PERFORMANCE LIMITATIONS OF NIOBIUM-BASED SUBMILLIMETER-WAVE QUASI-PARTICLE MIXERS OPERATING NEAR THE GAP FREQUENCY

We have measured the noise temperature of heterodyne receivers employi ng Nb/AlOx/Nb tunnel junction mixers at frequencies ranging from 70% t o 93% of the gap frequency of niobium (similar to 700 GHz). The sensit ivity of the receiver is decreased by the overlap of the n=l and n=2 p hoton steps of opposite sign. At bias voltages where these photon step s overlap, there is an increase in receiver noise up to 50%. Theoretic al calculations using the Tucker theory agree well with the observed m ixer performance. This overlap already affects the receiver operation for best performance at frequencies well below 700 GHz. (C) 1995 Ameri can institute of Physics.