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Trifonov, A., Tong, C. - Y. E., Lobanov, Y., Kaurova, N., Blundell, R., & Goltsman, G. (2016). Gap frequency and photon absorption in a hot electron bolometer. In Proc. 27th Int. Symp. Space Terahertz Technol. (121).
Abstract: The superconducting energy gap is a crucial parameter of a superconductor when used in mixing applications. In the case of the SIS mixer, the mixing process is efficient for frequencies below the energy gap, whereas, in the case of the HEB mixer, the mixing process is most efficient at frequencies above the gap, where photon absorption takes place more readily. We have investigated the photon absorption phenomenon around the gap frequency of HEB mixers based on NbN films deposited on silicon membranes. Apart from studying the pumped I-V curves of HEB devices, we have also probed them with microwave radiation, as previously described [1]. At frequencies far below the gap frequency, the pumped I-V curves show abrupt switching between the superconducting and resistive states. For the NbN HEB mixers we tested, which have critical temperatures of ~9 K, this is true for frequencies below about 400 GHz. As the pump frequency is increased beyond 400 GHz, the resistive state extends towards zero bias and at some point a small region of negative differential resistance appears close to zero bias. In this region, the microwave probe reveals that the device impedance is changing randomly with time. As the pump frequency is further increased, this random impedance change develops into relaxation oscillations, which can be observed by the demodulation of the reflected microwave probe. Initially, these oscillations take the form of several frequencies grouped together under an envelope. As we approach the gap frequency, the multiple frequency relaxation oscillations coalesce into a single frequency of a few MHz. The resultant square-wave nature of the oscillation is a clear indication that the device is in a bi-stable state, switching between the superconducting and normal state. Above the gap frequency, it is possible to obtain a pumped I-V curve with no negative differential resistance above a threshold pumping level. Below this pumping level, the device demonstrates bi-stability, and regular relaxation oscillation at a few MHz is observed as a function of pump power. The threshold pumping level is clearly related to the amount of power absorbed by the device and its phonon cooling. From the above experiment, we can derive the gap frequency of the NbN film, which is 585 GHz for our 6 μm thin silicon membrane-based device. We also confirm that the HEB mixer is not an efficient photon absorber for radiation below the gap frequency. 1. A. Trifonov et al., “Probing the stability of HEB mixers with microwave injection”, IEEE Trans. Appl. Supercond., vol. 25, no. 3, June 2015.
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Tretyakov, I., Kaurova, N., Voronov, B. M., & Goltsman, G. N. (2018). About effect of the temperature operating conditions on the noise temperature and noise bandwidth of the terahertz range NbN hot-electron bolometers. In Proc. 29th Int. Symp. Space Terahertz Technol. (113).
Abstract: Results of an experimental study of the noise temperature (Tn) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (Tb) and NbN bridge length are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (Tc) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature Tb close to Tc was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at Tb ≪ Tc for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.
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Arutyunov, K. Y., Ramos-Álvarez, A., Semenov, A. V., Korneeva, Y. P., An, P. P., Korneev, A. A., et al. (2016). Quasi-1-dimensional superconductivity in highly disordered NbN nanowires. arXiv:1602.07932v1 [cond-mat.supr-con]. Retrieved August 2, 2024, from https://arxiv.org/abs/1602.07932v1
Abstract: The topic of superconductivity in strongly disordered materials has attracted a significant attention. In particular vivid debates are related to the subject of intrinsic spatial inhomogeneity responsible for non-BCS relation between the superconducting gap and the pairing potential. Here we report experimental study of electron transport properties of narrow NbN nanowires with effective cross sections of the order of the debated inhomogeneity scales. We find that conventional models based on phase slip concept provide reasonable fits for the shape of the R(T) transition curve. Temperature dependence of the critical current follows the text-book Ginzburg-Landau prediction for quasi-one-dimensional superconducting channel Ic~(1-T/Tc)^3/2. Hence, one may conclude that the intrinsic electronic inhomogeneity either does not exist in our structures, or, if exist, does not affect their resistive state properties.
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Murphy, A., Semenov, A., Korneev, A., Korneeva, Y., Gol’tsman, G., & Bezryadin, A. (2014). Dark counts initiated by macroscopic quantum tunneling in NbN superconducting photon detectors. arXiv:1410.7689v2 [cond-mat.supr-con]. Retrieved August 2, 2024, from https://arxiv.org/abs/1410.7689v2
Abstract: We perform measurements of the switching current distributions of three w = 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijarvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors.
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Tretyakov, I., Seliverstov, S., Zolotov, P., Kaurova, N., Voronov, B., Finkel, M., et al. (2014). Noise temperature and noise bandwidth of hot-electron bolometer mixer at 3.8 THz. In Proc. 25th Int. Symp. Space Terahertz Technol. (77).
Abstract: We report on our recent results of double sideband (DSB) noise temperature and bandwidth measurements of quasi-optical hot electron bolometer (HEB) mixers at local oscillator frequency of 3.8 THz. The HEB mixers used in this work were made of a NbN thin film and had a superconducting transition temperature of about 10.3 K. To couple terahertz radiation, the NbN microbridge (0.2 μm long and 2 μm wide) was integrated with a planar logarithmic-spiral antenna. The mixer chip was glued to an elliptical Si lens clamped tightly to a mixer block mounted on the 4.2 K plate of a liquid helium cryostat. The terahertz radiation was fed into the HEB device through the cryostat window made of a 0.5 mm thick HDPE. A band-pass mesh filter was mounted on the 4.2 K plate to minimize the direct detection effect [1]. We used a gas discharge laser irradiating at 3.8 THz H 2 0 line as a local oscillator (LO). The LO power was combined with a black body broadband radiation via Mylar beam splitter. Our receiver allows heterodyne detection with an intermediate frequency (IF) of a several gigahertz which dictates usage of a wideband SiGe low noise amplifier [2]. The receiver IF output signal was further amplified at room temperature and fed into a square-law power detector through a band-pass filter. The DSB receiver noise temperature was measured using a conventional Y-factor technique at IF of 1.25 GHz and band of 40 MHz. Using wideband amplifiers at both cryogenic and room temperature stages we have estimated IF bandwidth of the HEB mixers used. The obtained results strengthen the position of the HEB mixer as one of the most important tools for submillimeter astronomy. This device operates well above the energy gap (at frequencies above 1 THz) where performance of state-of-the-art SIS mixers starts to degrade. So, HEB mixers are expected to be a device of choice in astrophysical observations (ground-, aircraft- and space-based) at THz frequencies due to its excellent noise performance and low LO power requirements. The HEB mixers will be in operation on Millimetron Space Observatory. References 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. Sander Weinreb, Life Fellow, IEEE, Joseph C. Bardin, Student Member, IEEE, and Hamdi Mani, “Design of Cryogenic SiGe Low-Noise Amplifiers”, IEEE Transactions on Microwave Theory and Techniques, 55, 11, 2007.
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Kinev, N. V., Filippenko, L. V., Ozhegov, R. V., Gorshkov, K. N., Gol’tsman, G. N., & Koshelets, V. P. (2014). Superconducting integrated receiver with HEB-mixer. In Proc. 25th Int. Symp. Space Terahertz Technol. (78).
Abstract: Detectors in THz range with high sensitivity are very essential nowadays in different fields: space technology, atmospheric research, medicine and security. The most sensitive heterodyne detectors below 1 THz are the SIS- mixers due to its extremely high non-linearity and low noise level. Nevertheless, their effective range is strongly limited by superconducting gap Δ (about 1 THz for NbN circuits). Above 1 THz the detectors based on HEB (hot electron bolometers) are more effective [1]; their operation frequency is not limited from above and can be up to 70 THz [2]. HEBs can perform as both direct and heterodyne detectors (mixers). All HEB-mixers are used with external heterodyne, most useful are synthesizer with multipliers, quantum cascade lasers or far infrared lasers and backward-wave oscillators. Superconducting integrated receiver (SIR) is based on implementation of both SIS-miser and flux flow oscillator (FFO) acting as heterodyne at single chip [3]. Such receiver has been successfully applied at TELIS balloon-borne instrument for study of atmospheric constituents [4] and looks as very promising device for other THz missions including space research. Thus, there is a task to expand its operating range to higher frequencies. The frequency range of the SIR the operation is limited by both the SIS-mixer and the FFO maximum frequencies. The idea of present work is implementation of the HEB as a mixer in the SIR instead of the SIS traditionally used. We introduce the first results of integrating the HEB-mixer coupled to planar slot antenna with the FFO on one chip. For properly FFO operation the SIS harmonic mixer is used to phase lock the oscillator. The scheme of the SIR based on the HEB- mixer is presented in fig. 1. We have demonstrated the principal possibility of integration of both the HEB-mixer and the flux-flow oscillator on a single chip and succeed with sufficient power coupling for properly receiver operation. We measured the direct response of the HEB coupled to the antenna at THz frequencies by the FTS setup and noise temperature of the receiver with standard Y- factor measuring technique. The SIR operating range 450-620 GHz was achieved with the best uncorrected noise temperature of about 1000 К. One should note that it is still quite low frequencies for effective operation of the HEB-mixer; therefore we expect to obtain the better results for frequencies above 700 GHz (up to 1.2 THz). Another additional task is to increase the FFO frequencies by using NbTiN electrodes instead of NbN; currently we are working on this issue. This work was supported by the RFBR grant, the Ministry of Education and Science of Russia and Russian Academy of Sciences. References 1. D. Semenov, H.-W. Hubers, J. Schubert, G. N. Gol’tsman, A. I. Elantiev, B. M. Voronov, E. M. Gershenzon, Design and performance of the lattice-cooled hot-electron terahertz mixer, J. Appl. Phys. 88, 6758, 2000. 2. Maslennikov S. N., Finkel M. I., Antipov S. V. et al. Spiral antenna coupled and directly coupled NbN HEB mixers in the frequency range from 1 to 70THz. Proc. 17 th international symposium on space terahertz technology. Paris, France: 2006.—may. Pp. 177 – 179. 3. V.P. Koshelets, S.V. Shitov. Integrated Superconducting Receivers. Supercond. Sci. Technol. Vol. 13. P. R53-R59. 2000. 4. Gert de Lange, Dick Boersma, Johannes Dercksen et.al. Development and Characterization of the Superconducting Integrated Receiver Channel of the TELIS Atmospheric Sounder. Supercond. Sci. Technol. vol. 23, No 4, 045016 (8pp). 2010.
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Shcherbatenko, M., Lobanov, Y., Finkel, M., Maslennikov, S., Pentin, I., Semenov, A., et al. (2014). Development of a 30 THz heterodyne receiver based on a hot-electron-bolometer mixer. In Proc. 25th Int. Symp. Space Terahertz Technol. (122).
Abstract: We present new Hot-Electron-Bolometer (HEB) mixers designed for mid-IR spectroscopy targeting astrophysical and geophysical observations where high sensitivity and spectral resolution are required. The mixers are made of an ultrathin NbN film deposited on GaAs substrates. Two entirely different types of the devices have been fabricated. The first type is based on a direct radiation coupling concept and the mixing devices are shaped as squares of 5×5 μm 2 (which corresponds to the diffraction limit at the chosen wavelength) and 10×10 μm 2 (which was used to establish a possible influence of the contact pads on the radiation absorption). The second type utilizes a spiral antenna designed with HFSS. The fabrication and layout of the devices as well as the performance comparison will be presented. During the experiments, the HEB mixer was installed on the cold plate of a LHe cryostat. A germanium window and an extended semi-spherical germanium lens are used to couple the radiation. The cryostat is equipped with a germanium optical filter of thickness 0.5 mm and with a center wavelength of 10.6 mμ. The incident power absorption is measured by using the isothermal method. As a Local Oscillator, a 10.6 micrometers line of a CO2 gas laser is used. We further characterize the frequency response of the spiral antenna with a FIR-spectrometer. The noise characteristics of the mixers are determined from a room temperature cold load and a heated black body at ~600 K as a hot load.
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Shurakov, A., Tong, C. -yu E., Blundell, R., & Gol’tsman, G. (2014). A microwave pumped HEB direct detector using a homodyne readout scheme. In Proc. 25th Int. Symp. Space Terahertz Technol. (129).
Abstract: We report the results of our study on the noise performance of a fast THz detector based on the repurpose of hot electron bolometer mixer (HEB). Instead of operating with an elevated bath temperature, microwave power is injected into the HEB device, which enhances the sensitivity of the detector and at the same time provide a mechanism for reading out impedance changes of the device induced by the modulated incident THz radiation [1]. We have demonstrated an improvement of the detector’s optical noise equivalent power (NEP). Furthermore, by introducing a homodyne readout scheme based on a room temperature microwave mixer, the dynamic range of the detector is increased. The HEB devices used in this work were made of 4 nm thick NbN film. The detector chips were installed into a waveguide mixer block fitted with a corrugated horn, mounted on the cold plate of a liquid helium cryostat. The HEBs were operated at a bath temperature of 4.2 K. The signal beam was terminated on black bodies at ambient and liquid nitrogen temperatures. A chopper wheel placed in front of the cryostat window operating at a frequency of 1.48 kHz modulated the input load temperature of the detector. A cold mesh filter, centered at 830 GHz, was used to define the input signal power bandwidth. Microwave was injected through a broadband directional coupler inside the cryostat. Our experiments were mostly conducted at a pump frequency of 1.5 GHz. The reflected microwave power from the HEB device was fed into a cryogenic low noise amplifier (LNA). The output of the LNA was connected to the RF input port of a room temperature microwave mixer, which beat the reflected signal from the HEB using a copy of the original 1.5 GHz injection signal in a homodyne demodulation scheme. The amplitude of the detected power was measured by a lock-in amplifier, which was synchronized to the chopper frequency. Preliminary results yield an optical NEP of ~1 pW/ Hz 1/2 which corresponds to an improvement of a factor of 3 compared to [1], driven mainly by a lowering of the system noise floor. The dynamic range was also increased by similar amount. References 1. A. Shurakov et al. “A Microwave Pumped Hot Electron Bolometric Direct Detector,” submitted on Oct 18, 2013 to Appl. Phys. Let.
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Tong, C. E., Trifonov, A., Blundell, R., Shurakov, A., & Gol’tsman, G. (2014). A digital terahertz power meter based on an NbN thin film. In Proc. 25th Int. Symp. Space Terahertz Technol. (170).
Abstract: We have further studied the effect of subjecting a superconducting Hot Electron Bolometer (HEB) element made from an NbN thin film to microwave radiation. Since the photon energy is weak, the microwave radiation does not simply heat the film, but generates a bi-static state, switching between the superconducting and normal states, upon the application of a small voltage bias. Indeed, a relaxation oscillation of a few MHz has previously been reported in this regime [1]. Switching between the superconducting and normal states modulates the reflected microwave pump power from the device. A simple homodyne setup readily recovers the spontaneous switching waveform in the time domain. The switching frequency is a function of both the bias voltage (DC heating) and the applied microwave power. In this work, we use a 0.8 THz HEB waveguide mixer for the purpose of demonstration. The applied microwave pump, coupled through a directional coupler, is at 1 GHz. Since the pump power is of the order of a few μW, a room temperature amplifier is sufficient to amplify the reflected pump power from the HEB mixer, which beats with the microwave source in a homodyne set-up. After further amplification, the switching waveform is passed onto a frequency counter. The typical frequency of the switching pulses is 3-5 MHz. It is found that the digital frequency count increases with higher microwave pump power. When the HEB mixer is subjected to additional optical power at 0.8 THz, the frequency count also increases. When we vary the incident optical power by using a wire grid attenuator, a linear relationship is observed between the frequency count and the applied optical power, over at least an order of magnitude of power. This phenomenon can be exploited to develop a digital power meter, using a very simple electronics setup. Further experiments are under way to determine the range of linearity and the accuracy of calibration transfer from the microwave to the THz regime. References 1. Y. Zhuang, and S. Yngvesson, “Detection and interpretation of bistatic effects in NbN HEB devices,” Proc. 13 th Int. Symp. Space THz Tech., 2002, pp. 463–472.
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Pentin, I. V., Smirnov, A. V., Ryabchun, S. A., Gol’tsman, G. N., Vaks, V. L., Pripolzin, S. I., et al. (2011). Heterodyne source of THz range based on semiconductor superlattice multiplier. In IRMMW-THz (pp. 1–2).
Abstract: We present the results of our studies of the possibility of developing a heterodyne receiver incorporating a hot-electron bolometer mixer as the detector and a semiconductor superlattice multiplier driven by a reference synthesizer as the local oscillator. We observe that such a local oscillator offers enough power in the terahertz range to pump the HEB into the operating state.
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