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Journal of Applied Physics

Untuned Josephson detectors of microwave and millimeter-wave radiation

C. K. So, William O. Hamilton

Published May 1, 1977• 1 citation
Publisher

Abstract

The intrinsic response of Josephson junctions to microwave and far-infrared radiation is studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junction. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22 to 105 GHz. The dependence of the responsivity of these currents on film thickness over a range from 50 to 500 Å is studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies. The observed responsivity of a 50-Å film junction at 22 GHz is (6±3) ×10−2 V/W. To facilitate comparison of our results with other experiments, a more conventional coupling configuration in which the wave electric field rather than the wave magnetic field is coupled to the junction is studied. The results are comparable with other experiments with the responsivity in the E field coupling case 106 times larger than in the B field coupling configuration. We demonstrate that for successful use of simple Josephson junction detectors in the microwave and millimeter region it seems crucial to impedance match the junctions to the radiation source, probably by making them a part of a resonant structure.

DOI
10.1063/1.323914

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Applied Sciences•Sep 17, 2020•7 citations
The intrinsic response of Josephson junctions to microwave and far-infrared radiation is studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junction. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22 to 105 GHz. The dependence of the responsivity of these currents on film thickness over a range from 50 to 500 Å is studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies. The observed responsivity of a 50-Å film junction at 22 GHz is (6±3) ×10−2 V/W. To facilitate comparison of our results with other experiments, a more conventional coupling configuration in which the wave electric field rather than the wave magnetic field is coupled to the junction is studied. The results are comparable with other experiments with the responsivity in the E field coupling case 106 times larger than in the B field coupling configuration. We demonstrate that for successful use of simple Josephson junction detectors in the microwave and millimeter region it seems crucial to impedance match the junctions to the radiation source, probably by making them a part of a resonant structure.
The response of Josephson junctions to microwave and far infrared radiation was studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junctions. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22GHz to 105GHz. The dependence of the responsivity of these current on film thickness over a range from 50 A to 500 A was studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies.
The coupling of millimeter-wave radiation from a long Josephson junction to a small-area junction is discussed. The coupling is obtained by means of thin-film technology and the design parameters allowing radiation coupling are discussed. Since the frequency of the oscillator radiation is smaller than the plasma frequency of the detector junction, the latter shows evidence of chaotic dynamics in the current-voltage characteristic. However, very stable zero-crossing bias steps appear for high values of the applied radiation power. The tunability of the oscillator power allows measuring the dependence of the detector critical current on the external radiation to be measured; from this measured dependence and from the observed amplitude of the photon-assisted tunneling steps it is estimated that maximum coupled power is the range of tens of nanowatts.
The Josephson junction radiator (JJR) is a thin-film superconducting device that converts DC voltage into coherent millimeter-wave (mm-wave) radiation. The source is frequency-tunable, instantaneously narrow-band, compact, lightweight, has no moving parts, and may share the same cryogenic and vacuum space as the device under test. With these favorable characteristics, the JJR may find use as a general purpose calibration tool for low temperature mm-wave detectors. We show that the JJR unambiguously emits mm-wave radiation and couples to a mm-wave detector. We use the JJR to determine the spectral response of a superconducting mm-wave detector that operates in two large frequency passbands centered at 150 GHz and 220 GHz. A comparison to measurements with a conventional Fourier Transform Spectrometer is presented, and the use of the JJR as a precision spectral calibration tool is discussed.
To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF 4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 – 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.
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.
Superconducting Josephson junctions have major advantages as detectors of millimeter wave radiation. Frequency of the radiation can be easily derived from the Shapiro steps of the current-voltage characteristics. However, system performance is highly sensitive to impedance mismatch between the antenna and the junction; therefore, optimization is essential. We analyzed and implemented an improved antenna structure, in which the junction is displaced from the antenna center and placed between the ends of two matching strips. Based on theoretical analysis and advanced electromagnetic simulations, we optimized strip dimensions, which affect both the detection magnitude and the frequency of the reflection coefficient dip. Accordingly, two Au bow-tie antennas with different matching strip widths were fabricated. Superconducting Yttrium Barium Copper Oxide (YBCO) thin films were deposited exactly at the bicrystal substrate misorientation points, forming Josephson junctions at the ends of two matching strips. We found a very high correlation between the simulations and the response to Radio Frequency (RF) radiation in the range of 145–165 GHz. Experimental results agree extremely well with the design, showing best performance of both antennas around the frequency for which impedance matching was derived.