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Superconductor Science and Technology

Terahertz detector with transmission-line type superconducting tunnel junctions

Seiichiro Ariyoshi, Koh Takahashi, Takashi Noguchi, Masahiko Kurakado, K Koga, Naohiro Furukawa, Chiko Otani

Published Jun 12, 2012• 1 citation
Publisher

Abstract

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.

DOI
10.1088/0953-2048/25/7/075011

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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.
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.
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.
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.
We are developing a broadband THz detectors using Nb-based superconducting tunnel junctions. We found two components in a current signal for 1-2 THz pulse input. From the comparison of results obtained for the signals, the frequency response for the different size of STJs and the STJs fabricated on different substrate materials, we demonstrate that they correspond to photons absorbed with the superconductor electrode (referred as 'direct' component) and phonons generated by photons absorbed with the substrate (refered as 'phonon-mediated' component).
Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Δν/ν<10-6). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.