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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Superconducting detectors for submillimeter-wave astronomy

Eric E. Bloemhof

Published Jul 15, 1993• 0 citations
Publisher

Abstract

Sensitive heterodyne receivers are being built at ever higher frequencies with superconducting (SIS) junctions as the first mixer. These devices have extremely sharp non-linearities in their current-voltage characteristics as a result of quantum-mechanical tunneling of electrons across thin insulating barriers. The low energy scale set by the magnitude of the superconducting energy gap implies very low local oscillator power requirements for heterodyne operation. Some general system design considerations for astrophysical receivers are reviewed. These principles are illustrated by discussing two specific applications: a 230 GHz SIS receiver recently installed as a facility instrument at the Swedish-ESO submillimeter telescope in Chile, and the broader receiver requirements of the 6-antenna submillimeter array (SMA), an interferometer now being designed at the Harvard-Smithsonian Center for Astrophysics. The SMA will require receivers at frequencies as high as 820 GHz, and will place some unique demands on detector performance.

DOI
10.1117/12.148071

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Sensitive heterodyne receivers are being built at ever higher frequencies with superconducting (SIS) junctions as the first mixer. These devices have extremely sharp non-linearities in their current-voltage characteristics as a result of quantum-mechanical tunneling of electrons across thin insulating barriers. The low energy scale set by the magnitude of the superconducting energy gap implies very low local oscillator power requirements for heterodyne operation. Some general system design considerations for astrophysical receivers are reviewed. These principles are illustrated by discussing two specific applications: a 230 GHz SIS receiver recently installed as a facility instrument at the Swedish-ESO submillimeter telescope in Chile, and the broader receiver requirements of the 6-antenna submillimeter array (SMA), an interferometer now being designed at the Harvard-Smithsonian Center for Astrophysics. The SMA will require receivers at frequencies as high as 820 GHz, and will place some unique demands on detector performance.
This text describes in some detail the design and operational history of an instrument used as the front-end receiver for a fast, broadband, high-resolution spectrometer for the 1.3 millimeter wavelength atmospheric window. Using only a single superconductor-insulator-superconductor (SIS) tunnel junction as its heterodyne detector, the receiver’s novel design achieved then unprecedented RF and IF bandwidths and incorporated several innovations which have since been widely adopted within the millimeter and submillimeter wave research communities. Although intended as a relatively simple technology demonstrator and starting point for more refined and sophisticated designs, the receiver turned out to be a useful astronomical instrument in its own right, and it was deployed as a de facto facility instrument for several years at the Caltech Submillimeter Observatory. Also described are the author’s contributions to another important aid to research and design efforts: the SuperMix software library for the analysis and optimization of high-frequency circuitry, especially developed to aid in the design of systems involving SIS and other superconducting elements. Finally, the text may serve as a useful introduction to the theory behind and methodology for modeling and design of SIS heterodyne mixers.
The progress made in relation to the development of sensitive submillimeter heterodyne receiver systems is reviewed. Sensitive superconductor-insulator-superconductor (SIS) tunnel junction mixers were demonstrated at frequencies of greater than 1 THz, with noise temperatures of below 1000 K. An alternative mixer technology is that of the superconducting hot electron bolometer, for which sensitivities of 1.2 THz and 2.5 THz were demonstrated. Local oscillator technology is under development. Gunn oscillators followed by Schottky varactor frequency multipliers have sufficient output power to drive SIS mixers at 1 THz. It is considered that optical photomixer local oscillators have potential for driving hot electron bolometers above 1 THz.
Observation and analysis of submillimeter-wave radiation (300GHz-3THz) in astronomy and atmospheric sciences requires increasingly performant receivers. The most sensitive receivers working in this range of electromagnetic spectrum use superconductor-insolator-superconductor (SIS) junctions. In order to increase the bandwidth and the sensitivity, we are developing a quantum-noise limited heterodyne receiver based on several parallel SIS junctions with broad (larger than 30%) fixed tuned bandwidth. These circuits can be viewed as passband filters which have been optimized by varying the spacings between junctions. We have designed such 5-junction arrays for operation in the range 480-640 GHz. Fabrication and heterodyne characterization of these devices has been done. The 1 μm2 junctions current density ranges from 4 to 13 kA/cm2, using optical lithography and Nb/Al2Nb5/Nb trilayer sputtering technology. The fabrication process and yield are presented in this paper, along with measured performances
Superconducting tunnel diodes (SISs) are investigated for both the generation and detection of submillimeter wavelengths. For detection, heterodyne receivers using SISs are investigated. Josephson currents are found to have a large effect on SIS mixers in the submillimeter, even though their effect is less at lower frequencies. At 492 GHz, fixer conversion gain increased by 4 dB when Josephson currents are not suppressed by a magnetic field. A much higher noise is seen in this gain mode, but further investigation of this recently discovered phenomenon is warranted. For submillimeter generation, a two-dimensional array of SISs is investigated. The ac Josephson effect in the SIS is expected to deliver submillimeter power into a free-space radiation mode traveling perpendicular to the array. No experimental devices have been developed yet, but the authors show computer simulations and microwave scale model measurements of array structures.
Superconductor-insulator-superconductor (SIS) mixers are the most sensitive heterodyne receivers for mm and submm wavelengths. SIS mixers are installed on all state of the art submm observatories such as the Atacama Large Millimeter array (ALMA) and the Atacama Pathfinder Experiment (APEX). Recently, an improved type of Nb/AlN/NbN SIS junction has become available that combines higher gap voltage with high current density. This opens new parametric space for further improvement of the SIS mixer's performance and bandwidth both for frequencies higher and lower than the gap frequency of Nb (700 GHz). In this contribution we will describe designs of SIS mixers based on a new type of SIS junction, both for high (~900 GHz) and low (~200 GHz) frequencies.
At the Smithsonian Astrophysical Observatory, we have been developing expertise in the construction of high-frequency heterodyne receivers using SIS junctions as their first mixing elements. This research has been carried out in support of the Submillimeter Array (SMA), a radio interferometer soon to be built on Mauna Kea that will require multiple reciever systems at a number of bands between 177 GHz and 920 GHz. In this paper, we describe research on the fabrication of all-refractory SIS junctions. To be useful as high-frequency mixers, SIS junctions must have small area and high critical current density. At the lower SMA frequencies, Nb/Al-Oxide/Nb ahs proven to be an ideal material system, and optical photolithography has sufficient resolution for junction definition. At the higher frequencies, NbN/MgO/NbN may enjoy an advantage due to its larger energy gap, and electron-beam lithography may be necessary for defining junctions much smaller than one micron in size. We report good initial results for Nb/Al-Oxide/Nb junctions, fabricated using a self-aligned optical technique, with area $AP 0.5 micrometers 2 and critical current density $AP 10,000 Amp/cm2.