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Conference on Spectral Technology and Applications (CSTA)

DC characteristics of terahertz superconducting SIS mixers under alternating magnetic field

Jinbiao Zhao, Dong E. Liu, Boliang liu, Ming Yao, Jing Li, Sheng‐Cai Shi

Published Dec 4, 2024• 0 citations
Publisher

Abstract

Terahertz Superconducting SIS (Superconductor-Insulator-Superconductor) mixers demonstrate ultra-high sensitivity close to the quantum limit. However, the DC characteristics of these mixes are vulnerable to interference, which may lead to decreased stability. In this study, the paper conducts a systematic experimental investigation into the impact of alternating magnetic field interference. It is observed that the DC bias voltage noise of superconducting SIS mixer under alternating magnetic field increases with the current derivative and is positively correlated with the coupling coefficient. Further experiments also reveal that measures such as increasing the distance between the bias source port and radiation source or reducing the area of the receiving circuit can decrease the coupling coefficient. The method can effectively suppress the interference caused by alternating magnetic field on DC characteristics of the terahertz superconducting SIS mixer.

DOI
10.1117/12.3037132

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Journal of Physics D Applied Physics•Oct 6, 2026•0 citations
Terahertz Superconducting SIS (Superconductor-Insulator-Superconductor) mixers demonstrate ultra-high sensitivity close to the quantum limit. However, the DC characteristics of these mixes are vulnerable to interference, which may lead to decreased stability. In this study, the paper conducts a systematic experimental investigation into the impact of alternating magnetic field interference. It is observed that the DC bias voltage noise of superconducting SIS mixer under alternating magnetic field increases with the current derivative and is positively correlated with the coupling coefficient. Further experiments also reveal that measures such as increasing the distance between the bias source port and radiation source or reducing the area of the receiving circuit can decrease the coupling coefficient. The method can effectively suppress the interference caused by alternating magnetic field on DC characteristics of the terahertz superconducting SIS mixer.
Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1 μm2 Nb/Al–Al2O3/Nb junctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing.
A detailed analysis of the effects of the quantum susceptance on the performance of superconductor-insulator-superconductor (SIS) mixers is performed. It is found that the principal effects of the quantum susceptance are to change the DC bias at which optimum coupling of the signal to the mixer occurs, and to change the output admittance at the IF frequency, thus changing the available gain. It is shown that at the signal port the principal effect is to change the bias voltage at which optimum coupling to the mixer occurs. A more dramatic effect occurs at the IF port, where the quantum susceptance leads to negative values of the output conductance, which causes the available gain to be infinite.
We have investigated the effect of a moderately strong magnetic field (up to 1200 Gauss, 13 quantum fluxes within a single junction) on the heterodyne mixing performance of a 385-500GHz superconductor-insulator-superconductor mixer. Both experimental and numerical results indicate that the mixer conversion gain can be significantly reduced by a magnetic field. Surprisingly, the mixer noise shows very weak dependence. The overall receiver noise increase is merely caused by the intermediate frequency noise contribution amplified by a factor of gain reduction. At the expense of little system noise degradation, a relatively strong magnet field can make the mixer more stable at the bias point corresponding to its noise minimum
The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.
Abstract The wide intermediate frequency (IF) bandwidth is a core performance metric for terahertz heterodyne receivers, and a wide-IF superconductor–insulator–superconductor (SIS) mixer is key to achieving this. This paper presents the simulation-based design, optimization, and performance analysis of a 345 GHz superconducting SIS mixer developed for wide-IF applications. The mixer employs a parallel-connected twin-junction tuning structure and integrates a dedicated IF impedance matching circuit, achieving wideband impedance matching between the mixer output port and low-noise amplifier. The design is based on two tunnel junction technologies, AlO x and AlN, with critical current densities of 1 kA·cm −2 and 25 kA·cm −2 , respectively. We systematically studied the effects of different junction materials and junction areas on the RF and IF bandwidth performance of the SIS mixer. It is found that for AlN-barrier junctions with diameters of 0.6 μ m and 0.8 μ m, the receiver’s IF noise temperature slowly decreased as the IF frequency increased at a high local oscillator frequency. Furthermore, we incorporated the calculated IF output port impedance into the mixer model for correction and introduced an integrated IF circuit approach, overcoming the original IF bandwidth limitation. Simulations show that the newly designed mixer achieves the stringent sensitivity and bandwidth requirements across the 320–370 GHz range, delivering an IF bandwidth of 4–22 GHz and a receiver noise temperature of 39–73 K. Notably, the −3 dB IF bandwidth is improved by more than 1.5 times compared to conventional AlO x -based designs. These findings provide new design guidelines and quantitative criteria for next-generation wide-IF receivers and other high-frequency applications requiring a wide IF bandwidth.