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Future prospects of superconducting direct detectors in terahertz frequency range
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750 www.elsevier.com/locate/nima
Future prospects of superconducting direct detectors in terahertz frequency range Hiroshi Matsuo National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Available online 4 January 2006
Abstract Large format arrays using high-sensitivity detectors are being fabricated using superconducting devices. TES bolometer array with SQUID readout is one example and superconducting direct detector, or SIS photon detector, is another example. I will discuss on merits and drawbacks of each detector technology and show advantages of the SIS photon detectors over TES bolometers. Some aspects of detectors will be discussed such as operating temperature, input coupling, sensitivity, noise sources, and readout electronics. Wavelength coverage, dynamic range, time response and immunity to interference are key parameters to identify detector for astronomy and industrial applications. Further improvements in detector performance are required for future space observatories in terahertz frequencies that are a strong driving force to develop high-sensitivity large format array detectors. r 2005 Elsevier B.V. All rights reserved. PACS: 95.55.Aq; 85.25.Pb; 07.57.Kp Keywords: Superconducting direct detectors; TES bolometers; Submillimeter wave; Imaging detectors; Astronomical applications
1. Introduction
2. SIS photon detectors
Development of detector technology is a key to the progress of submillimeter-wave and terahertz astronomy where formation sites of various astronomical objects are observed. For ground-based astronomical observations, two-dimensional arrays with more than thousand pixels are anticipated. Recently, superconducting detectors show excellent performance in submillimeter wave, such as TES bolometers, kineteic inductance detectors and SIS photon detectors. The detector technologies are reviewed in Zmuidzinas and Richards [1]. Submillimeter-wave cameras using these types of detectors have been proposed such as SCUBA2 for JCMT submillimeter-wave telescope and a submillimeter-wave camera for ASTE (Atacama Submillimeter Telescope Experiment) [2,3].
Superconductors have gap energy in terahertz frequencies. When incident photon energy is higher than the gap, cooper pair is broken by the photon and quasi-particles can be readout through tunnel junctions. When the photon energy is lower, photon-assisted tunneling is observed when the tunnel junction is biased and coupled to antenna efficiently. Here we discuss on the latter case in which one incident photon makes one electron tunnel through the junction, which can be called as superconducting photoconductors. Dominant noise source is shot noise of the leakage current. For niobium junctions, on cooling the detector from 4.2 K to less than 0.8 K, leakage current decreases 6 orders of magnitude and high-performance detector is realized [4]. 2.1. Merit of the detector
Tel.: +81 422 34 3915; fax: +81 422 34 3864.
E-mail address: [email protected]. 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.126
Because of the direct photo response, their response is fast and dynamic range is large. Voltage responsivity is
ARTICLE IN PRESS H. Matsuo / Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750
about 109 V/W due to the high dynamic impedance of about 10 MO [4]. Antenna coupling is easily maintained even with the high dynamic impedance because their input impedance of the photon-assisted tunneling can be an order of the normal impedance of the junction, which is about 10 O. Dynamic range of the detector is extraordinarily large because critical current of the tunnel junction is an order of 100 mA. Assuming that the saturation of an SIS photon detector occurs at about one tenth of the normal state current, I estimated the dynamic range to be about 109. Here the dynamic range is defined as the ratio between saturation current and noise current that is identical to the ratio between saturation power and NEP. Since the SIS photon detector is directly coupled to antenna, matching circuit design can form band-pass characteristics. Requirements on optical filter are greatly reduced.
2.2. Limitation of the detector The most difficult point will be cryogenic readout electronics. For a large format array, it is advantageous to use cryogenic readout electronics attached to the detectors, especially for high-impedance detectors. However, low-noise and low-power dissipation circuit device operating in less than 1 K is quite limited. SQUIDs cannot be applied to the SIS photon detectors since their current noise is much larger than the detector. Semiconductor device is superior in this case, which has low current noise. Si-MOSFET and GaAs-JFET are candidate devices. Development of Capacitive Transimpedance Amplifier using GaAs-JFETs is underway [5]. Another point is that the detector performance as an array is not yet evaluated, such as uniformity of detector response and noise. Frequency coverage of the SIS photon detector is limited by the superconducting material. For niobium junctions, 650 GHz is just below the gap, and low bias voltage result in high coupling efficiency and low leakage current at the same time. It is noted that the gap voltage can be tuned by proximity effect of superconductor and normal metal.
749
3. TES bolometers Temperature dependence around the critical temperature of thin film superconductor is used as a sensitive thermistor. Under voltage bias condition, a strong electro-thermal feedback makes the bolometers to respond fast and their linearity improves. SQUID readout circuit gives a multiplexed output signal either by time domain or frequency domain multiplexing. Owing to the recent thermister design, noise performance is improved [6]. 3.1. Merit of the detector Because of the strong electro-thermal feedback, performance of TES bolometer is greatly improved from the semiconductor composite bolometers. Coupled with recent micro-machining fabrication technique, bolometer element is rigid and light weighted enough to avoid microphonic noise. SQUID multiplexing have advantages for their low heat dissipation. Laboratory evaluation is extensively made using prototype instrument, and stable operation condition is realized. 3.2. Limitation of the detector Since TES bolometer is a thermal detector, it is essential to reduce the operating temperature to improve their performance. Care should be taken so that any input energy should be avoided to reach bolometer, such as vibration, electrical interference, and infrared to UV radiation, except for the incident radiation of interest. Optical efficiency of bolometer instrumentation tends to be lower due to the number of blocking filters required. One of the limitations is the dynamic range of the TES bolometer. Since NEP scales as square root of the thermal conductivity and saturation power scales linearly to the conductivity, the dynamic range scales as square root of the conductivity. Typical TES bolometer for ground-based observation would have operating temperature of 0.3 K, superconducting transition width of 0.01 K and thermal conductivity of 1 nW/K. Under this condition, the dynamic range becomes about 105. Fluctuation of atmospheric emission might limit the operation of TES bolometers (Table 1).
Table 1 Comparison of detector performance Type of detector
SIS photon detectors
TES bolometersa
Merit of SIS detectors
Operating temperature Voltage responsivity Noise equivalent power Time constant Dynamic range Read-out electronics Limiting factor of NEP
o0.8 K 109 V/W 1 10 16 W/Hz0.5 oms 109 GaAs-JFET CTIA Leakage current
o0.3 K 106 V/W o1 10 16 W/Hz0.5 ms 105 SQUID Thermal noise
Easy operation Less interference Background limited Enable fast switch Easy calibration Low cross-talk Further improvements
a
Thermal conductivity is assumed to be 1 nW/K.
ARTICLE IN PRESS 750
H. Matsuo / Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750
4. Future prospects of SIS photon detectors
References
The fast response and large dynamic range are the advantages of the SIS photon detectors over the TES bolometers. SIS photon detectors will be useful not only for astronomical observations but also for terahertz application in laboratory. NEP of the SIS photon detector is limited by the shot noise and decreases as square root of the leakage current. The leakage current decreases when the quality of insulation barrier is improved or by using smaller junctions. To achieve NEP of 10 19 W/Hz0.5 or less, leakage current should be decreased to less than 10 fA. Understanding the physics of leakage current at very low temperature is essential to improve the detector performance.
[1] J. Zmuidzinas, P.L. Richards, Proc. IEEE 92 (2004) 1597. [2] H. Matsuo, S. Ariyoshi, C. Otani, H. Ezawa, J. Kobayashi, Y. Mori, H. Nagata, H.M. Shimizu, M. Fujiwara, M. Akiba, I. Hosako, SPIE 5498 (2004) 371. [3] H. Ezawa, R. Kawabe, K. Kohno, S. Yamamoto, SPIE 5458 (2004) 763. [4] S. Ariyoshi, H. Matsuo, C. Otani, H. Sato, H.M. Shimizu, K. Kawase, T. Noguchi, IEEE Trans. Appl. Supercond. 14 (2005) 920. [5] H. Nagata, J. Kobayashi, H. Matsuo, M. Akiba, M. Fujiwara, Nucl. Instr. and Meth. A, submitted. [6] J.G. Staguhn, D.J. Benford, J.A. Chervenak, S.H. Moseley, C.A. Allen, R. Stevenson, W.-T. Hsieh, SPIE 5459 (2004) 390.
Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750 www.elsevier.com/locate/nima
Future prospects of superconducting direct detectors in terahertz frequency range Hiroshi Matsuo National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Available online 4 January 2006
Abstract Large format arrays using high-sensitivity detectors are being fabricated using superconducting devices. TES bolometer array with SQUID readout is one example and superconducting direct detector, or SIS photon detector, is another example. I will discuss on merits and drawbacks of each detector technology and show advantages of the SIS photon detectors over TES bolometers. Some aspects of detectors will be discussed such as operating temperature, input coupling, sensitivity, noise sources, and readout electronics. Wavelength coverage, dynamic range, time response and immunity to interference are key parameters to identify detector for astronomy and industrial applications. Further improvements in detector performance are required for future space observatories in terahertz frequencies that are a strong driving force to develop high-sensitivity large format array detectors. r 2005 Elsevier B.V. All rights reserved. PACS: 95.55.Aq; 85.25.Pb; 07.57.Kp Keywords: Superconducting direct detectors; TES bolometers; Submillimeter wave; Imaging detectors; Astronomical applications
1. Introduction
2. SIS photon detectors
Development of detector technology is a key to the progress of submillimeter-wave and terahertz astronomy where formation sites of various astronomical objects are observed. For ground-based astronomical observations, two-dimensional arrays with more than thousand pixels are anticipated. Recently, superconducting detectors show excellent performance in submillimeter wave, such as TES bolometers, kineteic inductance detectors and SIS photon detectors. The detector technologies are reviewed in Zmuidzinas and Richards [1]. Submillimeter-wave cameras using these types of detectors have been proposed such as SCUBA2 for JCMT submillimeter-wave telescope and a submillimeter-wave camera for ASTE (Atacama Submillimeter Telescope Experiment) [2,3].
Superconductors have gap energy in terahertz frequencies. When incident photon energy is higher than the gap, cooper pair is broken by the photon and quasi-particles can be readout through tunnel junctions. When the photon energy is lower, photon-assisted tunneling is observed when the tunnel junction is biased and coupled to antenna efficiently. Here we discuss on the latter case in which one incident photon makes one electron tunnel through the junction, which can be called as superconducting photoconductors. Dominant noise source is shot noise of the leakage current. For niobium junctions, on cooling the detector from 4.2 K to less than 0.8 K, leakage current decreases 6 orders of magnitude and high-performance detector is realized [4]. 2.1. Merit of the detector
Tel.: +81 422 34 3915; fax: +81 422 34 3864.
E-mail address: [email protected]. 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.126
Because of the direct photo response, their response is fast and dynamic range is large. Voltage responsivity is
ARTICLE IN PRESS H. Matsuo / Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750
about 109 V/W due to the high dynamic impedance of about 10 MO [4]. Antenna coupling is easily maintained even with the high dynamic impedance because their input impedance of the photon-assisted tunneling can be an order of the normal impedance of the junction, which is about 10 O. Dynamic range of the detector is extraordinarily large because critical current of the tunnel junction is an order of 100 mA. Assuming that the saturation of an SIS photon detector occurs at about one tenth of the normal state current, I estimated the dynamic range to be about 109. Here the dynamic range is defined as the ratio between saturation current and noise current that is identical to the ratio between saturation power and NEP. Since the SIS photon detector is directly coupled to antenna, matching circuit design can form band-pass characteristics. Requirements on optical filter are greatly reduced.
2.2. Limitation of the detector The most difficult point will be cryogenic readout electronics. For a large format array, it is advantageous to use cryogenic readout electronics attached to the detectors, especially for high-impedance detectors. However, low-noise and low-power dissipation circuit device operating in less than 1 K is quite limited. SQUIDs cannot be applied to the SIS photon detectors since their current noise is much larger than the detector. Semiconductor device is superior in this case, which has low current noise. Si-MOSFET and GaAs-JFET are candidate devices. Development of Capacitive Transimpedance Amplifier using GaAs-JFETs is underway [5]. Another point is that the detector performance as an array is not yet evaluated, such as uniformity of detector response and noise. Frequency coverage of the SIS photon detector is limited by the superconducting material. For niobium junctions, 650 GHz is just below the gap, and low bias voltage result in high coupling efficiency and low leakage current at the same time. It is noted that the gap voltage can be tuned by proximity effect of superconductor and normal metal.
749
3. TES bolometers Temperature dependence around the critical temperature of thin film superconductor is used as a sensitive thermistor. Under voltage bias condition, a strong electro-thermal feedback makes the bolometers to respond fast and their linearity improves. SQUID readout circuit gives a multiplexed output signal either by time domain or frequency domain multiplexing. Owing to the recent thermister design, noise performance is improved [6]. 3.1. Merit of the detector Because of the strong electro-thermal feedback, performance of TES bolometer is greatly improved from the semiconductor composite bolometers. Coupled with recent micro-machining fabrication technique, bolometer element is rigid and light weighted enough to avoid microphonic noise. SQUID multiplexing have advantages for their low heat dissipation. Laboratory evaluation is extensively made using prototype instrument, and stable operation condition is realized. 3.2. Limitation of the detector Since TES bolometer is a thermal detector, it is essential to reduce the operating temperature to improve their performance. Care should be taken so that any input energy should be avoided to reach bolometer, such as vibration, electrical interference, and infrared to UV radiation, except for the incident radiation of interest. Optical efficiency of bolometer instrumentation tends to be lower due to the number of blocking filters required. One of the limitations is the dynamic range of the TES bolometer. Since NEP scales as square root of the thermal conductivity and saturation power scales linearly to the conductivity, the dynamic range scales as square root of the conductivity. Typical TES bolometer for ground-based observation would have operating temperature of 0.3 K, superconducting transition width of 0.01 K and thermal conductivity of 1 nW/K. Under this condition, the dynamic range becomes about 105. Fluctuation of atmospheric emission might limit the operation of TES bolometers (Table 1).
Table 1 Comparison of detector performance Type of detector
SIS photon detectors
TES bolometersa
Merit of SIS detectors
Operating temperature Voltage responsivity Noise equivalent power Time constant Dynamic range Read-out electronics Limiting factor of NEP
o0.8 K 109 V/W 1 10 16 W/Hz0.5 oms 109 GaAs-JFET CTIA Leakage current
o0.3 K 106 V/W o1 10 16 W/Hz0.5 ms 105 SQUID Thermal noise
Easy operation Less interference Background limited Enable fast switch Easy calibration Low cross-talk Further improvements
a
Thermal conductivity is assumed to be 1 nW/K.
ARTICLE IN PRESS 750
H. Matsuo / Nuclear Instruments and Methods in Physics Research A 559 (2006) 748–750
4. Future prospects of SIS photon detectors
References
The fast response and large dynamic range are the advantages of the SIS photon detectors over the TES bolometers. SIS photon detectors will be useful not only for astronomical observations but also for terahertz application in laboratory. NEP of the SIS photon detector is limited by the shot noise and decreases as square root of the leakage current. The leakage current decreases when the quality of insulation barrier is improved or by using smaller junctions. To achieve NEP of 10 19 W/Hz0.5 or less, leakage current should be decreased to less than 10 fA. Understanding the physics of leakage current at very low temperature is essential to improve the detector performance.
[1] J. Zmuidzinas, P.L. Richards, Proc. IEEE 92 (2004) 1597. [2] H. Matsuo, S. Ariyoshi, C. Otani, H. Ezawa, J. Kobayashi, Y. Mori, H. Nagata, H.M. Shimizu, M. Fujiwara, M. Akiba, I. Hosako, SPIE 5498 (2004) 371. [3] H. Ezawa, R. Kawabe, K. Kohno, S. Yamamoto, SPIE 5458 (2004) 763. [4] S. Ariyoshi, H. Matsuo, C. Otani, H. Sato, H.M. Shimizu, K. Kawase, T. Noguchi, IEEE Trans. Appl. Supercond. 14 (2005) 920. [5] H. Nagata, J. Kobayashi, H. Matsuo, M. Akiba, M. Fujiwara, Nucl. Instr. and Meth. A, submitted. [6] J.G. Staguhn, D.J. Benford, J.A. Chervenak, S.H. Moseley, C.A. Allen, R. Stevenson, W.-T. Hsieh, SPIE 5459 (2004) 390.