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IEEE Transactions on Magnetics

Kinetic-inductance coupling scheme for superconducting stripline system and its device applications

Keiji Yoshida, Keisuke Kudo, Mohammad Sajjad Hossain, Keiji Enpuku

Published Mar 1, 1991• 5 citations
Publisher

Abstract

The properties and applications of the inductive coupling of a superconducting stripline system, which takes advantage of kinetic inductance of thin superconducting films, are studied. The system, composed of two striplines inductively coupled through a common electrode as a four-port circuit, was formulated by obtaining its scattering coefficients in a general form. The system is characterized with the coupled-mode theory. Based on the analysis, it is proposed that a novel type of kinetic inductance devices for microwave analog components, such as directional couplers, microwave switches, variable filters, and efficient coupling elements for striplines. Experiments on stripline coupling with this scheme are also discussed.

DOI
10.1109/20.133767

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A theoretical study is made of the inductive coupling of two superconducting striplines. The study takes advantage of the kinetic inductance, which is remarkable in superconducting films where the thickness is less than the London penetration depth. In the case of weak coupling where the coupled-mode description is most effective, it has been found that the forward-wave coupling leads to power exchange between two striplines with the efficiency of m= kappa 2 /( kappa 2 + Delta 2 ) at a distance of I= pi /2( kappa 2 + Delta 2 ) 1 2/. Here kappa 2 = beta 1 beta 2 M 2 /4L 1 L 2 is the coupling coefficient and delta =(1/2)(B 1 -B 2 ) is the phase mismatch parameter, where beta i , L i (i=1,2) and M are propagation constants, self-inductances and the mutual inductance of the two striplines, respectively. Numerical studies are also made for new types of kinetic inductance devices for microwave components, i.e. a directional coupler and microwave switch using the scattering coefficients in the general case.

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Applied Physics Letters•May 30, 2016•25 citations
The properties and applications of the inductive coupling of a superconducting stripline system, which takes advantage of kinetic inductance of thin superconducting films, are studied. The system, composed of two striplines inductively coupled through a common electrode as a four-port circuit, was formulated by obtaining its scattering coefficients in a general form. The system is characterized with the coupled-mode theory. Based on the analysis, it is proposed that a novel type of kinetic inductance devices for microwave analog components, such as directional couplers, microwave switches, variable filters, and efficient coupling elements for striplines. Experiments on stripline coupling with this scheme are also discussed.
A theoretical study is made of the inductive coupling of two superconducting striplines. The study takes advantage of the kinetic inductance, which is remarkable in superconducting films where the thickness is less than the London penetration depth. In the case of weak coupling where the coupled-mode description is most effective, it has been found that the forward-wave coupling leads to power exchange between two striplines with the efficiency of m= kappa 2 /( kappa 2 + Delta 2 ) at a distance of I= pi /2( kappa 2 + Delta 2 ) 1 2/. Here kappa 2 = beta 1 beta 2 M 2 /4L 1 L 2 is the coupling coefficient and delta =(1/2)(B 1 -B 2 ) is the phase mismatch parameter, where beta i , L i (i=1,2) and M are propagation constants, self-inductances and the mutual inductance of the two striplines, respectively. Numerical studies are also made for new types of kinetic inductance devices for microwave components, i.e. a directional coupler and microwave switch using the scattering coefficients in the general case.
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.
A theoretical and experimental study of the coupling of the flux-flow-type Josephson oscillator (FFO) to an external stripline has been carried out to overcome the large impedance mismatch between them. For efficient coupling and impedance matching at millimeter and submillimeter wavelengths, the authors have proposed a scheme where the Josephson oscillator is inductively coupled to the stripline by a thin superconducting electrode. Theoretical modeling of the coupling section and the design of the stripline impedance-transformer have been carried out. In a preliminary experiment, the authors obtained a power of about 10/sup -7/ W emitted from the FFO into the stripline in the frequency range from 184 GHz to 193 GHz.>
Microwave kinetic inductance detectors (MKID) are superconducting broadband direct detectors. If a superconducting thin film patterned into a resonator is illuminated with photons, Cooper pairs break up and the kinetic inductance of the film changes. This shifts the resonance frequency downwards and the dissipation of the resonator increases, which can be detected in the phase and amplitude of a transmitted signal. \n \nIn this thesis, a waveguide coupled MKID is designed and simulated. 350 GHz radiation is coupled into a copper waveguide via a horn antenna. A waveguide probe leads the signal onto a 9 µm silicon membrane where it is absorbed in a 40 nm superconducting aluminum film which is the inductive part of a superconducting resonator. The capacitor of the resonator is made of niobium. Two versions of the device are developed, a transmission line resonator and a lumped element resonator design. The new design will enable an accurate understanding of the optical sensitivity of MKIDs and allow comparison of different resonator materials. Since two different resonator types are used, the direct comparison of transmission line and lumped resonators will also be possible. \n \nA measurement setup is developed including an ADR cryostat to cool MKID devices down to below 40 mK. An analog readout system using either a vector network analyzer or an IQ mixer is designed and built. The necessary software to control the ADR, the temperature monitoring, the readout signal generation and detection and the signal acquisition of the setup is also developed. \n \nThe waveguide coupled MKID design is fabricated in our in-house microfabrication laboratory and measured in the new setup. In my measurements, the devices do not show a resonance feature. The VNA and DC measurements are analyzed to determine a plausible hypothesis of the failure. The devices are diagnosed for critical issues during fabrication which will be addressed in a future iteration. \n \nTo verify the measurement setup, a test device of the IRAM NIKA 2 MKID camera is measured and the complete analysis pipeline is exercised. The quasi-particle lifetime in the aluminum thin films is measured and a dark and optical noise equivalent power are determined. The results are within the uncertainty comparable to measurements of IRAM themselves. \n \nA test chip designed in my master thesis that contains eight NbTiN resonators is measured in the new setup and non-linear effects under strong readout power are studied. Parametric amplification and oscillation of a resonator are shown qualitatively.
It is pointed out that superconducting microstrip transmission lines operating in the kinetic-inductance limit have the potential to reduce the size and weight of microwave circuits by several orders of magnitude. Since the phase velocity of these guided wave structures is up to a hundred times less than in free space, the electrical length of distributed elements is similarly reduced. Simple transmission lines have been fabricated from both low- and high-transition-temperature superconductors while more complicated circuits, such as filters, with a good match to 50- Omega coax have been fabricated with the low-transition-temperature superconductors. Also, a novel coupled-line topology, compatible with these structures, has been studied analytically.>
We present a tunable coupler scheme that allows us to tune the coupling strength between a feedline and a superconducting resonator in situ over a wide range. In this scheme, we shunt the feedline with a 50-Ω lumped-element nonlinear transmission line made from a 20 nm NbTiN film. By injecting a DC current, the nonlinear kinetic inductance changes and the effective impedance shunting the resonator periodically varies from a short to an open, which tunes the coupling strength and coupling quality factor Qc. We have demonstrated Qc tuning over a factor of 40, between Qc∼5.5×104 and Qc∼2.3×106, for a 4.5 GHz resonator by applying a DC current less than 3.3 mA. Our tunable coupler scheme is easy to implement and may find broad applications in superconducting detector and quantum computing/information experiments.