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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Design considerations for high-gain SIS mixers

Larry R. D’Addario, Michael W. Cromar, F L Lloyd

Published Dec 1, 1985• 0 citations
Publisher

Abstract

It has been shown both theoretically [1] and experimentally [2,3] that the very sharp nonlinearity in the conductance of superconducting tunnel junctions can be used to implement millimeter wavelength mixers having high gain and very low noise, leading to the possibility of receivers that achieve the quantum noise limit. The junctions involved contain a thin insulating barrier between two superconductors, and are thus known as superconductor-insulator-superconductor (SIS) junctions. They are physically identical to Josephson junctions, but in this application the conductance is via the tunneling of quasiparticles (unpaired electrons), and so the Josephson effect is not involved.

DOI
10.1109/irmm.1985.9126604

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Applied Physics Letters•Mar 15, 1980•106 citations
It has been shown both theoretically [1] and experimentally [2,3] that the very sharp nonlinearity in the conductance of superconducting tunnel junctions can be used to implement millimeter wavelength mixers having high gain and very low noise, leading to the possibility of receivers that achieve the quantum noise limit. The junctions involved contain a thin insulating barrier between two superconductors, and are thus known as superconductor-insulator-superconductor (SIS) junctions. They are physically identical to Josephson junctions, but in this application the conductance is via the tunneling of quasiparticles (unpaired electrons), and so the Josephson effect is not involved.
Single-particle (quasi-particle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is being used to make very-low-noise detectors and mixers for millimeter wavelengths. The nonlinearity of theI-Vcurve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon-assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasi-particle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limitT_{M} =\bar{h}\omega/k. Both of these predictions have been experimentally realized at 36 GHz using superconductor-insulator-superconductor (SIS) junctions. It appears probable that these quasi-particle detectors and mixers will supercede the corresponding Josephson-effect devices at millimeter wavelengths.
It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network.< >
Single particle (quasiparticle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is being used to make very low noise detectors and mixers for millimeter wavelengths. The nonlinearity of the I-V curve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasiparticle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limit T sub M = h omega/k. Both of these predictions have been experimentally realized at 36GHz using superconductor-insulator-superconductor junctions. It appears probable that these quasiparticle detectors and mixers will supercede the corresponding Josephson effect devices at millimeter wavelengths. (Author)
Quasiparticle and Josephson tunneling currents in superconducting tunnel junctions provide nonlinear low-noise response up to terahertz frequencies. Many types of direct detector heterodyne mixer and parametric amplifier based on these effects have been explored. Only the SIS quasiparticle mixer has been sufficiently competitive to find significant applications. These mixers can have noise approaching the quantum limit TN = ℏω/k and gain larger than 10 dB. They are used at a number of observatories for molecular line radio astronomy at millimeter wavelengths. Similar applications at submillimeter wavelengths are beginning.
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.
A computer simulation is constructed to describe the performance of superconductor-insulator-superconductor (SIS) quasiparticle heterodyne receivers in terms of photon-assisted tunneling theory. The results predict that conversion efficiencies greater than unity can be realized at millimeter wave frequencies, with noise temperatures approaching the fundamental quantum limit. The nonlinearity responsible for the mixing is the extremely rapid onset of quasiparticle tunneling current in SIS junctions near the full gap voltage 2Δ/e, and is independent of the Josephson pair currents. Substantial gain is expected under a variety of conditions and for a wide range of source and load impedances.