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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Detection of Direct and Indirect Terahertz Waves using a Nb-based Superconducting Tunnel Junction

Chiko Otani, R. Nakano, Tohru Taino, Hiroaki Myoren, S. Takada, Takayuki Shibuya, Seiichiro Ariyoshi, H. Sato, Hirohiko M. Shimizu, Kodo Kawase

Published Jan 18, 2006• 0 citations
Publisher

Abstract

We are developing a broadband THz detectors using Nb-based superconducting tunnel junctions. We found two components in a current signal for 1-2 THz pulse input. From the comparison of results obtained for the signals, the frequency response for the different size of STJs and the STJs fabricated on different substrate materials, we demonstrate that they correspond to photons absorbed with the superconductor electrode (referred as 'direct' component) and phonons generated by photons absorbed with the substrate (refered as 'phonon-mediated' component).

DOI
10.1109/icimw.2005.1572509

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We are developing a broadband THz detectors using Nb-based superconducting tunnel junctions. We found two components in a current signal for 1-2 THz pulse input. From the comparison of results obtained for the signals, the frequency response for the different size of STJs and the STJs fabricated on different substrate materials, we demonstrate that they correspond to photons absorbed with the superconductor electrode (referred as 'direct' component) and phonons generated by photons absorbed with the substrate (refered as 'phonon-mediated' component).
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.
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 demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.
Superconducting SIS (Superconductor-Insulator-Superconductor) tunnel junctions are promising application prospect in the incoherent detection of astronomy observation. In this paper, the direct detection performances of Nb and NbN based superconducting SIS junctions, which cover about 250 GHz and 140 GHz centered at 600 GHz respectively, are characterized at a bath temperature of 4.2 K. The experiments show that Nb and NbN based SIS junctions have a current shot noise of 2.5×10-12and 3.1×10-12A/radicHz, a current responsivity of 303 and 187 A/W (corresponding to the quantum efficiency of 0.67 and 0.43) respectively. Thus their respective NEPs are 8.4×10-15and 1.7×10-14W/√Hz.