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IEEE Transactions on Microwave Theory and Techniques

Two-junction tuning circuits for submillimeter SIS mixers

Jonas Stasys Zmuidzinas, Henry George LeDuc, J. A. Stern, S.R. Cypher

Published Apr 1, 1994• 128 citations
Publisher

Abstract

The capacitance of superconducting tunnel junctions can seriously degrade the performance of quasiparticle (SIS) mixers operating in the submillimeter band, so it is essential to provide a circuit for tuning out this capacitance at the operating frequency. In this article, we present two new tuning circuits for SIS mixers which use a pair of SIS junctions connected by an inductance. Compared to previously proposed tuning circuits, ours have a broader bandwidth, are easier to scale to higher frequencies, and may be easier to fabricate. We have constructed quasi-optical mixers which employ these tuning circuits, using Nb/Al-Oxide/Nb SIS junctions defined by optical lithography. The performance of these devices is excellent, giving receiver noise temperatures of 113 K (DSB) at 490 GHz and 230 K DSB at 612 GHz. In addition to demonstrating the effectiveness of our tuning circuit, these results show that quasi-optical mixers can be competitive with or superior to waveguide mixers at submillimeter wavelengths. The mixers continue to perform well at frequencies up to 672 GHz, which is about 95% of the Nb gap frequency.>

DOI
10.1109/22.285084

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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.

Superconductor Science and Technology•Apr 1, 1996•5 citations
The capacitance of superconducting tunnel junctions can seriously degrade the performance of quasiparticle (SIS) mixers operating in the submillimeter band, so it is essential to provide a circuit for tuning out this capacitance at the operating frequency. In this article, we present two new tuning circuits for SIS mixers which use a pair of SIS junctions connected by an inductance. Compared to previously proposed tuning circuits, ours have a broader bandwidth, are easier to scale to higher frequencies, and may be easier to fabricate. We have constructed quasi-optical mixers which employ these tuning circuits, using Nb/Al-Oxide/Nb SIS junctions defined by optical lithography. The performance of these devices is excellent, giving receiver noise temperatures of 113 K (DSB) at 490 GHz and 230 K DSB at 612 GHz. In addition to demonstrating the effectiveness of our tuning circuit, these results show that quasi-optical mixers can be competitive with or superior to waveguide mixers at submillimeter wavelengths. The mixers continue to perform well at frequencies up to 672 GHz, which is about 95% of the Nb gap frequency.>
We report on the continuing development of submillimeter quasi-optical slot antenna SIS mixers which use two-junction tuning circuits [1]. The mixers use 10 kA/cm2 Nb/Al-Oxide/Nb junctions and Nb wiring, and have generated DSB receiver noise temperatures around 5hv/kB to 700 GHz, and 16hv/kB (620 K) at 798 GHz. We present Fourier-transform spectrometer (FTS) measurements and heterodyne measurements on several such devices. In general, the measured response at frequencies below the gap of Nb is in good agreement with the predicted performance. Introduction SIS mixers with Nb-trilayer tunnel junctions offer excellent performance at frequencies below 800 GHz, and should perform competitively at frequencies as high as 1.4 THz [2-5]. Our goal is to develop a suite of ultra-low-noise SIS mixers which cover frequencies from = 400 GHz to more than I THz. At frequencies below the Nb gap (< 700 GHz), existing devices perform quite well [6], but further improvements in noise temperatures remain important. In addition, it is quite useful to be able to predict device performance given the design parameters. One can then optimize the device design for a particular frequency band, and confirm that we indeed understand the physics of the device. Progress in the design and operation of devices in this frequency range is presented below. At frequencies above the Nb gap, RF photons can break Cooper pairs, resulting in greater signal loss and higher receiver noise temperatures. The loss is particularly important in the slender microstrip lines used to resonate the junction capacitance and to transform the antenna impedance down to the RF junction impedance. However, the actual coupling of radiation into Nb junctions above the gap should still be reasonably efficient -more than 30% at 800 GHz [7]. In addition to RF loss, one is also affected by increased dispersion at frequencies near the gap. The Nb microstriplines have therefore been carefully designed to ensure the desired bandpass is achieved. We demonstrate for the first time that all-Nb SIS receivers can have substantially better performance than GaAs Schottky receivers for the astronomically important CI ( P2 — P 1 ) and CO (7-6) transitions near 810 GHz. Receiver Configuration Since waveguide mixers become difficult to construct at short submillimeter wavelengths, we have adopted a quasi-optical approach. Here, lenses take the place of waveguide horns, and the incoming radiation is collected by a planar antenna on the SIS mixer substrate [8]. This offers several other advantages, such as on-chip broad-band silt a SS 114 SSE WA Wa Sta MX Dewar 120 Mylar Window SIS Mixer 295 IQ 77 K Mylar Load Bearnsplitter ICI 1= NZ IC =1 HEN1T Amplifier Polyethylene Lens
We report on low-noise heterodyne mixing with NbN/A1N/NbN tunnel junctions in the submillimeter-wave region for the first time. The receiver consists of the quasi-optical NbN SIS mixer with an integrated tuning circuit in which a radial short stub tuner is incorporated to resonate out the junction capacitance for RF matching and a balanced IF circuit for broadband IF matching. The prepared NbN SIS junction has a current density of 20 kA/cm 2 and is about 1 I1111 in diameter, supplying a small coC JRN product (o)CJRN = 3 at 300 GHz). The junctions showed good dc I-V characteristics, excellent submillimeter-wave responses and sensitive heterodyne mixing responses. From 254 to 350 GHz the average receiver noise temperature measured using by the standard Y-factor method was about 250 K (DSB) at 5 K. The lowest receiver noise temperature, 200 K (DSB), was obtained at around 303 GHz. Comparing NbN/A1N/NbN and Nb/A10x/Nb tunnel junction performance with the same tuning circuit showed that the frequency dependence of the receiver noise temperature agreed well in the two receivers. These results suggest that our well-controlled NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.
We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX=260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h∼1.2 THz.
A quasi optical SIS mixer for the frequency band 400-550 GHz has been designed and successfully testedin a laboratory receiver setup. The intrinsic SIS junction capacitance was tuned out by a novel pair (twin) SISjunction compensation circuit. The 4µm2 area Nb-A10x-Nb junctions, with (oRnC product as high as 7.4 at500 GHz, were patterned by optical lithography and integrated with a equiangular spiral antenna to give broad- band SIS mixer operation. The receiver demonstrated flat response within the tuning range of the available LOsource (440 - 520 GHz) with DSB noise temperatures below 300 K over the entire band and with the best re- ceiver noise temperature below 250 K DSB at 453 GHz (measured at 3.5 K physical temperature). No mechani- cal tuning was employed in the SIS mixer. These noise temperatures include all losses and was measured atan IF center frequency of 1.5 GHz.
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.>
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.