A broad-band THz radiation detector using a Nb-based superconducting tunnel junction

We have proposed and been developing a new broad-band terahertz (THz) radiation detector using a Nb-based superconducting tunnel junction (STJ). The STJ´s were fabricated on LiNbO₃ and LiTaO₃ mono-crystal substrates. We radiated monochromatic THz pulses in the range 1-2 THz with the pulse repetition rate of 49 Hz, and successfully detected the corresponding periodic signals via a charge-sensitive preamplifier. The current signal obtained by the derivative of the output signal gave the time response of about 15-20 microseconds. The frequency response was compared with that of a conventional pyro-electric sensor. We found their frequency response showed the similar shape in frequency, demonstrating the flat response of the STJ detector in 1-2 THz. These results show that the detector can be used as a broad-band THz and far-infrared radiation detector above the frequency corresponding to the energy gap of the superconductor used for the base electrode.