Numerical characterization of impedance transformers consisting of vertically stacked superconducting transmission lines

A novel coupling configuration between an SIS mixer, an antenna, and a local oscillator has been numerically characterized for an integrated receiver operated in the sub-THz region. The configuration consists of two vertically stacked superconducting striplines, where the thickness of their common electrode t/sub 2/ is of the order of its magnetic penetration depth /spl lambda//sub 2/. It has been found that the coupling efficiency between the antenna and the mixer decreases with decrease of t/sub 2///spl lambda//sub 2/, while that between the mixer and the local oscillator has a local maximum. It is also shown that the coupling efficiencies increase with decrease of Q/sub A/, the Q-factor due to the tunneling loss in the mixer. For Q/sub A/=5.0 and 16, 75-85% of the available signal power and sufficient power from the local oscillator can be absorbed in the tunnel conductance of the mixer at 1 THz. This coupling efficiency is equal to or larger than that of a conventional planar coupler.