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Superconductor Science and Technology

Detected power and linewidth of Josephson self-radiation from frequency-locked and microwave-coupled arrays in a waveguide detection system

Insang Song, Kwang‐Yong Kang, Gwangseo Park

Published Feb 1, 1998• 9 citations
Publisher

Abstract

Frequency-locked and microwave-coupled arrays of high- superconducting Josephson junctions have been developed. The Josephson junctions were prepared on bicrystal MgO substrates using a pulsed laser ablation method. The maximum detected power of Josephson self-radiation, which is directly detected by a superheterodyne receiver with a central frequency of 22 GHz, from the arrays was enhanced with increasing number of Josephson junctions. The Josephson self-radiation properties of the arrays revealed a good quality of frequency locking and exactly satisfied the Josephson voltage-frequency relationship. A narrower linewidth of the Josephson self-radiation was obtained by increasing the number of Josephson junctions in the array.

DOI
10.1088/0953-2048/11/2/004

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•Jan 1, 2000•2 citations
Frequency-locked and microwave-coupled arrays of high- superconducting Josephson junctions have been developed. The Josephson junctions were prepared on bicrystal MgO substrates using a pulsed laser ablation method. The maximum detected power of Josephson self-radiation, which is directly detected by a superheterodyne receiver with a central frequency of 22 GHz, from the arrays was enhanced with increasing number of Josephson junctions. The Josephson self-radiation properties of the arrays revealed a good quality of frequency locking and exactly satisfied the Josephson voltage-frequency relationship. A narrower linewidth of the Josephson self-radiation was obtained by increasing the number of Josephson junctions in the array.
We fabricated a Josephson junction array with an additional inductive coupling line to detect the high frequency and power of Josephson self-radiation. The YBa2Cu3O y Josephson junctions are fabricated by a pulsed laser deposition method on bicrystal MgO substrates. We measured the submillimeter wave and the Josephson self-radiation power in an external waveguide system. The Josephson junction arrays showed the resistively shunted junction (RSJ)-like current–voltage characteristics and good frequency-locking of Josephson self-radiation. As the number of Josephson junctions increased, the maximum detected power was increased. We observed the maximum frequency of above 6 THz for Josephson self-radiation in the array with 11 Josephson junctions from the Shapiro steps under 22 GHz irradiation.
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.
We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.
We have investigated the mutual phase interaction between a single detector junction and a phase-locked three-junction array of high-T/sub c/ Josephson junctions. The YBCO Josephson junctions were fabricated on a MgO bicrystal substrate. In the array junction structure, the dc bias current was fed to the junction array having parallel geometry with each pair of junctions shunted by a superconducting coplanar microstrip superconducting loop. The configuration of bias leads was a series connection of interlocking dc SQUIDs geometry which enables all junctions to oscillate at the same frequency. We measured the radiation power of a Josephson-junction array using an off-chip coupled to a radiometer and on-chip coupled to a single junction from centimeter- to millimeter-wave ranges. Clear self-induced Shapiro steps were observed in the current-voltage curves of the detector junction. The power transfered to the single detector junction was estimated from the modulation of Shapiro step height. The maximum detected power level was about 7 nW at frequency 150-250 GHz. The Josephson oscillation power excited the resonant mode coupled to the coplanar microstrip loops.
We proposed a novel approach to self-radiation of Josephson junction arrays. Bicrystal Josephson junction arrays fabricated on a bicrystal substrate were integrated with a half-wavelength dipole array and embedded in a Fabry–Perot resonator. Considered as a dielectric resonator, the substrate could be stimulated to resonate in an appropriate mode at the same frequency as the Fabry–Perot resonator, which realized a coupling between Josephson junctions as well as a coupling between the junctions and microwave structures by means of a resonant interaction between junctions and resonators. Both simulations and experiments were performed. A YBCO bicrystal Josephson junction array with 166 junctions was measured at liquid nitrogen temperature. The maximal self-radiation power of 10 pW was detected at 75.2 GHz, which was in agreement with the simulation results.
High-Tc Josephson junction arrays with coupling by superconducting short- circuited (5 µm wide and 0.1 µm thick) coupled microstrip lines, provided dc bias in parallel, were fabricated on bicrystal sapphire substrates for experimental investigation of the phase- locking phenomenon. Josephson self radiation, emitted by the array was measured by an external 90 GHz receiver for different applied dc magnetic fields. The numerical simulation of array shows that strong interaction between Josephson junctions and the standing waves within structure require small enough damping α≤ 10 -3 per coupling section for reduction of Josephson oscillation linewidth.