Hubers, H. - W., Semenov, A., Richter, H., Schwarz, M., Gunther, B., Smirnov, K., et al. (2004). Heterodyne receiver for 3-5 THz with hot-electron bolometer mixer. In J. Zmuidzinas, W. S. Holland, & S. Withington (Eds.), Proc. SPIE (Vol. 5498, pp. 579–586). SPIE.
Abstract: Heterodyne receivers for applications in astronomy and planetary research need quantum limited sensitivity. In instruments which are currently build for SOFIA and Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. The local oscillator and the mixer are the most critical components for a heterodyne receiver operating at 3-5 THz. The design and performance of an optically pumped THz gas laser optimized for this frequency band will be presented. In order to optimize the performance for this frequency hot electron bolometer mixers with different in-plane dimensions and logarithmic-spiral feed antennas have been investigated. Their noise temperatures and beam patterns were measured. Above 3 THz the best performance was achieved with a superconducting bridge of 2.0 x 0.2 μm2 incorporated in a logarithmic spiral antenna. The DSB noise temperatures were 2700 K, 4700 K and 6400 K at 3.1 THz, 4.3 THz and 5.2 THz, respectively. The results demonstrate that the NbN HEB is very well suited as a mixer for THz heterodyne receivers up to at least 5 THz.
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Cherednichenko, S., Khosropanah, P., Berg, T., Merkel, H., Kollberg, E., Drakinskiy, V., et al. (2004). Optimization of HEB mixer for the Herschel Space Observatory. In Proc. 15th Int. Symp. Space Terahertz Technol. (16).
Abstract: A mixer development for the HIFI instrument of the Herschel Space Observatory has come to the final stage. In our paper and conference presentation we will describe the most important details of the Band 6 Low and High Mixer Unit design. Special attention will be given to the optimization of the hot- electron bolometer mixer chip, which is based on 3.5nm NbN superconducting film on silicon. As the HEB’s local oscillator power requirements depend on the bolometer size, we have compared mixer noise temperature for different bolometer width- to- length ratio. A trade- off between mixer performance and local oscillator power requirements results in the mixer units equipped with optimized mixer chips, providing the largest coverage of the Band6 RF band with the lowest possible receiver noise. A short account of the beam pattern measurements of Band6 mixers will be given as well.
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Baselmans, J. J. A., Hajenius, M., Gao, J. R., Baryshev, A., Kooi, J., Klapwijk, T. M., et al. (2004). Hot electron bolometer mixers with improved interfaces: sensitivity, LO power and stability. In Proc. 15th Int. Symp. Space Terahertz Technol. (pp. 17–24).
Abstract: We study twin slot antenna coupled NbN hot electron bolometer mixers with an improved contact structure and a small volume, ranging from 1 µm × 0.1 µm to 2 × 0.3 µm. We obtain a DSB receiver noise temperature of 900 K at 1.6 THz and 940 K at 1.9 THz. To explore the practical usability of such small HEB mixers we evaluate the LO power requirement, the sensitivity and the stability. We find that the LO power requirement of the smallest mixers is reduced to about 240 nW at the Si lens of the mixer. This value is larger than expected from the isothermal technique and the known losses in the lens by a factor of 3-3.5. The stability of these receivers is characterized using a measurement of the Allan Variance. We find an Allan time of 0.5 sec. in an 80 MHz bandwidth. A small increase in stability can be reached by using a higher bias at the expense of a significant amount of sensitivity. The stability is sufficient for spectroscopic applications in a 1 MHz bandwidth at a 1 Hz chopping frequency.
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Semenov, A. D., Hübers, H. - W., Richter, H., Smirnov, K., Gol'tsman, G. N., & Voronov, B. M. (2004). Superconducting hot-electron bolometer mixer for terahertz heterodyne receivers. In Proc. 15th Int. Symp. Space Terahertz Technol. (164).
Abstract: A number of on-going astronomical and atmospheric research programs are aimed to the Terahertz (THz) spectral region. At frequencies above about 1.4 THz heterodyne receivers planned for these missions will use superconducting hot-electron bolometers as a mixers. We present current results on the development of superconducting NbN hot- electron bolometer mixer and quasioptical radiation coupling scheme for GREAT (German Receiver for Astronomy at Terahertz Frequencies, to be used aboard of SOFIA) and TELIS (Terahertz Limb Sounder). The mixer is incorporated into hybrid antenna consisting of a planar feed antenna, which has either logarithmic spiral or double-slot configuration, and hyperhemispherical silicon lens. For the log-spiral feed antenna, the double side-band receiver noise temperature of 5500 K was achieved at 4.3 THz. The noise temperature shows less than 3 dB increase in the intermediate frequency band from 4 GHz to 7 GHz. The hybrid antenna had almost frequency independent and symmetric radiation pattern with the beam-width slightly broader than expected for a diffraction limited pattern. Results of FTS measurements in the direct detection regime agreed with the spectral dependence of the noise temperature for spiral antennas with different spacing of inner terminals.
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Hajenius, M., Baselmans, J. J. A., Gao, J. R., Klapwijk, T. M., de Korte 2, P. A. J., Voronov, B., et al. (2004). Increased bandwidth of NbN phonon cooled hot electron bolometer mixers. In Proc. 15th Int. Symp. Space Terahertz Technol. (pp. 381–386).
Abstract: We study experimentally the IF gain bandwidth of NbN phonon-cooled hot-electron-bolometer (HEB) mixers for a set of devices with different contact structures but an identical NbN film. We observe that the IF bandwidth depends strongly on the exact contact structure and find an IF gain bandwidth of 6 GHz for a device with an additional superconducting layer (NbTiN) in between the active NbN film and the gold contact to the antenna. These results contradict the common opinion that the IF bandwidth is determined by the phonon-escape time between the NbN film and the substrate. Hence we calculate the IF gain bandwidth of a superconducting film using a two-temperature model. We find that the bandwidth increases strongly with operating temperature and is not limited by the phonon escape time. This is because of strong temperature dependence of the phonon specific heat in the NbN film.
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