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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Sensitivity improvements of SIS photon detectors

Hiroshi Matsuo, Sheng-Cai Shi, Takashi Noguchi, Hiromichi Akahori, Toshio Sato

Published Jul 31, 1998• 4 citations
Publisher

Abstract

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.

DOI
10.1117/12.317392

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•69 citations
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 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.
A Superconductor-Insulator-Superconductor (SIS) mixer using parallel array of N junctions have been studied. It has been shown by theoretical analysis that the performance of this type of device is excellent and that nearly quantum-limited performance of the mixer can be obtained. It has been demonstrated that the double sideband (DSB) noise temperature of a receiver employing parallel array of twin junctions, which is the most simplest case in the parallel array of N junctions, was approximately 3 to 5 times as large as the quantum limited photon noise up to the gap frequency of Nb (approximately 700 GHz). It has been also shown that the bandwidth performance of parallel junction arrays can be improved significantly with a large number of SIS junctions even if SIS junctions with a relatively low current density or a large (omega) RNCJ product are adopted in the arrays.
Apart from the great success of SIS-junctions as mixers in heterodyne receivers there are several other reasons to intensify investigations of SIS-junctions as direct detectors. These devices can be produced as single junctions and arrays for use. in waveguide systems or in planar antennas by standard lithographic techniques. Impedance matching structures and filter elements can be incorporated easily. Recent developments1,2 in junction technology show strong improvements of junction performance. Areas of 0.5μm2 and leakage currents as low as 50nA have been realized. Since the final sensitivity (NEP) of SIS-junctions as direct detectors is limited ultimately by the shot noise of the bias current (NEP∼√IL), a low leakage current IL, is highly important for these devices. Using these improved SIS-junctions, NEP's down to 10-17W/√Hz seem to be possible with He4-cooled devices. A crude estimate for the time constant of a SIS-direct detector yields the inverse of the gap frequency h/∆ or the sumgap frequency h/2∆, which gives very small values of 1 − 10psec.
Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Delta v/v<10(-6)). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.
Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Δν/ν<10-6). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.
We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be <0.1 pA, or of order 10/sup 5/ electrons/second, corresponding to an NEP of less than 10/sup -19/ W//spl radic/Hz at 500 microns (600 GHz). The photocurrent will be measured using a fast single-electron transistor (RF-SET), which allows a shot-noise-limited performance even for the very small currents delivered from this low capacitance and high impedance SIS junction. Results of initial fabrication and dc characterization of an integrated photodetector are also given.