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Asia-Pacific Microwave Conference (APMC)

Comparison of the performance of a 0.5-THz Nb and NbN superconducting SIS mixer

Jing Li, Masanori Takeda, Zhen Wang, Sheng‐Cai Shi

Published Dec 1, 2009• 1 citation
Publisher

Abstract

In THz regime, NbN Superconductor - Insulator - Superconductor (SIS) mixers are potentially applicable to astronomy and atmospheric research. We have developed a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and NbN/MgO/NbN tuning circuit. In this paper, we compare it with a 0.5-THz Nb SIS mixer, which has been successfully used on POST telescope. The design and fabrication parameters of the two mixers are introduced, and the noise temperatures are measured. In addition, the direct detection performance is analyzed, such as the shot noise, the current responsivity and the noise equivalent power. The detailed results are presented.

DOI
10.1109/apmc.2009.5385402

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In THz regime, NbN Superconductor - Insulator - Superconductor (SIS) mixers are potentially applicable to astronomy and atmospheric research. We have developed a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and NbN/MgO/NbN tuning circuit. In this paper, we compare it with a 0.5-THz Nb SIS mixer, which has been successfully used on POST telescope. The design and fabrication parameters of the two mixers are introduced, and the noise temperatures are measured. In addition, the direct detection performance is analyzed, such as the shot noise, the current responsivity and the noise equivalent power. The detailed results are presented.
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This paper mainly reports the development of a 0.5-THz waveguide SIS (superconductor – insulator – superconductor) mixer, which adopts twin NbN-based SIS junctions (NbN/AlN/NbN) connected in parallel. The 0.5-THz waveguide NbN SIS mixer is designed incorporating with a 51-μm thick MgO substrate and a waveguide mixer mount with no back-short cavity. Detailed simulation results, junction fabrication process and junction I-V characteristics are described. Preliminary FTS (Fourier Transform Spectrometer) measurement results will also be discussed.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AIO(x)/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region.
The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctions and an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated at frequencies above the gap frequency of Nb. To design the mixer, the complex refractive index of the MgO substrate was measured by using a THz time-domain spectrometer. The superconductive properties of NbTiN films on MgO were examined in detail. An uncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IF bandwidth of 4-12 GHz at a bath temperature of 4.2 K. By comparing the measured and calculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimated to be 0.98 dB at 810 GHz.
China is planning to build a 5-m THz telescope named DATE5 at Dome A in Antarctica. The telescope will operate in 0.85 THz and 1.5 THz frequency bands. A waveguide superconductor-insulator-superconductor (SIS) mixer is developed for the 0.85 THz frequency band by utilizing parallel-connected tunnel junctions (PCTJ) with Nb/AlN/NbN trilayer and NbTiN/SiO2/Al microstrip tuning circuit on quartz substrate. Its frequency response is measured by a Fourier-transform spectrometer (FTS), showing an FWHM frequency coverage of 762-925 GHz. The measured uncorrected double sideband (DSB) receiver noise temperature is approximately equal to 450 K at 840 GHz. With the vacuum window and beamsplitter losses corrected, the receiver noise temperature is less than five times the quantum-limited noise. Detailed mixer design and measurement results will be presented.