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IEEE Transactions on Applied Superconductivity

Fourier transform spectrometer studies (300-1000 GHz) of Nb-based quasi-optical SIS detectors

Victor Y. Belitsky, Stellan Jacobsson, Lyudmila V. Filippenko, C. Holmstedt, V. P. Koshelets, E. Kollberg

Published Jan 1, 1995• 8 citations
Publisher

Abstract

Three Nb/AlO/sub x//Nb SIS detectors, designed to operate in the 400-550, 550-700, and 600-750 GHz bands, have been studied in direct detection mode using a Fourier-transform spectrometer. All three detectors were of quasi-optical type and had on-chip-integrated-fixed tuned SIS junctions. The tuning ranges of the detectors were selected to cover the interesting region around the superconducting gap frequency of Nb (about 700 GHz). Measurements show detector responses at frequencies above the gap frequency, i.e., up to /spl ap/920 GHz, and that cooling the detectors to 3.1 K improved the direct detection responses about 15% below 700 GHz and about 50% for frequencies up to 800 GHz, compared to the responses at 4.2 K. The 500 GHz SIS detector was also studied in a 440-520 GHz heterodyne receiver set up. Good agreement between modeled tuning circuit characteristics, tuning range of the mixer and the direct detection response bandwidths were found. However, it is essential that the dispersion of the field penetration depth into the superconductor is included in the modeling of the tuning circuits when the detector is operated at frequencies above the superconducting gap.>

DOI
10.1109/77.413149

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Theoretical and Experimental Studies of Nb-Based Tuning Circuits for THz SIS Mixers

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Three Nb-A10x-Nb SIS mixers tuned to operate in the 400-550, 550-700 and 600-750 GHz frequency bands have been investigated by modeling and experimentally by employing Fourier Transform Spectrometer (FTS) technique and mixer measurements. The mixers were of quasioptical type with spiral antennas and twin junction tuning circuits integrated onto the apex area of the antenna arms. Modeling was employed for optimizing the tuning circuits and evaluating its performance. The comprehensive model for the SIS mixer bases on Tucker-Feldman theory of quasiparticle SIS mixer and Mattis-Bardeen theory of anomalous skin-effect. Very good agreement between modeling, FTS measurements and mixer experiment results was achieved. Accuracy in prediction of the tuning band and the coupling efficiency for the tuning circuits is better or about 15% within 400-1000 GHz band. Introduction The SIS mixer optimum performance can be reached when the intrinsic capacitance of the SIS junction is tuned out. Different integrated on chip tuning circuits have been developed for waveguide and open structure (quasioptical) Nb-A10x-Nb SIS mixers for 100-820 GHz band [16] providing broadband low-noise operation of the receivers [7-11]. These tuning circuits mainly utilize different microstrip lines, made from Nb with SiO or Si02 insulator, to resonate out SIS junction capacitance. The focus of this work is theoretical (by modeling) and experimental investigation of twin junction tuning circuit [4-6] for frequency band about 400-1000 GHz. Mattis-Bardeen theory was employed for the tuning circuit modeling. Fourier Transformer Spectrometer technique as well as mixer measurements of developed SIS mixers were used to determine tuning circuits performance, to compare experimental data and results of modeling. At the first step of the tuning circuit development simplified free-of-loss model has been employed for evaluating of the tuning circuit behavior and its optimization [11,191 The coupling efficiency of the spiral antenna output and the tuning circuit input was used as a quality parameter for loss-free circuitry (Fig. 1). However Fourier Transform Spectrometer (FTS) measurements of the tuning circuits [19] have shown noticeable discrepancy between tuning circuit coupling efficiency at above the gap frequency and model predicted performance. This associated with loss in Nb microstrip tuning circuitry. Near the gap frequency of Nb material (-690 GHz for film) the Nb based microstrip lines become dispersing [12, 13] and above this frequency having increasing losses [14, 15]. Mattis– Bardeen theory [16] of skin anomalous effect describes behavior of a superconductor at these frequencies in teillis of complex conductivity a: cr= G–ja2 , (1) where real a l and imaginary 32 components translate directly into the normal-electron and cooperpare currents in a superconductor. Following fundamental paper of R.L. Kautz [14] these components above can be presented in integral form: zo (7) Page 88 Sixth International Symposium on Space Terahertz Technology 2 E2±2 + E j = de[f (s)— f (e + hco)} O 7: ha) 2 2 1/2 2 2 A h + 1 ' er dep. 2f(hco hw i, 21 — A2 — A2} 112 (s2 _2 ) / (11(0 _ e) ha)

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The authors have tested the performance of planar SIS (superconductor-insulator-superconductor) mixers with log-periodic antennas at near millimeter and submillimeter wave frequencies from 90 to 360 GHz. The large omega R/sub N/C product ( approximately 10 at 90 GHz) of the Nb/NbO/sub x//Pb-In-Au junctions requires an integrated inductive tuning element to resonate the junction capacitance at the operating frequencies. Two types of integrated tuning element were used which were designed with the aid of measurements using a Fourier transform spectrometer. Preliminary results indicate that the tuning elements can give very good mixer performance up to at least 200 GHz. An inductive wire in parallel with a 5-junction array gives a minimum mixer noise temperature of 115 K (DSB) at 90 GHz with a FWHM (full width at half maximum) bandwidth of 8 GHz. An open-ended microstrip stub in parallel with a single junction gives minimum mixer noise temperatures at 150 and 200 K (DSB) near 90 and 180 GHz with FWHM bandwidths of 4 and 3 GHz, respectively. The relatively high mixer noise temperatures compared to those of waveguide SIS mixers in a similar frequency range are attributed mainly to the losses in the optical system.>

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Mei Bin, Michael C. Gaidis, Jonas Stasys Zmuidzinas +2 more

We recently reported (1996) a quasi-optical SIS mixer which used Nb/Al-oxide/Nb tunnel junctions and a normal-metal (Al) tuning circuit to achieve an uncorrected receiver noise temperature of 840 K (DSB) at 1042 GHz. Here we present results on several different device designs, which together cover the 300-1200 GHz frequency range. The mixers utilize an antireflection-coated silicon hyper-hemispherical lens, a twin-slot antenna, and a two-junction tuning circuit. The broad-band frequency response was measured using Fourier transform spectrometry (FTS), and is in good agreement with model calculations. Heterodyne tests were carried out from 400 GHz up to 1040 GHz, and these measurements agree well with the FTS results and with calculations based on Tucker's theory (1985).

IEEE Transactions on Applied Superconductivity•Jun 1, 1997•24 citations
Three Nb/AlO/sub x//Nb SIS detectors, designed to operate in the 400-550, 550-700, and 600-750 GHz bands, have been studied in direct detection mode using a Fourier-transform spectrometer. All three detectors were of quasi-optical type and had on-chip-integrated-fixed tuned SIS junctions. The tuning ranges of the detectors were selected to cover the interesting region around the superconducting gap frequency of Nb (about 700 GHz). Measurements show detector responses at frequencies above the gap frequency, i.e., up to /spl ap/920 GHz, and that cooling the detectors to 3.1 K improved the direct detection responses about 15% below 700 GHz and about 50% for frequencies up to 800 GHz, compared to the responses at 4.2 K. The 500 GHz SIS detector was also studied in a 440-520 GHz heterodyne receiver set up. Good agreement between modeled tuning circuit characteristics, tuning range of the mixer and the direct detection response bandwidths were found. However, it is essential that the dispersion of the field penetration depth into the superconductor is included in the modeling of the tuning circuits when the detector is operated at frequencies above the superconducting gap.>
Three Nb-A10x-Nb SIS mixers tuned to operate in the 400-550, 550-700 and 600-750 GHz frequency bands have been investigated by modeling and experimentally by employing Fourier Transform Spectrometer (FTS) technique and mixer measurements. The mixers were of quasioptical type with spiral antennas and twin junction tuning circuits integrated onto the apex area of the antenna arms. Modeling was employed for optimizing the tuning circuits and evaluating its performance. The comprehensive model for the SIS mixer bases on Tucker-Feldman theory of quasiparticle SIS mixer and Mattis-Bardeen theory of anomalous skin-effect. Very good agreement between modeling, FTS measurements and mixer experiment results was achieved. Accuracy in prediction of the tuning band and the coupling efficiency for the tuning circuits is better or about 15% within 400-1000 GHz band. Introduction The SIS mixer optimum performance can be reached when the intrinsic capacitance of the SIS junction is tuned out. Different integrated on chip tuning circuits have been developed for waveguide and open structure (quasioptical) Nb-A10x-Nb SIS mixers for 100-820 GHz band [16] providing broadband low-noise operation of the receivers [7-11]. These tuning circuits mainly utilize different microstrip lines, made from Nb with SiO or Si02 insulator, to resonate out SIS junction capacitance. The focus of this work is theoretical (by modeling) and experimental investigation of twin junction tuning circuit [4-6] for frequency band about 400-1000 GHz. Mattis-Bardeen theory was employed for the tuning circuit modeling. Fourier Transformer Spectrometer technique as well as mixer measurements of developed SIS mixers were used to determine tuning circuits performance, to compare experimental data and results of modeling. At the first step of the tuning circuit development simplified free-of-loss model has been employed for evaluating of the tuning circuit behavior and its optimization [11,191 The coupling efficiency of the spiral antenna output and the tuning circuit input was used as a quality parameter for loss-free circuitry (Fig. 1). However Fourier Transform Spectrometer (FTS) measurements of the tuning circuits [19] have shown noticeable discrepancy between tuning circuit coupling efficiency at above the gap frequency and model predicted performance. This associated with loss in Nb microstrip tuning circuitry. Near the gap frequency of Nb material (-690 GHz for film) the Nb based microstrip lines become dispersing [12, 13] and above this frequency having increasing losses [14, 15]. Mattis– Bardeen theory [16] of skin anomalous effect describes behavior of a superconductor at these frequencies in teillis of complex conductivity a: cr= G–ja2 , (1) where real a l and imaginary 32 components translate directly into the normal-electron and cooperpare currents in a superconductor. Following fundamental paper of R.L. Kautz [14] these components above can be presented in integral form: zo (7) Page 88 Sixth International Symposium on Space Terahertz Technology 2 E2±2 + E j = de[f (s)— f (e + hco)} O 7: ha) 2 2 1/2 2 2 A h + 1 ' er dep. 2f(hco hw i, 21 — A2 — A2} 112 (s2 _2 ) / (11(0 _ e) ha)
Nb-based superconductor - insulator - superconductor (SIS) mixers with Nb tuning circuits have demonstrated good results up to the Nb gap frequency. Above the gap frequency the performance is expected to degrade quickly because RF loss in Nb becomes significant. In this paper we present the results of an effort to extend Nb-based SIS mixers to THz frequencies by employing lower-loss normal-metal Al wiring and tuning structures. The SIS mixer has two Nb/Al-oxide/Nb junctions connected by an Al microstrip inductor. The direct detection response of the device was measured by a Fourier transform spectrometer. A double-side-band receiver noise temperature of 840 K was obtained at 1042 GHz when the device was operated at 2.5 K.
This thesis describes the development of low-noise heterodyne receivers at THz frequencies for submillimeter astronomy using Nb-based superconductor-insulator-superconductor (SIS) tunneling junctions. The mixers utilize a quasi-optical configuration which consists of a planar twin-slot antenna and antisymmetrically-fed two-junctions on an antireflection-coated silicon hyperhemispherical lens. On-chip integrated tuning circuits, in the form of microstrip lines, are used to obtain maximum coupling efficiency in the designed frequency band. To reduce the rf losses in the integrated tuning circuits above the superconducting Nb gap frequency (~ 700 GHz), normal-metal Al is used to replace Nb as the tuning circuits. To account the rf losses in the micros trip lines, we calculated the surface impedance of the AI films using the nonlocal anomalous skin effect for finite thickness films. Nb films were calculated using the Mattis-Bardeen theory in the extreme anomalous limit. Our calculations show that the losses of the Al and Nb microstrip lines are about equal at 830 GHz. For Al-wiring and Nb-wiring mixers both optimized at 1050 GHz, the RF coupling efficiency of Al-wiring mixer is higher than that of Nb-wiring one by almost 50%. We have designed both Nb-wiring and Al-wiring mixers below and above the gap frequency. A Fourier transform spectrometer (FTS) has been constructed especially for the study of the frequency response of SIS receivers. This FTS features large aperture size (10 inch) and high frequency resolution (114 MHz). The FTS spectra, obtained using the SIS receivers as direct detectors on the FTS, agree quite well with our theoretical simulations. We have also, for the first time, measured the FTS heterodyne response of an SIS mixer at sufficiently high resolution to resolve the LO and the sidebands. Heterodyne measurements of our SIS receivers with Nb-wiring or Al-wiring have yielded results which arc among the best reported to date for broadband heterodyne receivers. The Nb-wiring mixers, covering 400 - 850 GHz band with four separate fixed-tuned mixers, have uncorrected DSB receiver noise temperature around 5hv/kb to 700 GHz, and better than 540 K at 808 GHz. An Al-wiring mixer designed for 1050 GHz band has an uncorrected DSB receiver noise temperature 840 K at 1042 GHz and 2.5 K bath temperature. Mixer performance analysis shows that Nb junctions can work well up to twice the gap frequency and the major cause of loss above the gap frequency is the rf losses in the microstrip tuning structures. Further advances in THz SIS mixers may be possible using circuits fabricated with higher-gap superconductors such as NbN. However, this will require high-quality films with low RF surface resistance at THz frequencies.
This paper presents the direct-detection responses of a 500-GHz and a 660-GHz waveguide superconducting SIS (superconductor-insulator-superconductor) mixer measured with a Fourier transform spectrometer (FTS). The two superconducting SIS mixers adopt the same type of junction tuning circuit (i.e., parallel connected twin SIS junctions), but are of different designs for the junction feed point where RF signals are coupled and then transmitted to the SIS junction tuning circuit and have different mixer design criteria. The experimental results are discussed in detail.
The authors have tested the performance of planar SIS (superconductor-insulator-superconductor) mixers with log-periodic antennas at near millimeter and submillimeter wave frequencies from 90 to 360 GHz. The large omega R/sub N/C product ( approximately 10 at 90 GHz) of the Nb/NbO/sub x//Pb-In-Au junctions requires an integrated inductive tuning element to resonate the junction capacitance at the operating frequencies. Two types of integrated tuning element were used which were designed with the aid of measurements using a Fourier transform spectrometer. Preliminary results indicate that the tuning elements can give very good mixer performance up to at least 200 GHz. An inductive wire in parallel with a 5-junction array gives a minimum mixer noise temperature of 115 K (DSB) at 90 GHz with a FWHM (full width at half maximum) bandwidth of 8 GHz. An open-ended microstrip stub in parallel with a single junction gives minimum mixer noise temperatures at 150 and 200 K (DSB) near 90 and 180 GHz with FWHM bandwidths of 4 and 3 GHz, respectively. The relatively high mixer noise temperatures compared to those of waveguide SIS mixers in a similar frequency range are attributed mainly to the losses in the optical system.>
We recently reported (1996) a quasi-optical SIS mixer which used Nb/Al-oxide/Nb tunnel junctions and a normal-metal (Al) tuning circuit to achieve an uncorrected receiver noise temperature of 840 K (DSB) at 1042 GHz. Here we present results on several different device designs, which together cover the 300-1200 GHz frequency range. The mixers utilize an antireflection-coated silicon hyper-hemispherical lens, a twin-slot antenna, and a two-junction tuning circuit. The broad-band frequency response was measured using Fourier transform spectrometry (FTS), and is in good agreement with model calculations. Heterodyne tests were carried out from 400 GHz up to 1040 GHz, and these measurements agree well with the FTS results and with calculations based on Tucker's theory (1985).