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Soviet Journal of Quantum Electronics

Superconducting Nb3Sn point contact in the submillimeter range of electromagnetic radiation

É. M. Belenov, M. V. Danileǐko, V. E. Derkach, V. I. Romanenko, Alexander V. Uskov

Published May 31, 1988• 0 citations
Publisher

Abstract

An investigation was made of the influence of submillimeter radiation emitted by an HCN laser operating at a frequency νl = 891 GHz on a superconducting point contact made of Nb3Sn. Three steps of the electric current were recorded. The experimental results indicated that such a contact could be used for frequency multiplication up to 3 THz.

DOI
10.1070/qe1988v018n05abeh012260

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Experiments are described in which Josephson point contacts were used to generate and mix harmonics (up to the 96th) of microwave klystrons with the radiation from an HCN laser at 891 GHz. A qualitative analysis of mixing of two external radiations in a Josephson junction is given in order to understand the existence of mixing components independent of the Josephson oscillation frequency and linewidth. The spectral analysis of the HCN laser shows the importance of the microwave source spectral purity when frequency multiplication by large factors is considered.

Revue de Physique Appliquée•Jan 1, 1977•3 citations
An investigation was made of the influence of submillimeter radiation emitted by an HCN laser operating at a frequency νl = 891 GHz on a superconducting point contact made of Nb3Sn. Three steps of the electric current were recorded. The experimental results indicated that such a contact could be used for frequency multiplication up to 3 THz.
An experimental investigation was made of superconducting point contacts (SPC's), made of the hightemperature superconductor Nb 3 Sn, in external microwave fields. It was found that under nonequilibrium conditions these SPC's had a high degree of nonlinearity which made it possible to observe current steps corresponding to harmonics of the external radiation frequency at voltages exceeding the limiting value for these SPC's. Maximum frequency multiplication up to 4.2 × 10 12 Hz was recorded for an 8.9 × 10 9 Hz signal and up to 5 × 10 12 Hz for a 7.8 × 10 10 Hz signal. Investigations were made of the subharmonic structure of the current-voltage characteristics of SPC's in external microwave fields. Mixing of the microwave radiation with the contact self-radiation was observed experimentally at gap singularities.
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 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but 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. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
Recently there has been a marked upsurge in interest in the construction of multipliers and mixers for the SHF and IR regions. Of special interest in this respect are Josephson junctions fabricated from high-temperature superconductors. The present paper reports the results of experimental investigations of superconducting point junctions (SPJ) fabricated from the high-temperature superconductor Nb3Sn, and operating in regimes of frequency multiplication and mixing in the SHF spectral region. A controllable Nb3Sn Nb3 Sn SPJ that could be adjusted after being immersed in liquid helium was used in the experiments. The method of preparation of the SPJ was the same as the one that was employed in Ref. 1. The assembled experimental setup allowed the SPJ to be irradiated with three-centimeter and four-millimeter microwaves separately and simultaneously. The current-voltage characteristics (CVO) of the (tuned) SPJ were registered on an oscillograph and then recorded automatically at different DHF-power levels. The differential resistance RD = dU/dl of the junction as a function of mixing voltage UO was recorded with the aid of a lock-in amplifier2 co nstructed by us. All the measurements were carried out at the liquid-helium temperature. The SPJ in question had the following parameters: the critical current Ic = 0.1–2.5 rnA and the resistance in the normal state Rn = 0.1–10 Ω. When the SPJ was subjected to 8.9- and 87-GHz radiation separately, the CVC exhibited current steps at voltages corresponding to the fundamental frequency and the harmonics of the external radiation, as well as a sub harmonic structure. The multiplication of the fundamental frequency to 1 THz was recorded in the case of the irradiation of the SPJ by an 8.9–GHz signal and the registration of the dependence of RD on the mixing voltage U0. This differential CVC exhibited a subharmonic structure near the fundamental frequency at low SHF-power levels and near this frequency and its harmonics at higher levels. When the SPJ was irradiated by 87-GHz signals, the multiplication of the fundamental frequency of the signals to 2.2 THz was recorded on the differential CVC. The SHF-power dependence of the harmonics and subharmonics had an oscillatory character. The mechanism underlying the formation of the sub harmonic structure on the CVC of the SPJ can be explained if we take account of the effect of the action of the Josephson self-radiation on the junction itself.3 It should be noted that the manifestation of the Josephson properties in the frequency range ∼1 · 1012 Hz was observed for a SPJ with high Ic and Rn ⩾ 1 Ω. When the Nb3Sn–Nb3Sn SPJ was irradiated by two signals with frequencies 87 and 8.9 GHz simultaneously, there appeared near the steps for the 87-GHz signal , as the power of the 8.9-GHz signal was increased, steps corresponding to signals with frequencies equal to combinations of the two frequencies, i.e., (87m ± 8.9k)-Ghz combination steps appeared on the eve. Thus, the first experimental investigations of the multiplication and mixing of signals by Nb3Sn-Nb3Sn SPJ have shown that such SPJ operate quite well in regimes of mixing and multiplication of external SHF radiation.
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.
Experiments are described in which Josephson point contacts were used to generate and mix harmonics (up to the 96th) of microwave klystrons with the radiation from an HCN laser at 891 GHz. A qualitative analysis of mixing of two external radiations in a Josephson junction is given in order to understand the existence of mixing components independent of the Josephson oscillation frequency and linewidth. The spectral analysis of the HCN laser shows the importance of the microwave source spectral purity when frequency multiplication by large factors is considered.