Search

We have developed a new terahertz (THz) wave detector using an Nb-based superconducting tunnel junction (STJ). The STJ consists of a Nb/Al/AlO x /Al/Nb structure on a single-crystal LiNbO 3 substrate. Using LiNbO 3 as a substrate, an irradiating THz wave is converted to phonons in LiNbO 3 , which has a relatively high absorption coefficient. The phonons propagating in the substrate reach the Nb base electrode of the STJ and break Cooper pairs. As a result, the THz wave can be detected as tunneling current signals in the STJ. THz waves from a widely tunable THz-wave parametric oscillator (TPO) were irradiated on to the STJ, which acted as a THz wave detector. The STJs were cooled to an operating temperature of 0.4 K using a 3 He cryostat. We have observed STJ signals related to THz wave irradiations. In this paper, fabrication processes and characteristics of THz wave detectors using STJs are also discussed.

We have proposed and demonstrated a new broadband terahertz (THz) wave detector using a superconducting tunnel junction (STJ) fabricated on LiNbO3, LiTaO3, and sapphire substrates. These substrates have relatively high absorption efficiency for THz waves, and we have expected that incident THz waves from substrate side are absorbed and that generate many THz phonons which break Cooper pairs in the base superconductor electrode of the STJ. Using a frequency-tunable THz pulse source in 1–2 THz, we have confirmed that the current signal corresponding to the THz input was observed. In addition, we found two current components with different timescales for the first time. We interpret that the slow component corresponds to phonons absorbed with the substrate and that the fast one corresponds to the directly absorbed photons with the superconductor electrode of the STJs.

High quality STJs of type Nb/Al/AlxOy/Al/Nb and Nb/Al/AlxOy/Nb/NbN, in which Al and Nb act as quasiparticle traps , were realized and tested under 6 KeV X ray radiation. The experimental results show an enhancement of the collected charge with respect to basic Nb/Nb configuration. An energy resolution of 300 eV was achieved for a 50x50 μm2 NbN-based STJ.