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All-NbN Quasi-Optical SIS Mixers at Terahertz Frequencies

Yoshinori Uzawa, Then Wang, Akira Kawakami

Published Jan 1, 1997• 1 citation
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Abstract

We have theoretically and experimentally investigated submillimeter-wave receiver performances employing NbN/ALN/NbN tunnel junctions with high current density and high energy. gap. The receiver noise based on parameters for our typical NbN SIS junction were analyzed using Tucker's quantum theory of mixing, which showed that DSB receiver noise temperatures of below 3hv/K B can be achieved by using the NbN SIS junctions for frequencies up to 2 THz. An experimental receiver has been designed and tested for the 1 THz band. The mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, a NbN twin-slot antenna, and NbN tuning circuits. The prepared NbN/A1N/NbN junction size was about 0.5 !mut), and the current density was about 30 kA/cm 2 (o)CJRN = 11 at 1 THz). Although the junctions showed good dc I-V characteristics with a small sub-gap leakage current, the receiver noise temperature measured by the standard Y-factor method was about 2700 K(DSB) at 761 GHz, which is much larger than the theoretical prediction. The relative high noise performance may be caused by the large RF losses in the tuning circuit of polycristalline NbN on the SiO.

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Quasi-optical NbN/AlN/NbN mixers in submillimeter wave band

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We have designed, fabricated and evaluated a quasi-optical SIS receiver with NbN/AlN/NbN tunnel junctions in the submillimeter-wave region. The receiver consists of a planar self-complementary log-periodic antenna and MgO hyperhemispherical lens with an anti-reflection cap for RF optics, NbN SIS junctions and a radial short stub tuner to resonate out its capacitance for RF matching. The prepared NbN SIS junction has a current density of 20 kA/cm/sup 2/ and is about 1 /spl mu/m in diameter, supplying a small /spl omega/C/sub J/R/sub N/ product (/spl omega/C/sub J/R/sub N/=3 at 300 GHz). From 297 to 356 GHz the average double sideband (DSB) receiver noise temperature measured by using the standard Y-factor method was about 200 K. The lowest receiver noise temperature, 167 K (DSB), was obtained at around 303 GHz. These results suggest that our NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.

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We have developed a low-noise quasioptical NbN/AlN/NbN superconductor-insulator-superconductor (SIS) mixer that operates at terahertz frequencies. The mixer uses a MgO hyperhemispherical lens with an antireflection cap, a single-crystal NbN log-periodic antenna, and two-junction tuning circuits which employ Al/SiO/NbN microstriplines. The NbN/AlN/NbN junction size was about 0.9 μm in diameter, and the current density was about 45 kA/cm2. The frequency dependence of the receiver noise temperature was investigated by using an optically pumped far-infrared laser and a backward-wave oscillator as a local oscillator at several frequencies from 670 to 1082 GHz. The experimental results showed that the center frequency was around 800 GHz, and the receiver noise temperature measured by the standard Y-factor method was 457 K (DSB) at 783 GHz including a 9-μm-thick Mylar beam splitter loss and other optical losses at the physical bath temperature of 4.2 K. This is the first SIS mixer based on NbN with low-noise performance (12hν/kB) above the gap frequency of Nb.

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Performance of all-NbN quasi-optical SIS mixers for the terahertz band

Yoshinori Uzawa, Akira Kawakami, Shigehito Miki +1 more

We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.

IEEE Transactions on Applied Superconductivity•Mar 1, 2001•21 citations
We have theoretically and experimentally investigated submillimeter-wave receiver performances employing NbN/ALN/NbN tunnel junctions with high current density and high energy. gap. The receiver noise based on parameters for our typical NbN SIS junction were analyzed using Tucker's quantum theory of mixing, which showed that DSB receiver noise temperatures of below 3hv/K B can be achieved by using the NbN SIS junctions for frequencies up to 2 THz. An experimental receiver has been designed and tested for the 1 THz band. The mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, a NbN twin-slot antenna, and NbN tuning circuits. The prepared NbN/A1N/NbN junction size was about 0.5 !mut), and the current density was about 30 kA/cm 2 (o)CJRN = 11 at 1 THz). Although the junctions showed good dc I-V characteristics with a small sub-gap leakage current, the receiver noise temperature measured by the standard Y-factor method was about 2700 K(DSB) at 761 GHz, which is much larger than the theoretical prediction. The relative high noise performance may be caused by the large RF losses in the tuning circuit of polycristalline NbN on the SiO.
We have designed, fabricated and evaluated a quasi-optical SIS receiver with NbN/AlN/NbN tunnel junctions in the submillimeter-wave region. The receiver consists of a planar self-complementary log-periodic antenna and MgO hyperhemispherical lens with an anti-reflection cap for RF optics, NbN SIS junctions and a radial short stub tuner to resonate out its capacitance for RF matching. The prepared NbN SIS junction has a current density of 20 kA/cm/sup 2/ and is about 1 /spl mu/m in diameter, supplying a small /spl omega/C/sub J/R/sub N/ product (/spl omega/C/sub J/R/sub N/=3 at 300 GHz). From 297 to 356 GHz the average double sideband (DSB) receiver noise temperature measured by using the standard Y-factor method was about 200 K. The lowest receiver noise temperature, 167 K (DSB), was obtained at around 303 GHz. These results suggest that our NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.
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 quasioptical NbN/AlN/NbN superconductor-insulator-superconductor (SIS) mixer that operates at terahertz frequencies. The mixer uses a MgO hyperhemispherical lens with an antireflection cap, a single-crystal NbN log-periodic antenna, and two-junction tuning circuits which employ Al/SiO/NbN microstriplines. The NbN/AlN/NbN junction size was about 0.9 μm in diameter, and the current density was about 45 kA/cm2. The frequency dependence of the receiver noise temperature was investigated by using an optically pumped far-infrared laser and a backward-wave oscillator as a local oscillator at several frequencies from 670 to 1082 GHz. The experimental results showed that the center frequency was around 800 GHz, and the receiver noise temperature measured by the standard Y-factor method was 457 K (DSB) at 783 GHz including a 9-μm-thick Mylar beam splitter loss and other optical losses at the physical bath temperature of 4.2 K. This is the first SIS mixer based on NbN with low-noise performance (12hν/kB) above the gap frequency of Nb.
We demonstrate low-noise heterodyne mixing with NbN/AlN/NbN tunnel junctions in the submillimeter-wave region. The prepared junction size was 1 μm in diameter and the current density was about 20 kA/cm2, supplying a small ωCJRN product (ωCJRN=3 at 300 GHz). The mixer used a lens-coupled log-period antenna and an integrated tuning circuit which incorporated a radial short stub tuner for resonating out the junction capacitance. 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 the standard Y-factor method was about 250 K double sideband (DSB). The lowest receiver noise temperature, 200 K (DSB), was obtained around 303 GHz.
We report on the fabrication and properties of high current density NbN/AlN/NbN tunnel junctions for the application of superconducting SIS mixers in the submillimeter wave regions. Junctions having current densities as high as 50 kA/cm/sup 2/ exhibited a good Josephson tunneling behaviour, excellent terahertz response, and sensitive heterodyne mixing properties. Accurate values of junction capacitance were estimated by measuring dc-SQUID resonant steps for the design of tuning circuits. The measured specific capacitance gives values in the range of 20-200 fF//spl mu/m/sup 2/ for the current density range of 1-100 kA/cm/sup 2/. The NbN/AlN/NbN tunnel junctions were integrated with a NbN thin film antenna to investigate the terahertz responses and mixing tests in a quasi-optical testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1.02 THz, and a low-noise heterodyne mixing was demonstrated in the 300-GHz band.
We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.