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Japanese Journal of Applied Physics

Frequency Mixing of 50 GHz Microwave and 4.25 THz FIR Light by a Josephson Point Contact

Yukinobu Miki, Eiichi Sakuma

Published Sep 1, 1987• 7 citations
Publisher

Abstract

The harmonic mixing of 50 GHz microwave and 4.25 THz far-infrared (FIR) light, the highest order mixing ever reported in this frequency range, has been achieved using a Josephson point contact made of Nb. The conditions under which the mixing was performed are described. The conversion efficiency of the incident FIR light to the beat signal is evaluated and the performance limit of the Nb Josephson point contact is also discussed.

DOI
10.1143/jjap.26.l1482

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Japanese Journal of Applied Physics•Nov 1, 1985•1 citation
The harmonic mixing of 50 GHz microwave and 4.25 THz far-infrared (FIR) light, the highest order mixing ever reported in this frequency range, has been achieved using a Josephson point contact made of Nb. The conditions under which the mixing was performed are described. The conversion efficiency of the incident FIR light to the beat signal is evaluated and the performance limit of the Nb Josephson point contact is also discussed.
Point-contact-type Josephson junctions have been developed as a harmonic mixer to measure far-infrared (FIR) laser frequencies in a frequency chain up to the infrared region. The junctions were formed by mechanically pressing a niobium wire of 70 μm in diameter with a sharply etched tip against a niobium post of 2 mm in diameter. The voltage-current characteristics of the mixers were investigated under the irradiation of FIR lights from an optically pumped methyl-alcohol laser. The Shapiro steps for FIR lights up to 3.1 THz were observed. The mixing properties of the mixers were studied through harmonic mixing experiments of 50-GHz microwaves and the FIR lights up to the frequency of 3.1 THz. The conversion efficiency of the beat signals to the incident FIR power was found to vary as the — 4th power of the FIR frequency. From the mixing properties, the combination of the critical current Icof the junction and the product IcRnwhere Rnis the normal resistance of the junction, was found important to optimize the mixing performance.
1 Introduction Compared with its semiconductor counterpart, Josephson junction (J. J.) usually has larger nonlinearity, lower noise temperature, wider operating waveband and less microwave loss, thus making itself a good candidate for applications at submillimeter and far-infrared wavebands. Some harmonic mixing experiments were done previously using N_b point-contact junctions, with the highest signal frequency being about 4.25 THz and the largest harmonic number being 851. However, because of the requirement of liquid
For mixing in a Josephson junction at infrared frequencies, we have shown that the available power from the junction increases as the intermediate frequency is increased. Following this result an infrared receiver has been developed incorporating a 9-GHz maser preamplifier at the i.f. Using this system, the beat between the 401st harmonic of a high-quality microwave source and a 3.8-THz infrared laser has been observed. Also, for low-order mixing at 3.8 THz, a comparison of beat signals from a Josephson junction and a room-temperature mixer has been made.
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.
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.