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Electronics and Communications in Japan (Part I Communications)

Detection with net‐structure arrays of Josephson Junctions

Norio Fujimaki, Yoichi Okabes, Sogo Okamura

Published Nov 1, 1980• 0 citations
Publisher

Abstract

Abstract Several studies have been performed on fabrication of Josephson junction arrays to improve the voltage sensitivity and impedance matching of Josephson junctions to be used as high‐speed and high‐sensitivity detectors of microwaves and submillimeter‐waves. This paper proposes a method for arranging point‐contact Josephson junctions in a net structure and describes experimental results of detection. First, we measured the I‐V characteristic at super low temperature and found that the critical current can be adjusted from a few μA to a few mA by varying the contact pressure. Next, we obtained the equivalent circuit of this net‐structure array installed in an x‐band waveguide in such a way that the array is perpendicular to the wave traveling direction. The net‐structure array was treated as a uniform surface impedance and the series admittance due to the gap between the array and the waveguide wall was obtained by an extension of the analysis for a capacitive window. To compare the equivalent circuit parameters with measured values, we carried out an impedance measurement at room temperature. The results agreed well with theoretical values. Further, a preliminary experiment for a wideband detection was performed in the superconducting state and obtained detection voltages for 10‐GHz signals as well as HCN laser light of wavelength 337 μm.

DOI
10.1002/ecja.4410631109

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Abstract Several studies have been performed on fabrication of Josephson junction arrays to improve the voltage sensitivity and impedance matching of Josephson junctions to be used as high‐speed and high‐sensitivity detectors of microwaves and submillimeter‐waves. This paper proposes a method for arranging point‐contact Josephson junctions in a net structure and describes experimental results of detection. First, we measured the I‐V characteristic at super low temperature and found that the critical current can be adjusted from a few μA to a few mA by varying the contact pressure. Next, we obtained the equivalent circuit of this net‐structure array installed in an x‐band waveguide in such a way that the array is perpendicular to the wave traveling direction. The net‐structure array was treated as a uniform surface impedance and the series admittance due to the gap between the array and the waveguide wall was obtained by an extension of the analysis for a capacitive window. To compare the equivalent circuit parameters with measured values, we carried out an impedance measurement at room temperature. The results agreed well with theoretical values. Further, a preliminary experiment for a wideband detection was performed in the superconducting state and obtained detection voltages for 10‐GHz signals as well as HCN laser light of wavelength 337 μm.
Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide-mounted experimental point-contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18 W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small-signal modulation of the staircase like dc I-V characteristics. It was shown that a small-signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk-formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well-known step structure in the dc I-V characteristic of the point-contact Josephson junction. Mixing of the Lo and signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.
We have investigated the microwave response of nanotube Josephson junctions at 600–900 MHz at microwave powers corresponding to currents from 0 to 2 × IC in the junction. Compared with theoretical modeling, the response of the junctions corresponds well to the lumped element model of resistively and capacitively shunted junction. We demonstrate the operation of these superconducting FETs as charge detectors at high frequencies without any matching circuits. Gate-voltage-induced charge QG modifies the critical current IC, which changes the effective impedance of the junction under microwave irradiation. This change, dependent on the transfer characteristics dIC/dQG, modifies the reflected signal and it can be used for wide band electrometry. We measure a sensitivity of 3.1×10−5 e/Hz from a sample which has a maximum switching current of 2.6 nA.
The possibilities and physical mechanisms of detection of microwave X- and Ka-band radiation by high-temperature superconducting (HTS) YBa2Cu3O7-0.05 (YBCO) bicrystalline Josephson junctions (JJ) embedded in a surface wave resonator (SWR) are considered. Microwave detection was investigated experimentally by using a linear Josephson junction array (JJA) embedded in the SWR. Measurements were carried out starting from the critical temperature of HTS film (T = 92 K), down to the liquid nitrogen level (T = 77 K) and beyond (down to 20 K) using the Gifford-McMahon closed-cycle cryo-cooler. The sensitivity of such type of detectors was determined to be approximately 104 V/W. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We fabricated a Josephson junction array with an additional inductive coupling line to detect the high frequency and power of Josephson self-radiation. The YBa2Cu3O y Josephson junctions are fabricated by a pulsed laser deposition method on bicrystal MgO substrates. We measured the submillimeter wave and the Josephson self-radiation power in an external waveguide system. The Josephson junction arrays showed the resistively shunted junction (RSJ)-like current–voltage characteristics and good frequency-locking of Josephson self-radiation. As the number of Josephson junctions increased, the maximum detected power was increased. We observed the maximum frequency of above 6 THz for Josephson self-radiation in the array with 11 Josephson junctions from the Shapiro steps under 22 GHz irradiation.
A high temperature superconducting YBa2Cu3Oy array of five Josephson junctions designed with additional coupling lines has been developed to demonstrate the effects of frequency locking and impedance matching for applications such as oscillators, mixers, and detectors. The Josephson self-radiation power was directly detected by a superheterodyne receiver, and Shapiro steps were also measured. The Josephson self-radiation properties reveal good quality of phase locking and microwave coupling with external circuits. The maximum self-radiation power of our array is about 50 pW which is several ten times higher than that of a single Josephson junction, and its peak point exactly satisfies the Josephson current-voltage relation. The Shapiro-step measurements show that the behavior of current-voltage curve depends on the effective inductance of coupling lines which affects the total impedance of Josephson junction array and microwave coupling. The Josephson oscillation frequency was obtained up to about 880 GHz which is 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions. Experimental results show that this type of Josephson junction array can improve the Josephson self-radiation power and increase the maximum detectable frequency.
Sensitivity of high temperature superconducting (HTS) Josephson detectors at millimetre-wave band has been studied in detail. YBCO grain boundary Josephson junctions with the normal-state resistance ( R N ) around 1 Ω and I C R N product of less than 2 mV have been fabricated for this study, where I C is the critical current of the junction. A Gunn oscillator at the frequency of 98 GHz together with a set of calibrated attenuators with a total attenuation of 90 dB are used as a signal source. A quasi-optical system, which consists of a wideband planar log-periodic antenna encompassing the junction, parabolic mirrors and a hyperhemispherical silicon lens, has been used as an effective coupling system. Sensitivity increases with I C R N . Using a junction with I C R N of 300 µV, a sensitivity of about 9000 V W −1 has been obtained, which is higher than that of the Schottky diode detectors at the same frequency band and demonstrates that the HTS Josephson detectors are very sensitive at millimetre-wave band.