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IEEE Transactions on Applied Superconductivity

Enhanced microwave power from triangular arrays of small Josephson junctions

P. Caputo, Alexey Vladimirovich Ustinov, Stanford P. Yukon

Published Mar 1, 2001• 1 citation
Publisher

Abstract

We report on high frequency measurements of underdamped Josephson junction arrays consisting of two rows of parallel biased cells. The rf voltage generated on the junctions transverse to the bias current (horizontal junctions) is detected by a room temperature receiver in the W-band. In order to enhance the emitted power, it has been proposed (S.P.Y.) that the single horizontal junction in each cell be replaced by two (or more) horizontal junctions. This geometry has been proven to have better high frequency performance. Here we present comparative measurements of the output power from two row arrays of different geometries. Among the studied configurations, larger power is obtained from an array with square cells and two horizontal Junctions. In this array, the measured output power is up to about 2 times larger than that in the array with only one horizontal junction, and up to 20 times larger with respect to the array with conventional triangular cells. Experimental data are in good qualitative agreement with numerical simulations.

DOI
10.1109/77.919381

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Applied Physics Letters•Sep 1, 1985•12 citations
We report on high frequency measurements of underdamped Josephson junction arrays consisting of two rows of parallel biased cells. The rf voltage generated on the junctions transverse to the bias current (horizontal junctions) is detected by a room temperature receiver in the W-band. In order to enhance the emitted power, it has been proposed (S.P.Y.) that the single horizontal junction in each cell be replaced by two (or more) horizontal junctions. This geometry has been proven to have better high frequency performance. Here we present comparative measurements of the output power from two row arrays of different geometries. Among the studied configurations, larger power is obtained from an array with square cells and two horizontal Junctions. In this array, the measured output power is up to about 2 times larger than that in the array with only one horizontal junction, and up to 20 times larger with respect to the array with conventional triangular cells. Experimental data are in good qualitative agreement with numerical simulations.
A Josephson oscillator array was investigated through both DC and AC experiments. The simulations and analysis of Yukon and Lin suggested that arrays with three junctions per cell might have unique advantages as high frequency oscillators. These triangular arrays operate with a bias flux of half a flux quantum and power is coupled from the horizontal junctions. This configuration allows one to couple from a whole series of junctions that oscillate in-phase. The power was successfully measured from the horizontal junctions by two different methods. In collaboration with Dr. Cawthorne and Professor Lobb at University of Maryland, an on-chip detector junction was used and Shapiro steps were measured. Also, in collaboration with Dr. Caputo and Professor Ustinov at KFA in Juelich, Germany, a finline antenna was placed across the horizontal row of junctions and coupled power off-chip, to a room temperature receiver. Both experiments demonstrated a proof-of-concept, and future work should include improved matching to measure the maximum available power.
When a parallel external magnetic field is applied to underdamped Josephson-junction arrays, constant-voltage steps appear in their current-voltage characteristics. These steps correspond to different numbers of rows being switched to a new resonant state. If the number of switched rows is larger then a threshold number, the array radiates coherent microwave radiation. When the array is biased on a step, the number of radiating rows stays fixed and we can change the input power, P/sub DC/, by changing the bias current. We measure the output power, P/sub AC/ as a function of P/sub DC/. This dependence is linear at high powers with a slope /spl alpha/, while at low powers P/sub AC/ vanishes nonlinearly with P/sub DC/. For a given array, the slope /spl alpha/ is larger for steps that correspond to a larger number of switched rows. We present a systematic study of the dependence of the slope or on the size of the array and discuss its implications for obtaining optimal DC-to-AC conversion efficiency.
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.
This report results from a contract tasking University of Erlangen-Nuremberg as follows: contractor will investigate novel designs of triangular Josephson Junction arrays attempting to improve the high-frequency performance of the arrays in two ways; first, increase the operation frequencies and second, increase the output power of the millimeter-band radiation to level enabling practical application of the arrays. Three approaches will be investigated. First, trimming of currents at the array edges is expected to stabilize coherent oscillations and increase the operation margins (output power). Second, it appears promising to set up a traveling wave in a transmission line that contains an array comparable with the wavelength. Third, testing dipole-like active antennas to couple radiation out from arrays is a high priority task for emission in free space at high frequencies. The contractor will experimentally verify the above concepts by designing new circuits and performing experiments with Nb/AOx/Nb Josephson Junction arrays. For these experiments, the contractor will use a millimeter-wave band receiver as well an on-chip detection circuits up to 300 GHz. Spatially-resolved measurements using low temperature scanning laser microscopy will be performed in order to monitor array dynamics at the junction level. Furthermore, the contractor intends to design a dipole antenna and test it with an integrated receiver in 400-500 GHz frequency range. For this purpose, improvements for a flux-flow oscillator as a local oscillator may be needed. Finally, the contractor will perform experiments with shunted junction arrays in order to simulate intrinsically overdamped high-Tc junctions and explore a possibility of using high Tc layered junctions for oscillator application
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 have studied a new type of Josephson junction amplifier which is based on a two-junction array; the junctions are mutually phase locked at the Josephson self-oscillation frequency. With this frequency at 82 GHz, the voltages of the junctions remain equal (locked) for a bias current range as large as 60% of the critical current. Over a much smaller bias range, with an applied signal frequency of 1 kHz, a small-signal power gain of 19 dB was measured, accompanied by a negative resistance input impedance. This performance is consistent with a quasistatic theory of the amplifier.