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Siddiqi I, Prober DE. Nb–Au bilayer hot-electron bolometers for low-noise THz heterodyne detection. Appl Phys Lett. 2004;84(8):1404.
Abstract: The sensitivity of present Nb diffusion-cooled hot-electron bolometer (HEB) mixers is not quantum limited, and can be improved by reducing the superconducting transition temperature TC. Lowering TC reduces thermal fluctuations, resulting in a decrease of the mixer noise temperature TM. However, lower TC mixers have reduced dynamic range and saturate more easily due to background noise. We present 30 GHz microwave measurements on a bilayer HEB system, Nb–Au, in which TC can be tuned with Au layer thickness to obtain the maximum sensitivity for a given noise background. These measurements are intended as a guide for the optimization of THz mixers. Using a Nb–Au mixer with TC = 1.6 K, we obtain TM = 50 K with 2 nW of local oscillator (LO) power. Good mixer performance is observed over a wide range of LO power and bias voltage and such a device should not exhibit saturation in a THz receiver.
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Meledin D, Pavolotsky A, Desmaris V, Lapkin I, Risacher C, Perez V, et al. A 1.3-THz balanced waveguide HEB mixer for the APEX telescope. IEEE Trans. Microw. Theory Techn.. 2009;57(1):89–98.
Abstract: In this paper, we report about the development, fabrication, and characterization of a balanced waveguide hot electron bolometer (HEB) receiver for the Atacama Pathfinder EXperiment telescope covering the frequency band of 1.25–1.39 THz. The receiver uses a quadrature balanced scheme and two HEB mixers, fabricated from 4- to 5-nm-thick NbN film deposited on crystalline quartz substrate with an MgO buffer layer in between. We employed a novel micromachining method to produce all-metal waveguide parts at submicrometer accuracy (the main-mode waveguide dimensions are 90×180 μm). We present details on the mixer design and measurement results, including receiver noise performance, stability and “first-light†at the telescope site. The receiver yields a double-sideband noise temperature averaged over the RF band below 1200 K, and outstanding stability with a spectroscopic Allan time more than 200 s.
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Cherednichenko S, Drakinskiy V, Berg T, Kollberg EL, Angelov I. The direct detection effect in the hot-electron bolometer mixer sensitivity calibration. IEEE Trans. Microw. Theory Techn.. 2007;55(3):504–10.
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Burke PJ, Schoelkopf RJ, Prober DE, Skalare A, Karasik BS, Gaidis MC, et al. Mixing and noise in diffusion and phonon cooled superconducting hot-electron bolometers. J. Appl. Phys.. 1999;85(3):1644–53.
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Cherednichenko S, Drakinskiy V, Baubert J, Krieg J-M, Voronov B, Gol'tsman G, et al. Gain bandwidth of NbN hot-electron bolometer terahertz mixers on 1.5 μm Si3N4 / SiO2 membranes. J Appl Phys. 2007;101(12):124508 (1 to 6).
Abstract: The gain bandwidth of NbN hot-electron bolometer terahertz mixers on electrically thin Si3N4/SiO2 membranes was experimentally investigated and compared with that of HEB mixers on bulk substrates. A gain bandwidth of 3.5 GHz is achieved on bulk silicon, whereas the gain bandwidth is reduced down to 0.6–0.9 GHz for mixers on 1.5 μm Si3N4/SiO2 membranes. We show that application of a MgO buffer layer on the membrane extends the gain bandwidth to 3 GHz. The experimental data were analyzed using the film-substrate acoustic mismatch approach.
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