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Japanese Journal of Applied Physics

Efficient Higher Harmonic Generation in Current-Biased “Clean” Superconducting Point Contact

Yoshikazu Hayashi, Tatsuji Kato, Hideo Ogawa, Toshihiro Omodaka, Toshiaki Takano

Published Nov 1, 1980• 1 citation
Publisher

Abstract

Spectral analyses are carried out on the strong RF (1.2-1.4 GHz) voltage fluctuations associated with the sudden onset of DC voltage near the critical current in a "clean" superconducting point contact. The strong RF voltage is concluded to arise from the remarkably efficient frequency conversion effects of weak external low-frequency (below 100 MHz) interference. Strong higher harmonics of intentionally-applied signals of about 250 and 90 MHz are observed up to 2 GHz. The mechanism of efficient harmonic generation is discussed briefly.

DOI
10.1143/jjap.19.2283

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Spectral analyses are carried out on the strong RF (1.2-1.4 GHz) voltage fluctuations associated with the sudden onset of DC voltage near the critical current in a "clean" superconducting point contact. The strong RF voltage is concluded to arise from the remarkably efficient frequency conversion effects of weak external low-frequency (below 100 MHz) interference. Strong higher harmonics of intentionally-applied signals of about 250 and 90 MHz are observed up to 2 GHz. The mechanism of efficient harmonic generation is discussed briefly.
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The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb–Nb point-contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequencies fi of 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low-frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
We successfully exploit the parabolic shape of the dc voltage output dip around B = 0 of a superconducting quantum interference filter (SQIF) to mix weak external r f signals. The two-tone response of weak time harmonic electromagnetic fields has been detected on the spectral voltage output of the SQIF at frequency f0= f1- f2, for various frequencies f1and f2ranging from few MHz up to 20 GHz. The two-tone response is a characteristic function of static magnetic field B and of bias current Ib, related to the second derivative of the dc voltage output.
This Letter reports the generation of coherent radiation with voltage-biased superconducting point contacts at both 30 MHz and 10 KMHz. The method of voltage biasing stabilizes the frequency of the emitted radiation against external noise sources and is tunable over a wide frequency range. We have optimized the electromagnetic coupling of the superconducting generator to the receiver and detected at least 5% of the generated power at 30 MHz. The relation1 V = Φ0ν, where Φ0 is the flux quantum, is satisfied. Furthermore, we have observed harmonics, V = nΦ0ν, and subharmonics, nV = Φ0ν, of this relation. The frequency stability of the emitted radiation was limited by the stability of the bias supply and was measured to be 10−5 at 30 MHz.
The dc characteristics of superconducting point-contact junctions subjected to external noise currents are measured under the condition that the noise-frequency spectrum is below the Josephson frequency. It is shown that the curves are in agreement with a model based on modulation of the noise-free dc characteristic. Although in the low-bias-voltage region the shape of the curves is qualitatively similar to previous theoretical predictions for the case of high-frequency noise, for higher voltages the shapes are different. Quantitative differences exist over the whole range of bias voltage.