China is now building an astronomical observatory including a THz telescope at Dome A on the Antarctic plateau. The development of superconducting HEB (Hot Electron Bolometer) mixers, the best choice for coherent detection at frequencies beyond 1 THz, becomes increasingly crucial. We present in this manuscript the investigation of high-sensitivity superconducting HEB mixers using quasi-optical technology in the THz region. The main results of this study are as follows: 1. We measured the dielectric constant of high-resistivity silicon at 77 K and 295 K by a vector measurement system. The measured permittivity is adopted to simulate the far-field beam pattern of a quasi-optical superconducting HEB mixer. 2. We measured the far-field beam pattern of a superconducting HEB mixer by both near-field heterodyne measurement system and far-field direct detection measurement system. The measured results accord well with the simulation. We also investigated the effect of HEB bias voltages and bath temperature on the far-field beam-pattern. 3. We successfully developed a high-sensitivity superconducting HEB mixer at 1.3 THz. The uncorrected measured double-sideband noise temperature is 600 K, which is a state-of-the-art result at this frequency. We systematically characterized heterodyne performances such as RF noise, conversion gain and noise bandwidth. We also measured the frequency response and calibrated it with a new method. Furthermore, we investigated the performance of this superconducting HEB mixer integrated with different elliptical lenses. 4. Finally, we measured the noise performance and the frequency response of a HEB mixer coupled to a 0.1-1.5 THz spiral antenna, the frequency dependence of the noise temperature has also been characterized. This study has provided a further understanding of superconducting HEB mixers and allowed the realization of a HEB mixer with state-of-the-art sensitivity at 1.3 THz. Hence the results of this study can find good use in the 5 m terahertz telescope (DATE5) at Dome A.
China is now building an astronomical observatory including a THz telescope at Dome A on the Antarctic plateau. The development of superconducting HEB (Hot Electron Bolometer) mixers, the best choice for coherent detection at frequencies beyond 1 THz, becomes increasingly crucial. We present in this manuscript the investigation of high-sensitivity superconducting HEB mixers using quasi-optical technology in the THz region. The main results of this study are as follows: 1. We measured the dielectric constant of high-resistivity silicon at 77 K and 295 K by a vector measurement system. The measured permittivity is adopted to simulate the far-field beam pattern of a quasi-optical superconducting HEB mixer. 2. We measured the far-field beam pattern of a superconducting HEB mixer by both near-field heterodyne measurement system and far-field direct detection measurement system. The measured results accord well with the simulation. We also investigated the effect of HEB bias voltages and bath temperature on the far-field beam-pattern. 3. We successfully developed a high-sensitivity superconducting HEB mixer at 1.3 THz. The uncorrected measured double-sideband noise temperature is 600 K, which is a state-of-the-art result at this frequency. We systematically characterized heterodyne performances such as RF noise, conversion gain and noise bandwidth. We also measured the frequency response and calibrated it with a new method. Furthermore, we investigated the performance of this superconducting HEB mixer integrated with different elliptical lenses. 4. Finally, we measured the noise performance and the frequency response of a HEB mixer coupled to a 0.1-1.5 THz spiral antenna, the frequency dependence of the noise temperature has also been characterized. This study has provided a further understanding of superconducting HEB mixers and allowed the realization of a HEB mixer with state-of-the-art sensitivity at 1.3 THz. Hence the results of this study can find good use in the 5 m terahertz telescope (DATE5) at Dome A.
Heterodyne receivers for applications in astronomy need quantum limited sensitivity. In instruments which are currently under development for SOFIA or Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. We present results of the development of a phonon-cooled NbN HEB mixer for GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be flown aboard SOFIA. The mixer is a small superconducting bridge incorporated in a planar feed antenna and a hyperhemispherical lens. Mixers with logarithmic-spiral and double-slot feed antennas have been investigated with respect to their noise temperature, conversion loss, linearity and beam pattern. At 2.5 THz a double sideband noise temperature of 2200 K was achieved. The conversion loss was 17 dB. The response of the mixer was linear up to 400 K load temperature. The performance was verified by measuring an emission line of methanol at 2.5 THz. The measured linewidth is in good agreement with the linewidth deduced from pressure broadening measurements at millimeter wavelength. The results demonstrate that the NbN HEB is very well suited as a mixer for far-infrared heterodyne receivers.
We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. Terahertz (THz) radiation is quasioptically coupled to a HEB mixer with a lens and a spiral antenna. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an antireflection coated Si lens, we obtained a double sideband (DSB) receiver noise temperature (TrecDSB) of 1150 K, which is nine times hν/2k, where h is the Planck constant, ν the frequency, and k the Boltzmann constant. In addition, the measured far field beam patterns of the integrated lens antenna show nearly collimated beams from 2.5 to 5.3 THz that allow reliable measurement of TrecDSB using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, making it suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen line at 4.7 THz.
We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. The terahertz (THz) radiation is quasioptically coupled to the HEB mixer with a lens and a spiral antenna. Using a measurement setup with black body calibration sources and a beam splitter in vacuum, and an antireflection coated Si lens, we obtained a double sideband (DSB) receiver noise temperature ( ) of 1150 K, which is 4.5 times h/k (the quantum limit). In addition, the measured far field beam patterns of the integrated lens antenna from 2.5 THz to 5.3 THz show collimated beams that make it reliable to measure using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen (OI) line at 4.7 THz. DSB rec T
The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating. So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz. The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging. In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.
Purple Mountain Observatory as well as its collaborators is proposing to build a 5-m THz telescope (DATE5) at Dome A, Antarctic. Two THz heterodyne receivers, with one at 1.4 THz, are being considered as its first generation instrument. Here we report on the development of a 1.4-THz superconducting hot-electron bolometer (HEB) mixer for DATE5, which adopts a twin-slot antenna design. The direct-detection spectral (FTS) response of the 1.4-THz superconducting HEB mixer, with the HEB device fabricated at MSPU, has been measured, giving a frequency coverage of 0.8~1.5THz. And the measured FTS response appears in good agreement with the simulated one. Preliminary noise- temperature measurement at a lower frequency of 0.85 THz shows a receiver noise temperature of 1500 K. The estimated performance at 1.4 THz should be less than 1000 K in terms of the measured FTS response. Detailed simulation and experimental results will be presented.
Superconducting hot electron bolometer (HEB) mixers are the most sensitive heterodyne detectors at frequencies beyond 1 THz, and are used for many astronomical observations. HEB, consisting of a narrow and short superconducting thin film patch connected to an antenna via contact pads, is brought into a highly sensitive operating regime by applying a DC current and a certain amount of local-oscillator power. Potentially, superconducting HEB mixers are capable of exhibiting high sensitivity for frequencies up to several tens of THz, as their output noise and conversion gain are nearly frequency independent according to the mixing mechanism. Moreover, the understanding of mixing mechanism has evolved over time from lumped-element model to distributed hot-spot model. In this review, we provide an overview of theoretical and experimental studies on HEBs and their applications, especially in astronomical observations on ground, airborne and space telescopes.