradiated outflow walls, where a cavity etched out by the outflow allows protostellar FUV photons to irradiate and heat the envelope at larger distances driving the chemical reactions that produce these molecules.
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