Detection of direct and indirect terahertz waves using a Nb-based superconducting tunnel junction

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).