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Радиотехника и электроника

Investigation of superconducting transmission lines and tunnel junctions for detecting signals at frequencies above 1 THz.

Nickolay V. Kinev, Artem M. Chekushkin, Fedor V. Khan, Kirill I. Rudakov

Published Sep 1, 2023• 0 citations
Publisher

Abstract

Superconducting integrated circuits based on NbTiN/Al transmission lines at frequencies up to 1.1 THz have been developed and experimentally studied. Numerical modeling of two microcircuit topologies with an operating frequency range of 0.9…1.2 THz, containing a slot antenna, made in a thin NbTiN film and matched in output to a microstrip transmission line, and a tunnel junction of the “superconductor–insulator–superconductor” (SIS) type with an area of the order of 1 μm^2, acting as a terahertz detector. Experimental samples of the microcircuit were manufactured and tested; in the experimental setup, a backward wave lamp (BWL) with an output frequency of up to 1.1 THz was used as a source. A powerful pumping of the SIS detector was obtained, thereby demonstrating the applicability of the manufactured NbTiN/Al transmission lines for operation in superconducting circuits at frequencies above 750 GHz, where traditionally used Nb/Nb transmission lines do not operate due to high attenuation.

DOI
10.31857/s0033849423090127

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Superconducting integrated circuits based on NbTiN/Al transmission lines at frequencies up to 1.1 THz have been developed and experimentally studied. Numerical modeling of two microcircuit topologies with an operating frequency range of 0.9…1.2 THz, containing a slot antenna, made in a thin NbTiN film and matched in output to a microstrip transmission line, and a tunnel junction of the “superconductor–insulator–superconductor” (SIS) type with an area of the order of 1 μm^2, acting as a terahertz detector. Experimental samples of the microcircuit were manufactured and tested; in the experimental setup, a backward wave lamp (BWL) with an output frequency of up to 1.1 THz was used as a source. A powerful pumping of the SIS detector was obtained, thereby demonstrating the applicability of the manufactured NbTiN/Al transmission lines for operation in superconducting circuits at frequencies above 750 GHz, where traditionally used Nb/Nb transmission lines do not operate due to high attenuation.
Abstract Superconducting integrated circuits based on NbTiN/Al transmission lines at frequencies of up to 1.1 THz have been developed and experimentally investigated. The numerical simulation has been carried out for two topologies of a microcircuit with an operating frequency range of 0.9‒1.2 THz, which contains a slot antenna formed in the NbTiN thin film and output-matched with a microstrip transmission line and a superconductor‒insulator‒superconductor (SIS) tunnel junction with an area of ~1 µm2 acting as a terahertz detector. Experimental samples of the microcircuit have been fabricated and tested in an experimental setup utilizing a backward-wave oscillator with an output frequency of up to 1.1 THz used as a source. The powerful pumping of the SIS detector has been obtained to demonstrate the applicability of the fabricated NbTiN/Al transmission lines for operation in superconducting circuits at frequencies above 750 GHz, where the conventional Nb/Nb transmission lines cannot operate due to high losses.
The results of the development and testing of superconducting integrated microcircuits based on NbTiN/Al transmission lines at the frequencies up to 1.1 THz are presented. The integrated circuits consist of a slot planar antenna made of thin film of NbTiN coupled to a NbTiN/Al microstrip line, and a superconductor-insulator-superconductor (SIS) junction based on Nb/AlN/NbN operating as a terahertz (THz) detector. Two different designs having the operating range of 0.9-1.2 THz were numerically simulated, fabricated and experimentally tested. A strong pumping of SIS detector by a signal of the backward wave oscillator (BWO) around frequency of 1.05 THz was observed, which demonstrated the applicability of fabricated transmission lines at frequencies higher than 750 GHz, where the traditionally used Nb/Nb transmission lines cannot operate.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
Josephson junctions (JJs) with non-hysteretic I-V characteristic and high critical parameters are required for many applications; in particular, they are needed to create tunable THz range oscillators. We fabricated and studied shunted superconductor-insulator-superconductor (SIS) Nb-AlOx-Nb tunnel junctions with an area of about 1μm2, estimated their parameters, and compared them with theoretical models. To assess the performance of shunted junctions in the terahertz range, their characteristics were studied under the influence of a high-frequency signal; the estimated value of the characteristic voltage for the shunted junction at a high frequency is about 1 mV. To improve the circuit parameters, a new type of the shunting based on application of NIS junctions was proposed, which reduces the influence of parasitic inductance, expands the frequency range of the generator, and decreases the size of the structure. The JJs with a new type of shunting have been fabricated and studied. The niobium integrated circuits intended for matching the superconducting oscillator and an SIS junction as a microwave detector were designed, fabricated, and tested. Such structures are required for studying the spectral characteristics and analyzing the operation of the oscillator. The developed circuits have demonstrated efficient operation at frequencies from 300 to 700 GHz; the experimental results are in good agreement with the simulated characteristics. The results obtained will be implemented for further development of the THz range oscillators.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AIO(x)/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region.
We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.