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IEEE Transactions on Magnetics

Millimeterwave emission from Josephson oscillator through thin film junction electrode

Keiji Yoshida, Tadao Nagatsuma, S. Kumataka, Keiji Enpuku

Published Mar 1, 1987• 12 citations
Publisher

Abstract

An experiment on the coupling of the flux-flow type Josephson oscillator to SIS detectors by inductive coupling scheme has been carried out. Radiated powers from the Josephson oscillator were detected by an array of Josephson tunnel junctions fabricated on top of the upper electrode of the oscillator. The thickness of the common electrode was made thin enough to allow inductive coupling through the electrode. The maximum value of the obtained voltage amplitude of the emitted wave from the oscillator was 1.7mV, which is in reasonable agreement with the theoretical one. The present method can be an efficient coupling scheme between the Josephson oscillator and external circuits.

DOI
10.1109/tmag.1987.1064921

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In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.

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Y.M. Zhang, Dag Winkler, T. Claeson

The authors report on the coupling of millimeter and submillimeter waves from a long (3- mu m*350- mu m) Josephson junction to a small (2- mu m*2- mu m) superconductor-insulator-superconductor (SIS) junction by using a center-fed interdigital capacitor. Coupled power at around 100 GHz and 261 GHz was observed. Experiments confirm that the critical current density j/sub c/ of a long junction determines the fluxon motion. Coupled irradiation (>3nW) at around 100 GHz, which corresponds to resonant soliton oscillations in low j/sub c/ samples (250 A/cm/sup 2/ to 1000 A/cm/sup 2/), is detected by the SIS junction. The incident power to the SIS detector junction could be tuned with only little frequency pulling by adjusting the current bias of the oscillator. For j/sub c/>1000 A/cm/sup 2/, coupled radiation at around 100 GHz (about 3 nW) and 261 GHz (4.6 nW), from flux-flow oscillations, is also clearly seen. Flux-flow radiation at 350 GHz is also observed. It is concluded that a large j/sub c/ value is preferable for high-frequency flux-flow oscillators.>

IEEE Transactions on Applied Superconductivity•Mar 1, 1993•18 citations
An experiment on the coupling of the flux-flow type Josephson oscillator to SIS detectors by inductive coupling scheme has been carried out. Radiated powers from the Josephson oscillator were detected by an array of Josephson tunnel junctions fabricated on top of the upper electrode of the oscillator. The thickness of the common electrode was made thin enough to allow inductive coupling through the electrode. The maximum value of the obtained voltage amplitude of the emitted wave from the oscillator was 1.7mV, which is in reasonable agreement with the theoretical one. The present method can be an efficient coupling scheme between the Josephson oscillator and external circuits.
Power emission from a flux-flow-type Josephson oscillator with a thin-film coupling method is studied experimentally. The thickness of one electrode of the oscillator is made thin enough to allow electromagnetic fields generated by the oscillator to be emitted through the thin-film electrode. Radiation power from the oscillator is detected with superconductor-insulator-superconductor detectors, which are fabricated on top of the oscillator. Experimental results show that sufficient power can be obtained from the oscillator by using the thin-film coupling method when the thickness of the electrode is comparable to the London penetration depth. The results obtained are in reasonable agreement with theoretical predictions. The thin-film coupling method will be useful when the oscillator is connected to external circuits, such as an impedance matching circuit.
A theoretical and experimental study of the coupling of the flux-flow-type Josephson oscillator (FFO) to an external stripline has been carried out to overcome the large impedance mismatch between them. For efficient coupling and impedance matching at millimeter and submillimeter wavelengths, the authors have proposed a scheme where the Josephson oscillator is inductively coupled to the stripline by a thin superconducting electrode. Theoretical modeling of the coupling section and the design of the stripline impedance-transformer have been carried out. In a preliminary experiment, the authors obtained a power of about 10/sup -7/ W emitted from the FFO into the stripline in the frequency range from 184 GHz to 193 GHz.>
The coupling of millimeter-wave radiation from a long Josephson junction to a small-area junction is discussed. The coupling is obtained by means of thin-film technology and the design parameters allowing radiation coupling are discussed. Since the frequency of the oscillator radiation is smaller than the plasma frequency of the detector junction, the latter shows evidence of chaotic dynamics in the current-voltage characteristic. However, very stable zero-crossing bias steps appear for high values of the applied radiation power. The tunability of the oscillator power allows measuring the dependence of the detector critical current on the external radiation to be measured; from this measured dependence and from the observed amplitude of the photon-assisted tunneling steps it is estimated that maximum coupled power is the range of tens of nanowatts.
A theoretical study is made of a travelling-wave-type oscillator, which utilizes a flux flow in a long Josephson junction for use as a local oscillator in the integrated superconducting receiver system. An internal electromagnetic field of the oscillator junction in the flux-flow state is investigated both numerically and analytically. It is shown that the voltage amplitude of the internal oscillation increases gradually in the direction of the flux flow and reaches a maximum value at the junction end. An equivalent circuit of the oscillator is also obtained, which gives dependences of the emitted radiation on frequency, magnetic field, and load. It is shown that the output power attains the value of the order of 10−6 W in the frequency range between 100 and 500 GHz, and that the output power and the radiation frequency can be controlled by both the bias voltage and the applied magnetic field. These theoretical results explain quantitatively the experimental ones with a Pb-alloy long junction of length 24 λJ.
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.
The authors report on the coupling of millimeter and submillimeter waves from a long (3- mu m*350- mu m) Josephson junction to a small (2- mu m*2- mu m) superconductor-insulator-superconductor (SIS) junction by using a center-fed interdigital capacitor. Coupled power at around 100 GHz and 261 GHz was observed. Experiments confirm that the critical current density j/sub c/ of a long junction determines the fluxon motion. Coupled irradiation (>3nW) at around 100 GHz, which corresponds to resonant soliton oscillations in low j/sub c/ samples (250 A/cm/sup 2/ to 1000 A/cm/sup 2/), is detected by the SIS junction. The incident power to the SIS detector junction could be tuned with only little frequency pulling by adjusting the current bias of the oscillator. For j/sub c/>1000 A/cm/sup 2/, coupled radiation at around 100 GHz (about 3 nW) and 261 GHz (4.6 nW), from flux-flow oscillations, is also clearly seen. Flux-flow radiation at 350 GHz is also observed. It is concluded that a large j/sub c/ value is preferable for high-frequency flux-flow oscillators.>