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International Conference on Microwave and Millimeter Wave Technology (ICMMT)

Development of a 0.5-THz Waveguide NbN-Based SIS Mixer

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

Published Apr 1, 2007• 4 citations
Publisher

Abstract

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.

DOI
10.1109/icmmt.2007.381460

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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.
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.
With a gap frequency of 1.4 THz, all-NbN Superconductor-Insulator-Superconductor (SIS) junctions are potentially applicable to sensitive heterodyne mixers for THz astronomy and atmospheric research. In this paper, we report the design, fabrication, and performance characterization of a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and an NbN/MgO/NbN tuning circuit with no impedance transformers. The noise performance of two NbN SIS junctions with different tuning inductances is measured, compared with the simulated results without and with the multiple Andreev reflection (MAR) effect taken into account, and also compared with the video detection response. In addition, the IF-output-power stability and aging effect of fabricated NbN SIS junctions are investigated.
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/AlOx/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. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
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.
This paper describes the first experimental results for a waveguide-type all-NbN superconductor-insulator-superconductor (SIS) heterodyne mixer on an MgO substrate designed to operate over the gap frequency of Nb. The mixer consists of an NbN/MgO/NbN junction, which has a length of one wavelength at 880GHz as a tuning circuit, an NbN/MgO/NbN microstrip as a λ/4 impedance transformer, and an RF choke filter. The mixer chip was designed using a high-frequency-structure simulator. Its return-loss and embedding-impedance characteristics were examined using a 180-times-scaled mixer model. By optimizing the cutting and polishing processes for the MgO substrate, we were able to fabricate the mixer chip with an accuracy of less than 5 μm. We succeeded in mounting the chip on a mixer block and in estimating the receiver noise temperature. The uncorrected minimum double-sideband receiver noise temperature was 740 K at 824 GHz. A comparison of the receiver noise temperature in a quasi-optical SIS mixer fabricated on the same wafer as the waveguide mixer showed that input noise was the major contributor to receiver noise in the waveguide mixer.
We have designed and fabricated waveguide superconductor-insulator-superconductor (SIS) receivers at 870 GHz with NbN/AlN/NbN tunnel junctions on an MgO substrate. A waveguide probe was incorporated into the mixer chip for a waveguide-to-microstrip transition. As the tuning circuit for the SIS mixer, we designed a resonant distributed junction whose length is one wavelength and critical current density is 20 kA/cm/sup 2/. The mixing properties were calculated based on the quantum theory of mixing. The SSB mixer noise temperature should be less than 100 K and conversion gain larger than -8.0 dB in the frequency range from 840 to 920 GHz. We also observed the resonance steps of the fabricated SIS mixer. The tuning frequency was slightly higher than the designed frequency.