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SUBMILLMETER-WAVE DIRECT DETECTORS WITH Nb-BASED SUPERCONDUCTING TUNNEL JUNCTIONS

Peiheng Wu

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions(STJs) match the request of broadband spectral response and high sensitivity,and having capability of large format arrays. We aim to realize the fractional bandwidth of 10 percent(60 GHz) at 650 GHz.The broadband coupling was designed using 2 distributed junction(DJ) arrays,which were composed of six STJs connected in parallel by a superconducting microstrip line.By analyze the DJ arrays,we found N-resonance combination needs to connect N-set of twin-junctions with short inductances through(N—1)-long inductances,alternatively.And we got the experiment result at 4.2 K that the center frequency of the STJ is 655 GHz,bandwidth about 50GHz,and the sensitivity is 1A/ W.

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Submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions(STJs) match the request of broadband spectral response and high sensitivity,and having capability of large format arrays. We aim to realize the fractional bandwidth of 10 percent(60 GHz) at 650 GHz.The broadband coupling was designed using 2 distributed junction(DJ) arrays,which were composed of six STJs connected in parallel by a superconducting microstrip line.By analyze the DJ arrays,we found N-resonance combination needs to connect N-set of twin-junctions with short inductances through(N—1)-long inductances,alternatively.And we got the experiment result at 4.2 K that the center frequency of the STJ is 655 GHz,bandwidth about 50GHz,and the sensitivity is 1A/ W.
We have developed submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions (STJs), with broadband spectral response, high sensitivity and imaging capability. Spectral response peaked at 650 GHz with a fractional bandwidth of 14 percent, which fits one of the important atmospheric windows for an astronomical project. We also confirmed the linearity of the detector response in the dynamic range greater than 10/sup 6/. The measured coupling efficiency was about 0.2, which was lower than the expected value of 0.5. The measured noise current of the detector was 10 fA//spl radic/Hz, which agreed with the shot noise from the residual leakage current of 100 pA at 0.3 K. Resultant noise equivalent power (NEP) is 1.6/spl times/10/sup -16/ W//spl radic/Hz, that is less than the background photon fluctuation limit for ground-based submillimeter-wave observations.
Employing an anodization method into our fabrication process, we have successfully fabricated STJs with size of 4 /spl mu/m in diameter, Josephson critical current density of 1000 A/cm/sup 2/, low leakage current of less than 10 pA at 0.3 K and high fabrication yield of more than 90 percent. These STJs were applied to fabricate submillimeter-wave direct detectors using 12 linearly distributed junctions, each circular junction having a current density of 600 A/cm/sup 2/ and size of 4 /spl mu/m in diameter. Direct detection of submillimeter-waves was measured using a Fourier transform spectrometer, which shows a maximum sensitivity at 440 GHz and a fractional bandwidth of 10 percent.
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.
Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.
We report on tunable submillimeter-wave radiation sources based on micrometer-sized superconducting tunnel junction arrays optimized within a bandwidth of 350-520 GHz. The arrays consist of 10, 20 and 40 Superconductor-Insulator- Superconductor (SIS) parallel-connected Nb/AlOx/Nb junctions embedded in superconducting microstrip lines. A SIS twin-junction is integrated along with each array to detect output signals. The pumped detector’s I-V characteristic exhibits clearly photon-assisted quasiparticule steps when the arrays are biased upon corresponding Josephson resonances ranging from 370 to 520 GHz.
Terahertz direct detectors based on superconducting tunnel junctions (STJs) have the merits of short response time, large dynamic range and relatively high operating temperature, providing a good choice for terahertz detection. Compared with commonly used niobium STJs, niobium nitride (NbN) ones have a high transition temperature close to 16K. Hence they might be useful for terahertz detection operating at temperatures well above the LHe temperature of 4.2K. In this paper, we investigate the properties of NbN STJ detectors at different bath temperatures. Detailed experiment results are given.