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CIE International Conference on Radar (RADAR)

Terahertz oscillator of superconducting planar Josephson junction array

Pei Wang, Wenshuai Wang, Hongwei Gao, Jizhou Yu

Published Oct 1, 2016• 0 citations
Publisher

Abstract

This paper describes an array of superconducting planar Josephson junctions (JJs) for terahertz emission. The JJs with bowtie antennae are fabricated with superconducting thin film deposited on the bicrystal dielectric substrate and connected in series by superconducting striplines. The dielectric substrate is regarded as a dielectric resonator to improve the electromagnetic coupling between the JJs for coherent terahertz emission. With the CST electromagnetic software, the S parameters, E-field distribution and farfield pattern of the JJ array are studied by adjusting the width of the bowtie antennae. The results show that standing waves are formed in the dielectric substrate around 420 GHz, which could contribute to the coherent terahertz emission of the superconducting oscillators.

DOI
10.1109/radar.2016.8059292

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This paper describes an array of superconducting planar Josephson junctions (JJs) for terahertz emission. The JJs with bowtie antennae are fabricated with superconducting thin film deposited on the bicrystal dielectric substrate and connected in series by superconducting striplines. The dielectric substrate is regarded as a dielectric resonator to improve the electromagnetic coupling between the JJs for coherent terahertz emission. With the CST electromagnetic software, the S parameters, E-field distribution and farfield pattern of the JJ array are studied by adjusting the width of the bowtie antennae. The results show that standing waves are formed in the dielectric substrate around 420 GHz, which could contribute to the coherent terahertz emission of the superconducting oscillators.
The experimental results of mm-wave coherent emission from a high temperature superconducting bicrystal Josephson junction array embedded in a Fabry-Perot resonator are reported in this paper.The mm-wave coherent emission from the series junction array results from the interaction of the junctions with the substrate serving as the dielectric resonator as well as the Fabry-Perot resonator.The central frequency of emission of a Josephson junction array containing 166 bicrystal junctions was 75.84 GHz,and the detected emission power was about 10 pW at 77 K.
High temperature superconducting (HTS) Josephson junction (JJ) mixers are attractive candidates in receivers for terahertz (THz) communication due to their high sensitivity, wide bandwidth, low local oscillator (LO) power requirement and high harmonic numbers. The required LO frequencies and the cost can be greatly reduced when harmonic signals can be detected at high orders (more than 100). In this paper, up to 114thharmonic mixings is presented in a HTS bicrystal YBa2Cu3O7-δ(YBCO) JJ harmonic mixer coupled with a bowtie loaded meander antenna at 0.21 THz. HTS JJs will be promising high-order harmonic mixers for THz practical applications on reception or detection at THz bands.
High-Tc Josephson junction arrays with coupling by superconducting short- circuited (5 µm wide and 0.1 µm thick) coupled microstrip lines, provided dc bias in parallel, were fabricated on bicrystal sapphire substrates for experimental investigation of the phase- locking phenomenon. Josephson self radiation, emitted by the array was measured by an external 90 GHz receiver for different applied dc magnetic fields. The numerical simulation of array shows that strong interaction between Josephson junctions and the standing waves within structure require small enough damping α≤ 10 -3 per coupling section for reduction of Josephson oscillation linewidth.
In a quasi-optical system, the high temperature superconducting terahertz detector often suffers from a fundamental problem of low coupling efficiency with the terahertz signal, especially for the detector based on YBa2Cu3O7-δ (YBCO) bicrystal Josephson junction (JJ) due to a small normal-state resistance. Here, we developed a bowtie loaded meander antenna to enhance coupling efficiency. Differing from the conventional characterization confining on vector network analyzers, we applied three methods to evaluate the antenna, including the measurements of the maximal size of the first order Shapiro step under per incident power, the coupling efficiency between the antenna and the junction, and voltage responsivity. Furthermore, with simulation analysis, we propose that the inductive reactance of the YBCO bicrystal JJ is around 60 ohms under terahertz irradiation at 210 GHz, thus, the reactance is comparable as that of the antenna.
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.
We observed coherent emission at the frequency of 76 GHz from a large-area array of 7500 niobium junctions embedded in the quasioptical resonator at liquid helium temperature. The maximal detected power of 2 μW was measured by a room temperature superheterodyne receiver. To explain the coherent behavior of the array of Josephson junctions, electromagnetic field simulations were performed. Considering the substrate as a dielectric resonator antenna, the coherent emission was excited if the locations of each subarray coincided with the positions of the maximum electric field intensity in the substrate resonance mode.