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Left: Microscope photographs of two detectors with different junction sizes (50 and ). They have the layer structure on the sapphire substrate. Right: Typical curve of the -square junction at 1.5 K. The normal state resistance was , and the resulting critical current density was about .
Spectral response of the absorption-film STJ detectors (the two solid lines). The resolution of the spectrometer is 15 GHz. The broadband signals were observed from 0.7 up to 2.4 THz. The radiation above 2.4 THz was sharply cut by metal-mesh low-pass filters at the cryostat window. We have also excluded the data below 0.4 THz, where the source has a low spectral brightness. The spectrum of a PAT-STJ detector is superimposed for reference (dotted line) (Ref. 12). These three lines are normalized at their respective main peaks because of the different sensitivities of the detectors.
Absorptivity of a normal-conducting Nb film as a function of frequency and film thickness. The contour step is 0.1 in absorptivity. This calculation was carried out using a sapphire substrate with and . A measured Nb-film resistivity at 4.2 K, (Ref. 14), was adopted for this calculation.
Proposed design of the absorption-film STJ detector. The film thickness in this schematic is exaggerated for clarity.
Comparison of terahertz-wave STJ detectors.
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