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Infrared, Millimeter-Wave, and Terahertz Technologies

Design and analysis of 230 GHz all-NbN superconductor-insulator-superconductor mixer

Minran Chen, Peng Chen, Lin‐Jie Zhang, Liping Zuo, Yi Zhang, Xiaoyong He, Fangting Lin, Wangzhou Shi, Duo Cao, Feng Liu

Published Oct 11, 2024• 1 citation
Publisher

Abstract

In this work, a 230 GHz all-NbN superconductor-insulator-superconductor (SIS) mixer was designed and analyzed. The NbN/AlN/NbN tunnel junctions (with an energy gap of 5.7 meV) and NbN/MgO/NbN tuning circuits were utilized. Full-height waveguides and bow-tie waveguide probes were adopted. The signal coupling circuit was designed and optimized to be transmissible to the given band. The quantum mixing characteristics of the all-NbN mixers were optimized with different-size SIS junctions. The mixer with a junction diameter of 1 μm achieves a conversion gain close to 0 dB and a noise temperature close to 40 K. The proposed design and analysis will provide technical support for the recovery and upgrade of the heterodyne receiver for the Leighton Chajnantor Telescope (LCT).

DOI
10.1117/12.3035989

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 29, 2007•3 citations
In this work, a 230 GHz all-NbN superconductor-insulator-superconductor (SIS) mixer was designed and analyzed. The NbN/AlN/NbN tunnel junctions (with an energy gap of 5.7 meV) and NbN/MgO/NbN tuning circuits were utilized. Full-height waveguides and bow-tie waveguide probes were adopted. The signal coupling circuit was designed and optimized to be transmissible to the given band. The quantum mixing characteristics of the all-NbN mixers were optimized with different-size SIS junctions. The mixer with a junction diameter of 1 μm achieves a conversion gain close to 0 dB and a noise temperature close to 40 K. The proposed design and analysis will provide technical support for the recovery and upgrade of the heterodyne receiver for the Leighton Chajnantor Telescope (LCT).
In order to expand the detection range of the LCT telescope, it is attractive to develop a high sensitivity Superconductor–Insulator–Superconductor (SIS) mixer with operating frequency higher than 680 GHz. In this study, we designed a SIS mixer with a center operating frequency of 850 GHz (covering from 750 to 950 GHz) using a high gap voltage material of NbN (Vgap = 5.4 - 5.6 mV) fabricated on MgO substrates. Simulations of electromagnetic fields and quantum mixing were applied to evaluate the performance of the designed SIS mixer. The width and length of the feed point were optimized to reduce the input reflection of the Local Oscillator (LO) input port (S11). In the frequency range of 750-950 GHz, a lowest S11 of less than -40 dB can be achieved using a feed point with a width of 4 μm and a length of 2.6 μm. It was observed that the embedded impedance was easily affected by the open angle of the feed point and the MgO substrate thickness. On the contrary, the deviation of the feed point showed less impact on the embedded impedance. The calculation results of quantum mixing showed that the noise temperature of the NbN-based SIS mixer was 323.4 K at 850 GHz, indicating that the designed SIS mixer has the potential to meet the requirements of manufacturing low noise high frequency receivers.
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.
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.
Superconductor–insulator–superconductor (SIS) mixers remain the choice of heterodyne mixers for single-dish telescopes and interferometers at millimeter and submillimeter wavelengths. Compared with conventional Nb/AlOx/Nb superconducting tunnel junctions, Nb/AlN/NbN ones have larger gap voltage and may reach critical-current density beyond 10kA/cm2, which are both of particularly interest in developing broadband SIS mixers. Here we report on the design and measurement of an SIS mixer based on Nb/AlN/NbN parallel connected twin junctions (PCTJ) incorporating NbTiN/SiO2/Al microstrip circuit. The junctions have a gap voltage of 3.18mV and a critical-current density of 15kA/cm2. The measured receiver noise temperature reach 5hν/kB among 200-260GHz band, and the mixer’s fractional bandwidth is about 40% centered at 230GHz.
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.