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IEEE Transactions on Magnetics

Microwave detector response of the Josephson junction arrays

G. A. Ovsyannikov, G.E. Babaian, V. N. Laptev, V. I. Makhov

Published Mar 1, 1991• 2 citations
Publisher

Abstract

Microwave detector response ( eta ) for three types of superconducting structures-single edge-type Josephson junctions, arrays of series-connected junctions, and arrays of RF-series and DC parallel-connected junctions-were investigated in the frequency range 35-120 GHz. Experimental dependence of eta versus biasing voltage is mainly determined by the interaction of junctions in the array. The selective response, which means a narrow linewidth of Josephson radiation, is observed for the third type of superconducting structure in a small magnetic field changing the phase shift between closely-spaced junctions. These arrays have the advantage in coupling with the external microwave system.

DOI
10.1109/20.133766

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Japanese Journal of Applied Physics•Aug 1, 1996•1 citation
Microwave detector response ( eta ) for three types of superconducting structures-single edge-type Josephson junctions, arrays of series-connected junctions, and arrays of RF-series and DC parallel-connected junctions-were investigated in the frequency range 35-120 GHz. Experimental dependence of eta versus biasing voltage is mainly determined by the interaction of junctions in the array. The selective response, which means a narrow linewidth of Josephson radiation, is observed for the third type of superconducting structure in a small magnetic field changing the phase shift between closely-spaced junctions. These arrays have the advantage in coupling with the external microwave system.
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 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.
The intrinsic response of Josephson junctions to microwave and far-infrared radiation is studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junction. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22 to 105 GHz. The dependence of the responsivity of these currents on film thickness over a range from 50 to 500 Å is studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies. The observed responsivity of a 50-Å film junction at 22 GHz is (6±3) ×10−2 V/W. To facilitate comparison of our results with other experiments, a more conventional coupling configuration in which the wave electric field rather than the wave magnetic field is coupled to the junction is studied. The results are comparable with other experiments with the responsivity in the E field coupling case 106 times larger than in the B field coupling configuration. We demonstrate that for successful use of simple Josephson junction detectors in the microwave and millimeter region it seems crucial to impedance match the junctions to the radiation source, probably by making them a part of a resonant structure.
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.
Effects on microwave detection of a series-connected Josephson junction array are examined. The series-connected 10 step-edge Josephson junctions (10-SEJJs) are fabricated by Y1Ba2Cu3O7- x thin films on MgO substrates. Their responsivities as microwave broad-band detectors are evaluated by constructing microstrip circuits for measuring transmission and return power. 948 V/W is obtained at 17 K. This value is several times greater than 139 V/W of a single SEJJ at 17 K. Additionally, the return power from 10-SEJJs is less than that from the single SEJJ, as the bias current or the temperature increases. The small return power is attributed to the increase in the normal resistance of the SEJJ.