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Wild W, de Graauw T, Baryshev A, Bos A, Gao JR, Gunst A, et al. Terahertz technology for ESPRIT – a far-infrared space interferometer. In: Proc. 16th Int. Symp. Space Terahertz Technol. Göteborg, Sweden; 2005.
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Khosropanah P, Gao JR, Laauwen WM, Hajenius M, Klapwijk TM. Low noise NbN hot electron bolometer mixer at 4.3 THz. Appl Phys Lett. 2007;91:221111 (1 to 3).
Abstract: We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300 K at the optimum local oscillator (LO) power of 330 nW, which is about 12 times the quantum noise (hnu/2kB). Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations.
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Hajenius M, Baselmans JJA, Gao JR, Klapwijk TM, de Korte PAJ, Voronov B, et al. Improved NbN phonon cooled hot electron bolometer mixers. In: Proc. 14th Int. Symp. Space Terahertz Technol. Tucson, USA; 2003. p. 413–23.
Abstract: NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized with negligible contact resistance to Au pads. By adding either a 5 nm Nb or a 10 nm NbTiN layer between the Au and NbN, to preserve superconductivity in the NbN under the Au contact pad, superior noise temperatures have been obtained. Using DC I,V curves and resistive transitions in combination with process parameters we analyze the nature of these improved devices and determine interface transparencies.
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Barends R, Hajenius M, Gao JR, Klapwijk TM. Current-induced vortex unbinding in bolometer mixers. Appl Phys Lett. 2005;87:263506 (1 to 3).
Abstract: We present a description of the current-voltage characteristics of hot electron bolometers in terms of the current-dependent intrinsic resistive transition of NbN films. We find that, by including this current dependence, we can correctly predict the complete current-voltage characteristics, showing excellent agreement with measurements for both low and high bias and for small as well as large devices. It is assumed that the current dependence is due to vortex-antivortex unbinding as described in the Berezinskii–Kosterlitz–Thouless theory. The presented approach will be useful in guiding device optimization for noise and bandwidth.
Keywords: HEB mixer numerical model, HEB model, IV-curves, vortex-antivortex, Berezinskii–Kosterlitz–Thouless theory, diffusion cooling channel, diffusion channel, distributed HEB model, distributed model, self-heating effect, temperature profile
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Gao JR, Hovenier JN, Yang ZQ, Baselmans JJA, Baryshev A, Hajenius M, et al. Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer. Appl Phys Lett. 2005;86:244104 (1 to 3).
Abstract: We report the first demonstration of an all solid-stateheterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.
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Klapwijk TM, Barends R, Gao JR, Hajenius M, Baselmans JJA. Improved superconducting hot-electron bolometer devices for the THz range. In: Proc. SPIE. Vol 5498.; 2004. p. 129–39.
Abstract: Improved and reproducible heterodyne mixing (noise temperatures of 950 K at 2.5 THz) has been realized with NbN based hot-electron superconducting devices with low contact resistances. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, has been used to understand the physical conditions during the mixing process. We find that the mixing is predominantly due to the exponential rise of the local resistivity as a function of electron temperature.
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Zhang W, Miao W, Yao QJ, Lin ZH, Shi SC, Gao JR, et al. Spectral response and noise temperature of a 2.5 THz spiral antenna coupled NbN HEB mixer. Phys Procedia. 2012;36:334–7.
Abstract: We report on a 2.5 THz spiral antenna coupled NbN hot electron bolometer (HEB) mixers, fabricated with in-situ process. The receiver noise temperature with lowest value of 1180 K is in good agreement with calculated quantum efficiency factor as a function of bias voltage. In addition, the measured spectral response of the spiral antenna coupled NbN HEB mixer shows broad frequency coverage of 0.8-3 THz, and corrected response for optical losses, FTS, and coupling efficiency between antenna and bolometer falls with frequency due to diffraction-limited beam of lens/antenna combination.
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Zhang W, Miao W, Zhong JQ, Shi SC, Hayton DJ, Vercruyssen N, et al. Temperature dependence of superconducting hot electron bolometers. In: Not published results: 24th international symposium on space terahertz technology.; 2013.
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Baselmans JJA, Baryshev A, Hajenius M, Gao JR, Klapwijk TM, Voronov B, et al. Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers [abstract]. In: Proc. 17th Int. Symp. Space Terahertz Technol.; 2006. 81.
Abstract: We present a detailed experimental study of the direct detection effect in a small volume (0.15pm x lpm) NbN hot electron bolometer mixer. It is a quasioptical mixer with a twin slot antenna designed for 700 GHz and the measurement was done at a LO frequency of 670 GHz. The direct detection effect is characterized by a change in the mixer bias current when switching broadband radiation from a 300 K hot load to a 77 K cold load in a standard Y factor measurement. The result is, depending on the receiver under study, an increase or decrease in the receiver noise temperature. We find that the small signal noise temperature, which is the noise temperature that would be observed without the presence of the direct detection effect, and thus the one that is relevant for an astronomical observation, is 20% lower than the noise temperature obtained using 300 K and 77 K calibration loads. Thus, in our case the direct detection effect reduces the mixer sensitivity. These results are in good agreement with previous measurement at THz frequencies [1]. Other experiments report an increase in mixer sensitivity [2]. To analyze this discrepancy we have designed a separate set of experiments to find out the physical origin of the direct detection effect. Possible candidates are the bias current dependence of the mixer gain and the bias current dependence of the IF match. We measured directly the change in mixer IF match and receiver gain due to the direct detection effect. From these measurements we conclude that the direct detection effect is caused by a combination of bias current reduction when switching form the 77 K to the 300 K load in combination with the bias current dependence of the receiver gain. The bias current dependence of the receiver gain is shown to be mainly caused by the current dependence of the mixer gain. We also find that an increase in receiver sensitivity due to the direct detection effect is only possible if the noise temperature change due to the direct detection is dominated by the mixer-amplifier IF match. [1] J.J.A. Baselmans, A. Baryshev, S.F. Reker, M. Hajenius, J.R. Gao, T.M. Klapwijk, Yu.Vachtomin, S. Maslennikov, S. Antipov, B. Voronov, and G. Gol'tsman., Appl. Phys. Lett. 86, 163503 (2005). [2] S. Svechnokov, A. Verevkin, B. Voronov, E. Menschikov. E. Gershenzon, G. Gol'tsman, 9th Int. Symp. On Space THz. Techn., 45, (1999).
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Gao JR, Hajenius M, Tichelaar FD, Voronov B, Grishina E, Klapwijk TM, et al. Can NbN films on 3C-SiC/Si change the IF bandwidth of hot electron bolometer mixers? In: Proc. 17th Int. Symp. Space Terahertz Technol.; 2006. p. 187–9.
Abstract: We realized ultra thin NbN films sputtered grown on a 3C-SiC/Si substrate. The film with a thickness of 3.5-4.5 nm shows a 1', of 11.8 K, which is the highest I`, observed among ultra thin NbN films on different substrates. The high-resolution transmission electron microscopy (HRTEM) studies show that the film has a monocrystalline structure, confirming the epitaxial growth on the 3C-SiC. Based on a two-temperature model and input parameters from standard NbN films on Si, simulations predict that the new film can increase the IF bandwidth of a HEB mixer by about a factor of 2 in comparison to the standard films. In addition, we find standard NbN films on Si with a T c of 9.4 K have a thickness of around 5.5 nm, being thicker than expected (3.5 nm).
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