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IEEE Transactions on Applied Superconductivity

Optimized Nonuniform Nb SIS Parallel Josephson Junction Array Oscillators

Faouzi M. Boussaha, Christine Chaumont, A. Féret, Jean-Guy Caputo, T. Vacelet

Published Mar 1, 2016• 3 citations
Publisher

Abstract

We developed submillimeter superconducting oscillators using Josephson effects in the Nb-based superconductor-insulator-superconductor (SIS) parallel array. The radiation emitted from the array is detected using integrated SIS twin junctions. Despite the small junction number (N = 21), the (I-V ) characteristics of the detector clearly exhibit photon-assisted quasiparticle steps when the array is biased upon its Josephson resonances, ranging from 370 to 500 GHz. The array has a moderate current density (Jc≈= 6 kA/cm2) but provides an output power around 0.28 μW, which would be sufficient in achieving sensitive heterodyne reception. The impedance array can be matched easier to microwave circuits than the usual long Josephson junctions. Furthermore, the parallel array approach can be also extended up to about 1.4 THz using Nb-based SIS junctions.

DOI
10.1109/tasc.2016.2536604

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We developed submillimeter superconducting oscillators using Josephson effects in the Nb-based superconductor-insulator-superconductor (SIS) parallel array. The radiation emitted from the array is detected using integrated SIS twin junctions. Despite the small junction number (N = 21), the (I-V ) characteristics of the detector clearly exhibit photon-assisted quasiparticle steps when the array is biased upon its Josephson resonances, ranging from 370 to 500 GHz. The array has a moderate current density (Jc≈= 6 kA/cm2) but provides an output power around 0.28 μW, which would be sufficient in achieving sensitive heterodyne reception. The impedance array can be matched easier to microwave circuits than the usual long Josephson junctions. Furthermore, the parallel array approach can be also extended up to about 1.4 THz using Nb-based SIS junctions.
We report heterodyne measurements of Josephson microwave radiation emitted by a parallel array of small superconductor-insulator-superconductor (SIS) junctions at submillimeter-wave frequencies. The array consists of five Nb/Al–AlOx/Nb junctions nonevenly distributed in a niobium superconducting stripline, and is optimized for rf coupling in the 450–640 GHz range. We observed Fiske-like resonant steps in its I-V curve in the presence of magnetic field. The device was placed in a waveguide mount, and its radiation was quasioptically coupled out of the cryostat, to a SIS-mixer spectrometer in the same frequency range, with a 4–8 GHz band for spectral analysis. We detected a coherent signal in the spectra when the array was biased on the first and third steps, respectively, at the first harmonic frequency of 242 GHz and at the fundamental frequency of 493 GHz, both being the Josephson frequencies associated with their dc voltages. This strongly suggests that this type of parallel arrays optimized for wideband rf coupling, though strongly discretized, hosts dynamic fluxon regimes similar to long Josephson junctions, which could find applications in superconducting digital electronics or integrated heterodyne submillimeter-wave receivers.
We report on tunable submillimeter-wave radiation sources based on micrometer-sized superconducting tunnel junction arrays optimized within a bandwidth of 350-520 GHz. The arrays consist of 10, 20 and 40 Superconductor-Insulator- Superconductor (SIS) parallel-connected Nb/AlOx/Nb junctions embedded in superconducting microstrip lines. A SIS twin-junction is integrated along with each array to detect output signals. The pumped detector’s I-V characteristic exhibits clearly photon-assisted quasiparticule steps when the arrays are biased upon corresponding Josephson resonances ranging from 370 to 520 GHz.
Observation and analysis of submillimeter-wave radiation (300GHz-3THz) in astronomy and atmospheric sciences requires increasingly performant receivers. The most sensitive receivers working in this range of electromagnetic spectrum use superconductor-insolator-superconductor (SIS) junctions. In order to increase the bandwidth and the sensitivity, we are developing a quantum-noise limited heterodyne receiver based on several parallel SIS junctions with broad (larger than 30%) fixed tuned bandwidth. These circuits can be viewed as passband filters which have been optimized by varying the spacings between junctions. We have designed such 5-junction arrays for operation in the range 480-640 GHz. Fabrication and heterodyne characterization of these devices has been done. The 1 μm2 junctions current density ranges from 4 to 13 kA/cm2, using optical lithography and Nb/Al2Nb5/Nb trilayer sputtering technology. The fabrication process and yield are presented in this paper, along with measured performances
We report on the first direct detection of submillimeter waves emitted by small parallel tunnel junction arrays. The arrays made up of 10 and 20 Nb/Al-AlOx/Nb junctions of 6 μm2is integrated and coupled in RF to Nb/Al-AlOx/Nb twin junction-based detector by a microstrip/slotline transition. The detector's I-V curve exhibits clearly photonassisted steps when the array is biased on the corresponding Josephson resonance at 390 and 480 GHz. These results are of particular significance in the light of previous work done on this type of devices. The arrays are specially suited for applications such as clock for RSFQ electronics or as local oscillator for THz superconducting integrated receivers.
A Superconductor-Insulator-Superconductor (SIS) mixer using parallel array of N junctions have been studied. It has been shown by theoretical analysis that the performance of this type of device is excellent and that nearly quantum-limited performance of the mixer can be obtained. It has been demonstrated that the double sideband (DSB) noise temperature of a receiver employing parallel array of twin junctions, which is the most simplest case in the parallel array of N junctions, was approximately 3 to 5 times as large as the quantum limited photon noise up to the gap frequency of Nb (approximately 700 GHz). It has been also shown that the bandwidth performance of parallel junction arrays can be improved significantly with a large number of SIS junctions even if SIS junctions with a relatively low current density or a large (omega) RNCJ product are adopted in the arrays.
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.