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Superconductor Science and Technology

HTS Josephson junctions arrays for high-frequency mixing

Aleksei Sharafiev, M. Malnou, C. Feuillet-Palma, C. Ulysse, Thomas Wolf, F. Couëdo, Pascal Febvre, Jérôme Lesueur, N. Bergeal

Published Nov 27, 2017• 18 citations
Publisher

Abstract

Abstract We designed, fabricated and measured short one-dimensional arrays of masked ion-irradiated YBa 2 Cu 3 O 7 Josephson junctions embedded into log-periodic spiral antennas. They consist of four or eight junctions separated either by 960 nm or 80 nm long areas of pristine material. Large spacing arrays show ‘giant’ Shapiro steps in the hundreds-GHz band at 66 K and are tested as Josephson mixers with improved impedance matching. On the contrary, small spacing arrays behave as one junction with a lower superconducting transition temperature, hence forming a single weak link on distances up to 880 nm. Such design opens a new way to increase the I c R n product of the devices, and therefore the efficiency of Josephson mixers. Hints on the origin of the observed long range proximity effect are proposed.

DOI
10.1088/1361-6668/aa9d48

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We present Josephson junctions fabricated from high transition temperature superconductor YBa2Cu3O7coupled to spiral THz antennas. The fabrication process involves creating a Josephson junction at the center of a superconducting bridge embedded into a THz spiral antenna using irradiation from a focused helium ion beam. For low doses of helium irradiation, the junctions have metallic barriers and operate at temperatures as high as 67 K. For higher doses of irradiation, insulator barriers are created. These devices have higher resistance but require cooling to lower temperatures (~10 K) due to smaller critical currents. Through optimization of the dose and by trimming the width of the junctions, we have created high resistance devices to improve the impedance match to a spiral antenna. Under 90-GHz radiation, we observe up to 17 Shapiro steps in the I-V curve out to approximately 3 mV at a temperature of 10 K.
Here we report on the millimeter-wave performance of high-Tc superconducting (HTS) Josephson junctions on MgO substrates, fabricated using a focused Ga ion beam (FIB) technique. The 6 junctions spaced 1.5 /spl mu/m apart from each other occupied 7.5 /spl mu/m. The 6 junctions can modulate even the 1000 GHz electromagnetic wave with nearly equal phase. We applied the YBCO series junction to a millimeter wave mixer with a log-periodic antenna. The HTS junction mixer and the log-periodic antenna are expected to have a function of flat frequency response up to THF. First, we measure wide frequency band responsibility for mixing function below 100 GHz. Next, we observe the HTS mixer-antenna performance as fundamental and harmonic mixers in the wide frequency range up to 100 GHz. Finally, the IF output phases can be changed within 1 dB of IF output power differences.
A high temperature superconducting YBa2Cu3Oy array of five Josephson junctions designed with additional coupling lines has been developed to demonstrate the effects of frequency locking and impedance matching for applications such as oscillators, mixers, and detectors. The Josephson self-radiation power was directly detected by a superheterodyne receiver, and Shapiro steps were also measured. The Josephson self-radiation properties reveal good quality of phase locking and microwave coupling with external circuits. The maximum self-radiation power of our array is about 50 pW which is several ten times higher than that of a single Josephson junction, and its peak point exactly satisfies the Josephson current-voltage relation. The Shapiro-step measurements show that the behavior of current-voltage curve depends on the effective inductance of coupling lines which affects the total impedance of Josephson junction array and microwave coupling. The Josephson oscillation frequency was obtained up to about 880 GHz which is 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions. Experimental results show that this type of Josephson junction array can improve the Josephson self-radiation power and increase the maximum detectable frequency.
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Abstract This paper presents a wideband terahertz (THz) mixer based on a thin-film antenna-coupled high-temperature superconducting (HTS) YBa 2 Cu 3 O 7−x (YBCO) step-edge Josephson junction. The HTS mixer enables the flexible harmonic mixing operation at multiple THz bands with the same microwave local oscillator (LO) source, and features very wide intermediate-frequency or instantaneous bandwidth. In order to optimize the frequency down-conversion performance of the mixer, systematic electromagnetic design and analysis have been carried out to improve the power coupling of THz radiation as well as wideband transmission of microwave signals. Experimental characterization of a fabricated device prototype has demonstrated that the mixer exhibits good performance at both the 200 GHz and 600 GHz bands. Detailed measurement results including the DC characteristics, LO pumping requirement, frequency response, mixing linearity and conversion gain are presented in this paper.