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Semenov, A. D., Hübers, H. –W., Schubert, J., Gol'tsman, G. N., Elantiev, A. I., Voronov, B. M., et al. (2000). Frequency dependent noise temperature of the lattice cooled hot-electron terahertz mixer. In Proc. 11th Int. Symp. Space Terahertz Technol. (pp. 39–48).
Abstract: We present the measurements and the theoretical model on the frequency dependent noise temperature of a lattice cooled hot electron bolometer (HEB) mixer in the terahertz frequency range. The experimentally observed increase of the noise temperature with frequency is a cumulative effect of the non-uniform distribution of the high frequency current in the bolometer and the charge imbalance, which occurs near the edges of the normal domain and contacts with normal metal. In addition, we present experimental results which show that the noise temperature of a HEB mixer can be reduced by about 30% due to a Parylene antireflection coating on the Silicon hyperhemispheric lens.
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Semenov, A. D., Hübers, H. - W., Schubert, J., Gol'tsman, G. N., Elantiev, A. I., Voronov, B. M., et al. (2000). Design and performance of the lattice-cooled hot-electron terahertz mixer. J. Appl. Phys., 88(11), 6758–6767.
Abstract: We present the measurements and the theoreticalmodel of the frequency-dependent noise temperature of a superconductor lattice-cooled hot-electron bolometer mixer in the terahertz frequency range. The increase of the noise temperature with frequency is a cumulative effect of the nonuniform distribution of the high-frequency current in the bolometer and the charge imbalance, which occurs at the edges of the normal domain and at the contacts with normal metal. We show that under optimal operation the fluctuation sensitivity of the mixer is determined by thermodynamic fluctuations of the noise power, whereas at small biases there appears additional noise, which is probably due to the flux flow. We propose the prescription of how to minimize the influence of the current distribution on the mixer performance.
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Cherednichenko, S., Kroug, M., Merkel, H., Kollberg, E., Loudkov, D., Smirnov, K., et al. (2001). Local oscillator power requirement and saturation effects in NbN HEB mixers. In C. Iit.u.t.e of T. Jet Propulsion Laboratory (Ed.), Proc. 12th Int. Symp. Space Terahertz Technol. (pp. 273–285). San Diego, CA, USA.
Abstract: The local oscillator power required for NbN hot-electron bolometric mixers (P LO ) was investigated with respect to mixer size, critical temperature and ambient temperature. P LO can be decreased by a factor of 10 as the mixer size decreases from 4×0.4 µm 2 to 0.6×0.13 µm 2 . For the smallest volume mixer the optimal local oscillator power was found to be 15 nW. We found that for such mixer no signal compression was observed up to an input signal of 2 nW which corresponds to an equivalent input load of 20,000 K. For a constant mixer volume, reduction of T c can decrease optimal local oscillator power at least by a factor of 2 without a deterioration of the receiver noise temperature. Bath temperature was found to have minor effect on the receiver characteristics.
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Kawamura, J., Hunter, T. R., Tong, C. Y. E., Blundell, R., Papa, D. C., Patt, F., et al. (2002). Ground-based terahertz CO spectroscopy towards Orion. A&A, 394(1), 271–274.
Abstract: Using a superconductive hot-electron bolometer heterodyne receiver on the 10-m Heinrich Hertz Telescope on Mount Graham, Arizona, we have obtained velocity-resolved 1.037 THz CO () spectra toward several positions along the Orion Molecular Cloud (OMC-1) ridge. We confirm the general results of prior observations of high-J CO lines that show that the high temperature, , high density molecular gas, , is quite extended, found along a ~ region centered on BN/KL. However, our observations have significantly improved angular resolution, and with a beam size of we are able to spatially and kinematically discriminate the emission originating in the extended quiescent ridge from the very strong and broadened emission originating in the compact molecular outflow. The ridge emission very close to the BN/KL region appears to originate from two distinct clouds along the line of sight with and ≈ . The former component dominates the emission to the south of BN/KL and the latter to the north, with a turnover point coincident with or near BN/KL. Our evidence precludes a simple rotation of the inner ridge and lends support to a model in which there are multiple molecular clouds along the line of sight towards the Orion ridge.
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Hübers, H. - W., Semenov, A. D., Richter, H., Schubert, J., Hadjiloucas, S., Bowen, J. W., et al. (2001). Antenna pattern of the quasi-optical hot-electron bolometric mixer at terahertz frequencies. In Proc. 12th Int. Symp. Space Terahertz Technol. (pp. 286–296). San Diego, CA, USA.
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Schwaab, G. W., Sirmain, G., Schubert, J., Hubers, H. - W., Gol'tsman, G., Cherednichenko, S., et al. (1999). Investigation of NbN phonon-cooled HEB mixers at 2.5 THz. IEEE Trans. Appl. Supercond., 9(2), 4233–4236.
Abstract: The development of superconducting hot electron bolometric (HEB) mixers has been a big step forward in the direction of quantum noise limited mixer performance at THz frequencies. Such mixers are crucial for the upcoming generation of airborne and spaceborne THz heterodyne receivers. In this paper we report on new results on a phonon-cooled NbN HEB mixer using e-beam lithography. The superconducting film is 3 nm thick. The mixer is 0.2 μm long and 1.5 μm wide and it is integrated in a spiral antenna on a Si substrate. The device is quasi-optically coupled through a Si lens and a dielectric beam combiner to the radiation of an optically pumped FIR ring gas laser cavity. The performance of the mixer at different THz frequencies from 0.69 to 2.55 THz with an emphasis on 2.52 THz is demonstrated. At 2.52 THz minimum DSB noise temperatures of 4200 K have been achieved at an IF of 1.5 GHz and a bandwidth of 40 MHz with the mixer mounted in a cryostat and a 0.8 m long signal path in air.
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Kawamura, J., Blundell, R., Tong, C. - Y. E., Papa, D. C., Hunter, T. R., Paine, S. N., et al. (2000). Superconductive hot-electron-bolometer mixer receiver for 800-GHz operation. IEEE Trans. Microw. Theory Techn., 48(4), 683–689.
Abstract: In this paper, we describe a superconductive hot-electron-bolometer mixer receiver designed to operate in the partially transmissive 350-μm atmospheric window. The receiver employs an NbN thin-film microbridge as the mixer element, in which the main cooling mechanism of the hot electrons is through electron-phonon interaction. At a local-oscillator frequency of 808 GHz, the measured double-sideband receiver noise temperature is TRX=970 K, across a 1-GHz intermediate-frequency bandwidth centered at 1.8 GHz. We have measured the linearity of the receiver and the amount of local-oscillator power incident on the mixer for optimal operation, which is PLO≈1 μW. This receiver was used in making observations as a facility instrument at the Heinrich Hertz Telescope, Mt. Graham, AZ, during the 1998-1999 winter observing season.
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Il'in, K. S., Lindgren, M., Currie, M. A., Semenov, D., Gol'tsman, G. N., Sobolewski, R., et al. (2000). Picosecond hot-electron energy relaxation in NbN superconducting photodetectors. Appl. Phys. Lett., 76(19), 2752–2754.
Abstract: We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time Ï„e–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time Ï„es decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, Ï„e–ph and Ï„es, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of Ï„e–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/–3) GHz if one eliminates the bolometric phonon-heating effect.
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Yagoubov, P., Kroug, M., Merkel, H., Kollberg, E., Gol'tsman, G., Svechnikov, S., et al. (1998). Noise temperature and local oscillator power requirement of NbN phonon-cooled hot electron bolometric mixers at terahertz frequencies. Appl. Phys. Lett., 73(19), 2814–2816.
Abstract: In this letter, the noise performance of NbN-based phonon-cooled hot electron bolometric quasioptical mixers is investigated in the 0.55–1.1 THz frequency range. The best results of the double-sideband <cd><2018>DSB<cd><2019> noise temperature are: 500 K at 640 GHz, 600 K at 750 GHz, 850 K at 910 GHz, and 1250 K at 1.1 THz. The water vapor in the signal path causes significant contribution to the measured receiver noise temperature around 1.1 THz. The devices are made from 3-nm-thick NbN film on high-resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are typically 0.2Ï«2 um. The amount of local oscillator power absorbed in the bolometer is less than 100 nW.
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Kawamura, J., Blundell, R., Tong, C. - Y. E., Golts'man, G., Gershenzon, E., & Voronov B. (1996). Superconductive NbN hot-electron bolometric mixer performance at 250 GHz. In Proc. 7th Int. Symp. Space Terahertz Technol. (pp. 331–336).
Abstract: Thin film NbN (<40 A) strips are used as waveguide mixer elements. The electron cooling mechanism for the geometry is the electron-phonon interaction. We report a receiver noise temperature of 750 K at 244 GHz, with / IF = 1.5 GHz, Af= 500 MHz, and Tphysical = 4 K. The instantaneous bandwidth for this mixer is 1.6 GHz. The local oscillator (LO) power is 0.5 1.tW with 3 dB-uncertainty. The mixer is linear to 1 dB up to an input power level 6 dB below the LO power. We report the first detection of a molecular line emission using this class of mixer, and that the receiver noise temperature determined from Y-factor measurements reflects the true heterodyne sensitivity.
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