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Journal of Applied Physics

Fundamental and harmonic mixing at 500 GHz using a superconductor-insulator-normal metal junction

Lawrence M. Chernin, R. Blundell, Edward Tong, Ronald Earle Miller

Published Oct 15, 1992• 4 citations
Publisher

Abstract

We have demonstrated fundamental and harmonic mixing using a PbInAu/InOxPbOx/Ag superconductor-insulator-normal metal tunnel junction mixer designed to operate over the 450–550 GHz frequency range. Direct mixing experiments at up to 1.8 times the gap frequency of the tunnel junction have shown that the mixer works in both fundamental and second harmonic modes, with the highest conversion from the fundamental mode obtained at bias voltages near the gap voltage and the highest conversion from the harmonic mode obtained at zero bias. The measured mixer conversion curves are consistent with our five-port model simulations based on Tucker’s quantum mixer theory [IEEE J. Quantum. Electron. QE-15, 1234 (1979)].

DOI
10.1063/1.352284

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Journal of Communications Technology and Electronics•Jun 1, 2011•10 citations
We have demonstrated fundamental and harmonic mixing using a PbInAu/InOxPbOx/Ag superconductor-insulator-normal metal tunnel junction mixer designed to operate over the 450–550 GHz frequency range. Direct mixing experiments at up to 1.8 times the gap frequency of the tunnel junction have shown that the mixer works in both fundamental and second harmonic modes, with the highest conversion from the fundamental mode obtained at bias voltages near the gap voltage and the highest conversion from the harmonic mode obtained at zero bias. The measured mixer conversion curves are consistent with our five-port model simulations based on Tucker’s quantum mixer theory [IEEE J. Quantum. Electron. QE-15, 1234 (1979)].
A theoretical analysis of harmonic mixing in a quasiparticle tunnel junction at Vbias=0 shows that harmonic mixing for submillimeter frequencies is a viable alternative to fundamental mixing. It is also shown that the signal gain from the first harmonic above the pump frequency to the intermediate frequency is independent of the embedding admittance at the pump frequency. In fact, we show that the five-port model reduces to the three-port model as a result of the inversion symmetry of the I-V curve and, moreover, no large-signal analysis is necessary. Full simulations of harmonic mixing using a five-port model confirm these results. The simulations have been done for the case of a superconductor-insulator-normal junction in which there is no pair current interference. For this type of harmonic mixer we show that the local oscillator (LO) power requirement can be less than a fundamental mixer operating at the same signal frequency, which is particularly useful in circumventing the problems of high-frequency LO power generation.
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.
The noise and gain of a heterodyne waveguide mixer using Nb/Al/sub 2/O/sub 3//Nb superconducting tunnel junctions were measured in the 400-500-GHz frequency range. Three different arrays of two junctions in series are analyzed. The minimum receiver noise temperature is 120 K DSB at 480 GHz, measured with an array having integrated tuning stubs. The authors compare data of the pumped I-V curves with the Werthammer-Tucker theory and demonstrate an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs. a mixer noise temperature of 90+or-30 K and a DSB mixer gain of -12.5+or-0.6 dB were measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows good qualitative agreement, indicating the applicability of this theory to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage is 220 K at 495 GHz. This frequency is 85% of the reduced gap frequency, indicating that Nb superconductor insulator-superconductor (SIS) mixers can be used up at least the gap frequency of 680 GHz.>
We experimentally investigate different mixing regi-mes in Nb/AlOx/Nb and Nb/AlN/NbN tunnel superconductor-insulator-superconductor (SIS) junctions. The SIS mixers were studied with regard to the following modes of operation: in an extremely low-frequency range (0.1-20 GHz) and as high harmonic mixers; n ≈ 20-30 at signal frequencies of about 500 GHz that requires high power of a local oscillator. The quasiparticle and Josephson mixing regimes have been compared. We demonstrate that, in some applications requiring high conversion efficiency, the Josephson regime can be more preferable than the quasiparticle regime due to a larger output signal and better signal-to-noise ratio. As an example of such an application, we consider the phase locking of a THz oscillator using a cryogenic harmonic phase detector, which only ought to realize all of its benefits under the Josephson mixing regime. In addition, we demonstrate that the presence of the Josephson effect results in considerable increase in conversion efficiency at the expense of the dynamic range decrease; it was measured for low-frequency up- and down-convertors, which are promising for cryogenic multiplexing systems.