Impedance mismatch elimination for improved THz detection by superconducting Josephson junctions

Superconducting Josephson junctions as radiation detectors dramatically reduce the complexity of THz detection systems. Rather than employing a complicated high frequency RF measurement system, the intensity and frequency of the RF radiation can be easily derived from the Shapiro current steps in the I-V characteristics of the junction. The system performance is highly sensitive to impedance mismatch between the antenna and the junction. The mismatch is much more problematic with superconducting THz detection due to the extremely low impedance of the junction, only a few Ohms. We performed a theoretical analysis of the interrelations between the input impedance, the antenna impedance, and the Josephson junction equivalent impedance, through a cascade, chain connection, of two "2 port network" model. To obtain matching, a new structure was designed, in which the junction is placed between the ends of two matching strips, remote from the antenna gap. Accordingly, extensive simulation of the detection system, for various dimensions of the strips, and for various bow-tie antenna parameters, were performed. High impedance matching was obtained for the range of 260 to 280GHz. Simulation results show high directionality, with radiation gain of 3.22dB and low return loss of up to 29dB.