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IEEE Transactions on Applied Superconductivity

Characterization of an STJ-Based Direct Detector of Submillimeter Waves

Seiichiro Ariyoshi, Hiroshi Matsuo, Chiko Otani, Y. Satou, Hirohiko M. Shimizu, Kodo Kawase, Takashi Noguchi

Published Jun 1, 2005• 22 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2005.850119

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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.
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.
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.
Development of two types of submillimeter-wave superconducting direct detectors are described. One is the SIS video detector and the other is the SIS photon detector. Using 500 GHz parallel connected twin junction device, video detector performance is eluted. High quantum efficiency in wide frequency range in submillimeter-wave and low noise performance is achieved. Noise equivalent power of 6 X 10-14 W/(root)Hz is measured at operating temperature of 1.5 K and bias voltage of 0.5 mV. Further improvements using distributed junction array is addressed. Large area niobium SIS junction (100 micrometer X 50 micrometer X 2) is used as a photon detection device and their thermal, electrical and optical performance is evaluated. Leakage current decrease more than three orders of magnitude at 0.5 K compared to that at 4.2 K. Noise of the device is limited by the shot noise.
We describe recent work at Yale on Superconducting Tunnel Junction (STJ) direct detectors that have been developed for submillimeter astronomy. To monitor the response of the detector with large readout bandwidth and excellent sensitivity, we use a novel readout based on radio frequency (RF) reflectometry, like the readout invented for the RF-SET. For calibration of the detector, we have developed an in-situ, on-chip, hot-cold photon source. This is a voltage biased gold microbridge. Noise emitted by the microbridge couples via a coplanar stripline to the detector. This provides a calibrated blackbody photon source with near unity coupling, fast chopping, and calculable output. We present recent detection results in the range 100-160 GHz. These demonstrate the expected good responsivity, high sensitivity, and fast response. This approach is easily used with a frequency-multiplexed readout, allowing economy of cold electronics. Ultimate sensitivity is in the range 1 times 10-19W/(Hz)1/2.