We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.
"NbN superconducting thin films have been optimized for the application of HEB mixers and as a result have been chosen for use on such space missions as Herschel HIFI. The work on NbN technology has produced mixers with noise temperature below 1000 K. "
[Show abstract][Hide abstract] ABSTRACT: Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.
"Previously, the noise temperature of NbN HEB mixers was reported to be sensitive to the bath temperature , , increasing almost immediately as the bath temperature rises. Similar behavior has been observed for a MgB mixer with a low . "
[Show abstract][Hide abstract] ABSTRACT: A noise bandwidth (NBW) of 6-7 GHz was obtained for hot-electron bolometer (HEB) mixers made of 10 nm MgB2 films. A systematic investigation of the (IF) gain bandwidth as a function of the MgB2 film thickness (30, 15, and 10 nm) is also presented. The gain bandwidth (GBW) of 3.4 GHz was measured for a 10 nm film, corresponding to a mixer time constant of 47 ps. For 10 nm films a reduction of the GBW was observed with the reduction of the critical temperature (T-c). Experimental data were analyzed using the two-temperature model. From the theoretical analysis, the electron-phonon time (tau(e-ph)) -, the phonon escape time (tau(esc)) and the electron and phonon specific heats (c(e), c(ph)) were extrapolated giving the first model for HEB mixers of MgB2 films.
IEEE Transactions on Terahertz Science and Technology 07/2013; 3(4):409-415. DOI:10.1109/TTHZ.2013.2252266 · 2.18 Impact Factor
"Superconducting hot-electron bolometer (HEB) mixers are so far the most sensitive heterodyne detectors at THz frequencies above 1.5 THz, and have been successfully used to detect spectral lines up to 2 THz from ground based    and space  telescopes. The HEB mixers become the detector of choice in the upper THz frequency range (3-6 THz) for high-resolution spectroscopic observations for astronomy. "
[Show abstract][Hide abstract] ABSTRACT: We report the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a tight spiral antenna at 5.3 THz. 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 receiver noise temperature of 1150 K, which is 4.5 times hnu/kB (quantum limit). Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that HEB mixer can work well at least up to 6 THz, suitable for next generation of high-resolution spectroscopic of the neutral atomic oxygen (OI) line at 4.7 THz.
Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.857683 · 0.20 Impact Factor
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