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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Characterization of Nb-based superconducting microstrip lines around the gap frequency

Sheng-Cai Shi, Chi-Chung Chin, Wen Zhang

Published Nov 7, 2002• 2 citations
Publisher

Abstract

Submillimeter-wave SIS mixers usually adopt junctions associated with an integrated tuning circuit, which tunes out the junction's capacitance and is typically a thin-film superconducting microstrip line. This paper mainly investigates the characteristic of Nb-based superconducting microstrip lines around the junction's gap frequency and its effect on the performance of SIS mixers.

DOI
10.1109/icimw.2000.893101

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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)•Sep 1, 2006•0 citations
Submillimeter-wave SIS mixers usually adopt junctions associated with an integrated tuning circuit, which tunes out the junction's capacitance and is typically a thin-film superconducting microstrip line. This paper mainly investigates the characteristic of Nb-based superconducting microstrip lines around the junction's gap frequency and its effect on the performance of SIS mixers.
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.
A 600-700GHz SIS mixer with the twin-junction tuning circuit has been designed and fabricated [1] The surface impedance of thin-film superconducting microstrip lines, based on Mattis-Bardeen theory [2], is included in the optimization of RF impedance matching. The receiver noise temperature measured in the frequency range of 630-660GHz is below 200K and the lowest receiver noise temperature of 181K is achieved at 656GHz. The FTS response of the SIS mixer shows a good RF coupling from 600GHz to 700GHz. Both the noise performance and the FTS response can be quantitatively described by numerical results using the quantum theory of mixing. Some detail considerations on the mixer model calculation, such as spreading inductance around the junction tuning structure and mixer's embedding impedance, are discussed. Introduction Nb-based superconductor-insulator-superconductor (SIS) mixers have been used very successfully in sub-mm wavelength detection. Their receiver noise temperature has reached to three times the quantum limit, —3 hfrkB, below the junction's gap frequency. However, at high frequency bands, Nb SIS mixers need to be designed carefully because the photon energy is close to or higher than the gap energy of Nb, A-1.4meV. When the frequency band covers the gap frequency of Nb, —670GHz, the RF loss of Nb superconducting transmission lines increases dramatically and the surface impedance has strong frequency dependence. Usually, the RF properties of a superconducting transmission line can be described by the Mattis-Bardeen theory [2] in good accuracy. From results reported so far [4-9], the best receiver noise temperature is close to 5 hi7kB in the band of 600-700GHz. Despite of the input loss contributed by the experimental setup, some other properties of SIS mixers should be taken into account. In this paper, we characterize the performance of a 600-700GHz mixer by comparing the experimental and simulated results.
At frequencies above the gap frequency of niobium (710 GHz), the performance of Nb/A10x/Nb SIS mixers degrades and the losses in the niobium films of the superconductive Nb/SiO/Nb microstrip circuits used to provide a good RF impedance match severely limit the performance. Therefore, we are investigating the properties of NbN mixers, and in particular of NbN/SiO/NbN microstrip circuits integrated with submicron IlbN/Mg0/NbN tunnel junctions. The very large penetration depth of NbN (3000-4000 A) makes the circuit design difficult and also puts critical constraints on alignment during fabrication. We present here the design of several rf tuning circuits for 630 GHz including novel symmetric designs which ease the alignment tolerances. In addition, a new approach for determining experimentally the penetration depth in the NbN films is presented.
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.