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Applied Physics Express

Three Quanta Sensitivity Superconductor–Insulator–Superconductor Mixer for the 0.78–0.95 THz Band

Takafumi Kojima, M. Kroug, Masanori Takeda, Yoshinori Uzawa, Wenlei Shan, Yasunori Fujii, Zhen Wang, Hideo Ogawa

Published Oct 9, 2009• 26 citations
Publisher

Abstract

We have successfully demonstrated a low-noise 0.78–0.95-THz waveguide mixer based on Nb/AlO x /Nb superconductor–insulator–superconductor (SIS) junctions integrated with an NbTiN/SiO 2 /Al superconducting inverted microstrip. The mixer employs a twin-junction tuning circuit and an impedance transformer which are designed to achieve both low noise and wideband operation. Corrected for optical input losses in front of the mixer, the double-sideband (DSB) noise temperature is below 120 K (<3 hf / k B ) around the local oscillator (LO) frequency of 0.88 THz and at an operating temperature of 2 K. The results represent state-of-the-art performance for mixers in the terahertz frequency range.

DOI
10.1143/apex.2.102201

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We have successfully demonstrated a low-noise 0.78–0.95-THz waveguide mixer based on Nb/AlO x /Nb superconductor–insulator–superconductor (SIS) junctions integrated with an NbTiN/SiO 2 /Al superconducting inverted microstrip. The mixer employs a twin-junction tuning circuit and an impedance transformer which are designed to achieve both low noise and wideband operation. Corrected for optical input losses in front of the mixer, the double-sideband (DSB) noise temperature is below 120 K (<3 hf / k B ) around the local oscillator (LO) frequency of 0.88 THz and at an operating temperature of 2 K. The results represent state-of-the-art performance for mixers in the terahertz frequency range.
China is planning to build a 5-m THz telescope named DATE5 at Dome A in Antarctica. The telescope will operate in 0.85 THz and 1.5 THz frequency bands. A waveguide superconductor-insulator-superconductor (SIS) mixer is developed for the 0.85 THz frequency band by utilizing parallel-connected tunnel junctions (PCTJ) with Nb/AlN/NbN trilayer and NbTiN/SiO2/Al microstrip tuning circuit on quartz substrate. Its frequency response is measured by a Fourier-transform spectrometer (FTS), showing an FWHM frequency coverage of 762-925 GHz. The measured uncorrected double sideband (DSB) receiver noise temperature is approximately equal to 450 K at 840 GHz. With the vacuum window and beamsplitter losses corrected, the receiver noise temperature is less than five times the quantum-limited noise. Detailed mixer design and measurement results will be presented.
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.
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.
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.
We present the design and the performance of a fixed-tuned, all niobium (Nb) Superconductor-Insulator-Superconductor (SIS) mixer covering RF frequencies from 163 to 211 GHz with 4-8 GHz IF bandwidth. The mixer uses two Nb/Al-AlOx/Nb junctions of size 3 μm2each with theRnAproduct of 30 Ω·μm2, in a twin junction configuration. An local oscillator (LO) directional coupler made of superconducting lines with slots in the ground plane is integrated on the SIS mixer chip. The isolated port of the LO coupler is terminated using a wide-band resistive load with a sheet resistance of 12 Ω/□. The SIS tuning circuitry is optimized to achieve best power match between the twin junctions and the embedding circuitry. The measurement of the double sideband mixer noise at 171 GHz shows that the receiver noise temperature is approximately 17 K, which is just 2hf/k. The estimated noise contribution from the RF loss and the IF chain at 176 GHz LO frequency is 8 K and 7.5 K respectively, resulting in a SIS mixer noise ofhf/2k. The mixer noise performance across the entire RF band varies between 19 to 25 K. This is to our knowledge the best reported noise temperature at these frequencies till now.
Heterodyne mixers incorporating Nb SIS junctions and NbTiN-SiO/sub 2/-Al microstrip tuning circuits offer the lowest reported receiver noise temperatures to date in the 0.8-0.96- and 0.96-1.12-THz frequency bands. In particular, improvements in the quality of the NbTiN ground plane of the SIS devices' on-chip microstrip tuning circuits have yielded significant improvements in the sensitivity of the 0.96-1.12-THz mixers relative to previously presented results. Additionally, an optimized RF design incorporating a reduced-height waveguide and suspended stripline RF choke filter offers significantly larger operating bandwidths than were obtained with mixers that incorporated full-height waveguides near 1 THz. Finally, the impact of junction current density and quality on the performance of the 0.8-0.96-THz mixers is discussed and compared with measured mixer sensitivities, as are the relative sensitivities of the 0.8-0.96- and 0.96-1.12-THz mixers.