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Revue de Physique Appliquée

The stripline coupling of thin-film josephson junctions to external circuits at microwave frequencies

Y. Sirkeinen, P. Somervuo, T. Wiik

Published Jan 1, 1974• 5 citations
Publisher

Abstract

A certain way of coupling to the thin-film Josephson tunneling junction has been studied. The coupling structure comprises a superconducting stripline which is terminated to the edge of the Nb-oxide-Pb junction. A mismatch of 23 dB has been obtained so far between the signal source and the junction at X-band compared to the earlier estimates of 40 dB between conventional size outer circuits and the junction. Prospects for improving the coupling are good. Methods of estimating the voltage and power which are coupled to the junction are given.

DOI
10.1051/rphysap:0197400901013100

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Journal of Applied Physics•Sep 1, 1981•0 citations
A certain way of coupling to the thin-film Josephson tunneling junction has been studied. The coupling structure comprises a superconducting stripline which is terminated to the edge of the Nb-oxide-Pb junction. A mismatch of 23 dB has been obtained so far between the signal source and the junction at X-band compared to the earlier estimates of 40 dB between conventional size outer circuits and the junction. Prospects for improving the coupling are good. Methods of estimating the voltage and power which are coupled to the junction are given.
A promising method for microwave coupling to thin-film Josephson devices via microstripline techniques has been developed which has significant advantages over more traditional waveguide techniques. In particular, direct determination of intrinsic junction cavity parameters such as the geometrical resonance frequencies and Q values are made practical and compact cryogenic multi-octave microwave holders readily realized. The results of coupling experiments with Pb-Pb oxide-Pb and Nb-Nb oxide-Pb tunnel junctions are discussed and applications of these results to the design and construction of shielded single junction 2e/h devices and small multi-junction arrays are described.
The inductive coupling between a Josephson junction and a superconducting stripline is calculated. In the overlapping region they share a superconducting film, the structure having superconducting (S) and insulating layers (I) sandwiched S—I—S—I—S. The coupling arises from the magnetic field penetration through the common superconductor. An equivalent circuit representation is exhibited which has the same modes and coupling as the original system. Using the circuit analog, the power transfer is calculated for several cases of interest: (a) power is radiated outward in the stripline from a biased junction and (b) power is absorbed by the junction from a source coupled to the stripline. The criterion for matching the tunnel junction to the stripline is discussed.
The fabrication of a planar fin-line circuit for rf-coupling to Josephson tunnel junctions and SIS devices is reported. E-field coupling is achieved by means of tapered transitions from a rectangular waveguide to a fin-line and from this line to the active device. Its low impedance can be matched by an antipodal fin-line with a fin-separation in the μm-range. Circuit parameters have been determined for Nb-Pb-devices mounted in a split Ka-band waveguide. Measurements were performed in the frequency range from 33 GHz and 38 GHz. Mismatches as low as 10 dB have been observed.
A major factor in the practical utilization of any Josephson device for microwave and higher frequency applications (such as the generation and detection of electromagnetic radiation) is the success with which the junction device can be coupled to an external transmission line. As part of an experimental investigation of the electrodynamics of small arrays of Josephson tunnel junctions, the properties of the coherent radiation emitted both by individual junctions and by coupled junction systems have been studied at frequencies between 2 and 12 GHz. Near 9 GHz, more than 10-9 W of coherent radiation has been detected from a single Pb-Pb oxide-Pb junction coupled via waveguide. The development of thin film devices in which the tunnel junctions form part of a microstrip line have made it feasible to observe directly (in a single device) the radiation emitted both at the fundamental geometrical frequency and at the low order harmonics. The observed temperature dependence of the junction cavity Q indicate the low temperature losses are primarily due to cavity loading of the external transmission line.
The design, operation and details of construction of a fully shielded microwave-integrated-circuit holder suitable for use with thin-film superconducting microstrip circuitry including couplers, filters, impedance transformers and devices such as Josephson junctions are described. Several simple microwave coupling schemes for use with Josephson junction 2e/h frequency-to-voltage transducers illustrate the flexibility of the microstrip approach for coupling to thin-film Josephson junctions and semiconductor devices at cryogenic temperatures.
Multisection superconducting microstrip transformers with designed output impedances below 0.1 Ω have been fabricated via precise photolithographic techniques to investigate the electromagnetic properties of Nb-Nb oxide-Pb tunnel junctions. The low-impedance transformer sections incorporate a rf sputtered thin-film Ta-oxide dielectric, and the reproducible external coupling achievable with this type of geometry makes possible the systematic investigation of electromagnetic device parameters as a function of tunneling oxide thickness.