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Bowtie loaded meander antenna for a high-temperature superconducting terahertz detector and its characterization by the Josephson effect

Mei Yu, Haifeng Geng, Shoulu Jiang, Tao Hua, Deyue An, Weiwei Xu, Zhi Ning Chen, Jun Li, Huabing Wang, Jian Chen, Peiheng Wu

Published Apr 19, 2020• 7 citations

Abstract

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.

DOI: 10.1364/oe.390997

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The high-Tcsuperconducting (HTS) bicrystal YBa2Cu3O7-δ(YBCO) zero-bias-operation Josephson junction (JJ) harmonic mixer coupled with a bowtie loaded meander antenna (BLMA) is proposed and studied at 0.21 THz. When the JJ mixer coupled with the BLMA, the maximal harmonic number increases up to 146 with the conversion efficiency of -63.6 dB at the bath temperature around 4.6 K compared to a mixer coupled with a log-periodic antenna (LPA) of only a maximal harmonic number of 46 with the conversion efficiency of -71.6 dB. The mixer coupled with the BLMA has a higher order harmonic mixing with a lower local oscillator (LO) power of 1.26 dBm, which further illustrates its superiority to that coupled with a LPA with the LO power of 5.59 dBm. The experiment verifies the merits of the JJ harmonic mixer operating at zero bias as a promising device in terahertz regions.

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.

This paper presents a terahertz (THz) spectrometer based on high temperature superconducting Josephson junction. The YBa2Cu3O7(YBCO) bi-crystal Josephson junctions (JJS) with log-periodic antennas are used as detectors. In the low temperature, with the signal measurement, data acquisition and human-computer interactive system, the automatic measurement system of THz spectrometer is finally accomplished and realizes the measurement of the wide frequency range from 0.1THz to 2.5THz. The frequency resolution reaches 0.04GHz (@ 114GHz) and 2GHz (@ 1.78THz). The spectral line width (Δf) is preliminarily analyzed.

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.

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.