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Advanced Photonics Research

Design of Waveguide NbN‐Based Superconductor–Insulator–Superconductor Mixer for 650 GHz

Lianhao He, Fangting Lin, Xiaoyong He

Published Apr 10, 2025• 1 citation
Publisher

Abstract

The submillimeter band accounts for a significant proportion of the photon energy in the universe, and an atmospheric window in the 600–720 GHz band is essential for astronomical detection because it contains a large number of molecular and atomic spectral lines. The superconductor–insulator–superconductor (SIS) mixer is one of the most sensitive coherent detectors. In order to improve the accuracy of detection and sensitivity and reduce noise temperature, a novel SIS mixer is proposed. It includes the design of the SIS junction, the signal coupling circuit, and the mixing circuit. After the initial optimization of the NbN‐based SIS junction's parameters, a signal coupling circuit is designed, with a return loss of less than −16 dB and an embedding impedance of 60 Ω. The mixing circuit is composed of the impedance transformer and tuning circuit. It is investigated using the SuperMix library, including the analysis of the effects of transformer width and tuner length. The return loss is less than −7.5 dB, and it is reduced by about 34% when the transformer width is decreased from 2.6 to 2.0 μm, indicating a good transmission performance. The research results can provide a reference for the development of high‐frequency and high‐sensitivity SIS mixers.

DOI
10.1002/adpr.202400197

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The Journal of Korean Institute of Electromagnetic Engineering and Science•Jan 1, 2004•0 citations
The submillimeter band accounts for a significant proportion of the photon energy in the universe, and an atmospheric window in the 600–720 GHz band is essential for astronomical detection because it contains a large number of molecular and atomic spectral lines. The superconductor–insulator–superconductor (SIS) mixer is one of the most sensitive coherent detectors. In order to improve the accuracy of detection and sensitivity and reduce noise temperature, a novel SIS mixer is proposed. It includes the design of the SIS junction, the signal coupling circuit, and the mixing circuit. After the initial optimization of the NbN‐based SIS junction's parameters, a signal coupling circuit is designed, with a return loss of less than −16 dB and an embedding impedance of 60 Ω. The mixing circuit is composed of the impedance transformer and tuning circuit. It is investigated using the SuperMix library, including the analysis of the effects of transformer width and tuner length. The return loss is less than −7.5 dB, and it is reduced by about 34% when the transformer width is decreased from 2.6 to 2.0 μm, indicating a good transmission performance. The research results can provide a reference for the development of high‐frequency and high‐sensitivity SIS mixers.
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.
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.
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).
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.
A fixed-tuned SIS(Superconductor-Insulator-Superconductor) mixer across 120∼180 GHz band has been developed. This mixer employs an SIS chip fabricated by Nobeyama radio observatory which consists of a series array of 6 Nb/Al-AlO/Nb junctions in a microstrip line on a fused quartz substrate. The SIS chip is placed at the center of the half-height waveguide mixer mount to have a good incoming signal coupling over the whole frequency band. No mechanical tuner was used in the SIS mixer and the RF signal and local oscillator power are injected to the mixer via a cooled cross-guide coupler. In order to prevent the IF signal loss, the If output impedance of the SIS mixer was matched to the 50 input impedance of the IF chain. Measured double sideband noise temperatures of a receiver using the SIS mixer are 32∼131 K over 120∼180 GHz band. The developed SIS mixer is now in use for radio astronomical observations on the TRAO 14 m radio telescope.