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Vercruyssen, N., Verhagen, T. G. A., Flokstra, M. G., Pekola, J. P., & Klapwijk, T. M. (2012). Evanescent states and nonequilibrium in driven superconducting nanowires. Phys. Rev. B, 85, 224503(1–10).
Abstract: We study the nonlinear response of current transport in a superconducting diffusive nanowire between normal reservoirs. We demonstrate theoretically and experimentally the existence of two different superconducting states appearing when the wire is driven out of equilibrium by an applied bias, called the global and bimodal superconducting states. The different states are identified by using two-probe measurements of the wire, and measurements of the local density of states with tunneling probes. The analysis is performed within the framework of the quasiclassical kinetic equations for diffusive superconductors.
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Gao, J. R., Hovenier, J. N., Yang, Z. Q., Baselmans, J. J. A., Baryshev, A., Hajenius, M., et al. (2005). Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer. Appl. Phys. Lett., 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, T. M., Barends, R., Gao, J. R., Hajenius, M., & Baselmans, J. J. A. (2004). Improved superconducting hot-electron bolometer devices for the THz range. In Proc. SPIE (Vol. 5498, pp. 129–139).
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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Bueno, J., Coumou, P. C. J. J., Zheng, G., de Visser, P. J., Klapwijk, T. M., Driessen, E. F. C., et al. (2014). Anomalous response of superconducting titanium nitride resonators to terahertz radiation. Appl. Phys. Lett., 105, 192601 (1 to 5).
Abstract: We present an experimental study of kinetic inductance detectors (KIDs) fabricated of atomic layer deposited TiN films and characterized at radiation frequencies of 350 GHz. The responsivity to radiation is measured and found to increase with the increase in radiation powers, opposite to what is expected from theory and observed for hybrid niobium titanium nitride/aluminium (NbTiN/Al) and all-aluminium (all-Al) KIDs. The noise is found to be independent of the level of the radiation power. The noise equivalent power improves with higher radiation powers, also opposite to what is observed and well understood for hybrid NbTiN/Al and all-Al KIDs. We suggest that an inhomogeneous state of these disordered superconductors should be used to explain these observations.
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Baselmans, J. J. A., Baryshev, A., Reker, S. F., Hajenius, M., Gao, J. R., Klapwijk, T. M., et al. (2006). Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers. J. Appl. Phys., 100(8), 084510 (1 to 7).
Abstract: We present a detailed experimental study of the direct detection effect in a small volume (0.15μm×1μm×3.5nm) quasioptical NbN phonon cooled hot electron bolometer mixer at 673GHz. We find that the small signal noise temperature, relevant for an astronomical observation, is 20% lower than the noise temperature obtained using 300 and 77K calibration loads. In a separate set of experiments we show that the direct detection effect is caused by a combination of bias current reduction when switching from the 77 to the 300K
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.
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Kooi, J. W., Baselmans, J. J. A., Baryshev, A., Schieder, R., Hajenius, M., Gao, J. R., et al. (2006). Stability of heterodyne terahertz receivers. J. Appl. Phys., 100(6), 064904 (1 to 9).
Abstract: In this paper we discuss the stability of heterodyne terahertz receivers based on small volume NbN phonon cooled hot electron bolometers (HEBs). The stability of these receivers can be broken down in two parts: the intrinsic stability of the HEB mixer and the stability of the local oscillator (LO) signal injection scheme. Measurements show that the HEB mixer stability is limited by gain fluctuations with a 1∕f spectral distribution. In a 60MHz noise bandwidth this results in an Allan variance stability time of ∼0.3s. Measurement of the spectroscopic Allan variance between two intermediate frequency (IF) channels results in a much longer Allan variance stability time, i.e., 3s between a 2.5 and a 4.7GHz channel, and even longer for more closely spaced channels. This implies that the HEB mixer 1∕f noise is strongly correlated across the IF band and that the correlation gets stronger the closer the IF channels are spaced. In the second part of the paper we discuss atmospheric and mechanical system stability requirements on the LO-mixer cavity path length. We calculate the mixer output noise fluctuations as a result of small perturbations of the LO-mixer standing wave, and find very stringent mechanical and atmospheric tolerance requirements for receivers operating at terahertz frequencies.
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Gao, J. R., Hajenius, M., Tichelaar, F. D., Klapwijk, T. M., Voronov, B., Grishin, E., et al. (2007). Monocrystalline NbN nanofilms on a 3C-SiC∕Si substrate. Appl. Phys. Lett., 91(6), 062504 (1 to 3).
Abstract: The authors have realized NbN (100) nanofilms on a 3C-SiC (100)/Si(100) substrate by dc reactive magnetron sputtering at 800°C. High-resolution transmission electron microscopy (HRTEM) is used to characterize the films, showing a monocrystalline structure and confirming epitaxial growth on the 3C-SiC layer. A film ranging in thickness from 3.4to4.1nm shows a superconducting transition temperature of 11.8K, which is the highest reported for NbN films of comparable thickness. The NbN nano-films on 3C-SiC offer a promising alternative to improve terahertz detectors. For comparison, NbN nanofilms grown directly on Si substrates are also studied by HRTEM.
The authors acknowledge S. V. Svetchnikov at National Centre for HRTEM at Delft, who prepared the specimens for HRTEM inspections. This work was supported by the EU through RadioNet and INTAS.
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Hajenius, M., Yang, Z. Q., Gao, J. R., Baselmans, J. J. A., Klapwijk, T. M., Voronov, B., et al. (2007). Optimized sensitivity of NbN hot electron bolometer mixers by annealing. IEEE Trans. Appl. Supercond., 17(2), 399–402.
Abstract: We report that the heterodyne sensitivity of superconducting hot-electron bolometers (HEBs) increases by 25-30% after annealing at 85degC in high vacuum. The devices studied are twin-slot antenna coupled mixers with a small area NbN bridge of 1 mum times 0.15 mum, above which there is a SiO 2 passivation layer. The mixer noise temperature, gain, and resistance versus temperature curve of a HEB before and after annealing are compared and analysed. We show that the annealing reduces the intrinsic noise of the mixer by 37% and makes the superconducting transition of the bridge and the contacts sharper. We argue that the reduction ofthe noise is mainly due to the improvement of the transparency of the contact/film interface. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and at a bath temperature of 4.2 K.
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Kardakova, A. I., Coumou, P. C. J. J., Finkel, M. I., Morozov, D. V., An, P. P., Goltsman, G. N., et al. (2015). Electron–phonon energy relaxation time in thin strongly disordered titanium nitride films. IEEE Trans. Appl. Supercond., 25(3), 1–4.
Abstract: We have measured the energy relaxation times from the electron bath to the phonon bath in strongly disordered TiN films grown by atomic layer deposition. The measured values of τ eph vary from 12 to 91 ns. Over a temperature range from 3.4 to 1.7 K, they follow T -3 temperature dependence, which are consistent with values of τ eph reported previously for sputtered TiN films. For the most disordered film, with an effective elastic mean free path of 0.35 nm, we find a faster relaxation and a stronger temperature dependence, which may be an additional indication of the influence of strong disorder on a superconductor.
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Finkel, M., Thierschmann, H., Galatro, L., Katan, A. J., Thoen, D. J., de Visser, P. J., et al. (2017). Performance of THz components based on microstrip PECVD SiNx technology. IEEE Trans. THz Sci. Technol., 7(6), 765–771.
Abstract: We present a performance analysis of passive THz components based on Microstrip transmission lines with a 2-μmthin plasma-enhanced chemical vapor deposition grown silicon nitride (PECVD SiNX) dielectric layer. A set of thru-reflect-line calibration structures is used for basic transmission line characterizations. We obtain losses of 9 dB/mm at 300 GHz. Branchline hybrid couplers are realized that exhibit 2.5-dB insertion loss, 1-dB amplitude imbalance, and -26-dB isolation, in agreement with simulations. We use the measured center frequency to determine the dielectric constant of the PECVD SiN x , which yields 5.9. We estimate the wafer-to-wafer variations to be of the order of 1%. Directional couplers are presented which exhibit -12-dB transmission to the coupled port and -26 dB to the isolated port. For transmission lines with 5-μm-thin silicon nitride (SiN x ), we observe losses below 4 dB/mm. The thin SiN x dielectric membrane makes the THz components compatible with scanning probe microscopy cantilevers allowing the application of this technology in on-chip circuits of a THz near-field microscope.
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Klapwijk, T. M., & Semenov, A. V. (2017). Engineering physics of superconducting hot-electron bolometer mixers. IEEE Trans. THz Sci. Technol., 7(6), 627–648.
Abstract: Superconducting hot-electron bolometers are presently the best performing mixing devices for the frequency range beyond 1.2 THz, where good-quality superconductor-insulator-superconductor devices do not exist. Their physical appearance is very simple: an antenna consisting of a normal metal, sometimes a normal-metal-superconductor bilayer, connected to a thin film of a narrow short superconductor with a high resistivity in the normal state. The device is brought into an optimal operating regime by applying a dc current and a certain amount of local-oscillator power. Despite this technological simplicity, its operation has found to be controlled by many different aspects of superconductivity, all occurring simultaneously. A core ingredient is the understanding that there are two sources of resistance in a superconductor: a charge-conversion resistance occurring at a normal-metal-superconductor interface and a resistance due to time-dependent changes of the superconducting phase. The latter is responsible for the actual mixing process in a nonuniform superconducting environment set up by the bias conditions and the geometry. The present understanding indicates that further improvement needs to be found in the use of other materials with a faster energy relaxation rate. Meanwhile, several empirical parameters have become physically meaningful indicators of the devices, which will facilitate the technological developments.
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Gao, J. R., Hajenius, M., Yang, Z. Q., Baselmans, J. J. A., Khosropanah, P., Barends, R., et al. (2007). Terahertz superconducting hot electron bolometer heterodyne receivers. IEEE Trans. Appl. Supercond., 17(2), 252–258.
Abstract: We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and-structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.
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Floet, D. W., Baselmans, J. J. A., Klapwijk, T. M., & Gao, J. R. (1998). Resistive transition of niobium superconducting hot-electron bolometer mixers. Appl. Phys. Lett., 73(19), 2826.
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Ganzevles, W. F. M., Gao, J. R., de Korte, P. A. J., & Klapwijk, T. M. (2001). Direct response of microstrip line coupled Nb THz hot-electron bolometer mixers. Appl. Phys. Lett., 79(15), 2483–2485.
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Dorenbos, S. N., Reiger, E. M., Perinetti, U., Zwiller, V., Zijlstra, T., & Klapwijk, T. M. (2008). Low noise superconducting single photon detectors on silicon. Appl. Phys. Lett., 93(13), 131101.
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