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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Series-Connected Josephson-Effect Mixers For Millimeter-Wave Radiation

Hiroshi Ohta, Toshiaki Matsui, Bokuji Komiyama

Published Nov 18, 1988• 1 citation
Publisher

Abstract

Millimeterwave detectors have been made of a series-connected array of Josephson junctions. Each junction of the array responds in phase to radiation and therefore the series-connected array as a whole behaves as if it were a single Josephson junction. When the Josephson junctions have a high critical current density, the IF outputs have sharp peaks at the voltages V = 2NΔ/ne where N is the number of the Josephson junctions in series connection and n is an integer. The subharmonic energy gap structures in voltage-current curves of Josephson junctions come from the Andreev-reflection-assisted-tunneling according to the recent Arnold theories.

DOI
10.1117/12.978373

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Millimeterwave detectors have been made of a series-connected array of Josephson junctions. Each junction of the array responds in phase to radiation and therefore the series-connected array as a whole behaves as if it were a single Josephson junction. When the Josephson junctions have a high critical current density, the IF outputs have sharp peaks at the voltages V = 2NΔ/ne where N is the number of the Josephson junctions in series connection and n is an integer. The subharmonic energy gap structures in voltage-current curves of Josephson junctions come from the Andreev-reflection-assisted-tunneling according to the recent Arnold theories.
Mutual phase locking has been demonstrated in series arrays of two and four Josephson junctions at millimeter-wave frequencies. Experimental observations are in good agreement with theory reported earlier. This technique increases the output power available from a Josephson junction source. Available output power is expected to be proportional to the square of the number of junctions until the array impedance approaches the load impedance. The output frequency is voltage tunable over as much as an octave. Theory indicates that the technique can be extended to even larger arrays.
Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.
Recent coupled Josephson junction experiments in our laboratory have demonstrated that high critical current density tunnel junctions can serve as effective local oscillators at frequencies up to and in excess of the gap sum frequency of the junction, i.e. well above 1 Terahertz for a niobium or niobium compound tunnel junction. While the details of the behavior of such a THz. oscillator were found not to be in accord with the predictions of the accepted theory of the A.C. Josephson effect in the gap region significant radiation could be capacitively coupled from the oscillator junction to an adjacent junction, sufficient for SIS mixer experiments at Terahertz frequencies. Research efforts are now under way to further extend and ex-pand these studies. A high critical current density all NbN tunnel junction system is now under development for Terahertz applications and a new set of coupled Josephson oscillator - SIS detector experiments is being initiated using NbN tunnel junc-tions. In this paper we will review the original coupled junction high frequency experiments and report on the recent progress of the current NbN tunnel junction experiments.
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.
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.