Optical Performance of SIS Photon Detectors at Terahertz Frequencies
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Optical Performance of SIS Photon Detectors at Terahertz Frequencies Hajime Ezawa1,2 · Hiroshi Matsuo1,2 · Masahiro Ukibe3 · Go Fujii3 · Shigetomo Shiki3 Received: 20 August 2019 / Accepted: 1 August 2020 / Published online: 29 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Fast photon detectors will be one of the key technologies to introduce new observing methods. Utilizing our recent development of low leakage SIS junctions with Nb/ Al/AlOx/Al/Nb, we designed and fabricated an antenna-coupled SIS detector. The detector exhibited a leakage current of 2 pA at T ≤ 0.8 K . Fourier transform spectrometer measurements show a clear resonance peak, although the peak frequency and bandwidth were shifted from the designed values. Even though the detector efficiency was low, the noise measurement suggests that the detector would allow us to read out single terahertz photon by introducing the photon-counting scheme. Keywords Superconducting tunnel junction · Terahertz detector · Photon counting
1 Introduction Observational astronomy and astrophysics have been continuously seeking for observing methods with higher angular resolution and better sensitivity. Fast photon detection would be one of the key technologies to advance the observing capability, which may improve the signal-to-noise ratio by resolving each photon, or may lead to photon statistics for high-precision measurements in photon-counting mode [1]. Thermal photons are governed by Bose–Einstein statistics. The photon number fluctuation can be described in terms of noise equivalent power (NEP) as: � √ � √ NEP = 2P(h𝜈 + kT) W∕ Hz (1)
* Hajime Ezawa [email protected] 1
National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan
2
The Graduate University for Advanced Studies, SOKENDAI, Mitaka, Tokyo, Japan
3
National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
1Vol:.(1234567890) 3
Journal of Low Temperature Physics (2020) 200:226–232
227
Here, the term with hν corresponds to Poisson or statistical fluctuation, while the term with kT corresponds to the bunching or thermal fluctuation [1]. Figure 1 shows the NEP described in Eq. (1) for thermal radiations with some specific temperatures. The photon statistics, or the second-order correlation function [2], may be a precise measure of radiation temperature when the contribution of statistical and thermal fluctuation become comparable: These frequencies reside in the terahertz range for typical astronomical cases, such as 10 THz for radiation at 300 K or 100 GHz at 3 K. Fast photon detectors will be a strong tool to evaluate the photon statistics with high signal-to-noise ratio, to determine the radiation temperature accurately. The initial design and performance of such detector will be discussed in the subsequent sections.
2 Design and Specifications We have chosen SIS junctions (or STJ) for a fast terahertz photon detector. The requirements for use of the SIS junction as a photon-counting detector with phot
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