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Applied Physics Letters

Quantum limited heterodyne detection in superconducting non-linear transmission lines at sub-millimeter wavelengths

Edward Tong, R. Blundell, B. Bumble, Jeffrey A. Stern, Henry George LeDuc

Published Aug 28, 1995• 37 citations
Publisher

Abstract

A non-linear superconducting transmission line has been successfully employed in heterodyne detection of sub-millimeter waves. In our experiments an Nb/Al/AlOx/Nb tunnel junction, measuring 0.15×40 μm, with a critical current density of about 2500 A/cm2 is used as a distributed mixer element. We have demonstrated that quantum limited sensitivity can be achieved with this type of device. At 460 GHz, a Y-factor of 2.5 has been recorded, corresponding to a double-side-band receiver noise temperature of 80 K. The double-side-band conversion loss is about 1 dB and the mixer noise temperature is estimated to be 18(±10) K.

DOI
10.1063/1.114405

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•Jan 1, 2008•0 citations
A non-linear superconducting transmission line has been successfully employed in heterodyne detection of sub-millimeter waves. In our experiments an Nb/Al/AlOx/Nb tunnel junction, measuring 0.15×40 μm, with a critical current density of about 2500 A/cm2 is used as a distributed mixer element. We have demonstrated that quantum limited sensitivity can be achieved with this type of device. At 460 GHz, a Y-factor of 2.5 has been recorded, corresponding to a double-side-band receiver noise temperature of 80 K. The double-side-band conversion loss is about 1 dB and the mixer noise temperature is estimated to be 18(±10) K.
A new class of distributed mixer based on the non-linear quasiparticle tunnel current in a superconductor-insulator-superconductor transmission line has been proposed and tested. The Nb/Al/AlOx/Nb junctions used in our mixers are typically 200 nm wide and are between 1 and 2 guided wavelengths long. Compared with the traditional sub-millimeter lumped element quasiparticle mixer, the distributed mixer requires lower critical current densities, lower magnetic field as well as simpler tuning circuitry. Experiments have been performed in various sub-millimeter frequency bands. Receiver noise temperatures of 66 K at 460 GHz, 210 K at 630 GHz and 600 K at 780 GHz have been measured. These noise temperatures are comparable to or slightly better than the noise temperatures of the corresponding lumped element mixers.
The non-linear quasiparticle current flowing in a superconductor- insulatorsuperconductor (SIS) transmission line has successfully been used in low-noise heterodyne receivers at sub-millimeter wavelengths. These distributed mixer receivers have been implemented in both the 400 and the 600 GHz frequency bands and double-side-band receiver noise temperatures of 66 K at 455 GHz and 210 K at 630 Gliz have been measured. Such performance is comparable to, or slightly better than the corresponding lumped element 515 mixerreceivers. The transmission lines used are fabricated from Nb/Al/A10x/Nb trilayer material, and are typrically 200 nm wide and 1-2 Ag long. The distributed mixer chips have been designed with a simple linear model. We present design rules derived from this model and discuss some difficulties in mixer implementation.
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.
Quantum-limited mixer noise temperature has been achieved in Superconductor-Insulator-Superconductor (SIS) resonant mixer in the 600 — 720 GHz frequency range. Our mixer employs a single full-wave Nb/A1N/Nb tunnel junction resonator, fed by a quarter-wave transformer. The devices have low critical current density (-5 kA/cm 2 ). The mixers were tested in a fixed-tuned waveguide mixer mount. Double-side-band receiver noise temperatures equivalent to a few quanta have been measured for a number of different devices. Using a 0.55 x 25 Inn resonator, a noise temperature of 141 K was recorded at an LO frequency of 700 GHz with the mixer at 4.2 K. The noise temperature dropped to 111 K when the helium bath was pumped down to 2.8 K. High sensitivity has attained over reasonably wide RF bandwidth, —17%. The IF bandwidth of these mixers has also been investigated.
In this work we report the current status in our development of sensitive heterodyne detectors for efficient down-conversion of submillimeter-wave signals to D-band. These detectors use the strong non-linearity of Nb/Al-AlO/sub x//Nb tunnel junctions and employ preferably quasi-optical techniques to couple local-oscillator and radio-frequency signals to the superconducting junctions via a log-periodic antenna and hyper-hemispherical lens combination. Several devices have been fabricated and tested.
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.