Quasi-optical Terahertz Sis Mixer

The performance of quasi-optical (QO) SIS mixers designed for operation around 1 THz is evaluated. Mixers incorporating Si elliptical lens with either double slot-line or double dipole antennas with back reflectors have been fabricated and measured. Nb/AlAlOx/Nb superconductor-insulator-superconductor (SIS) tunnel junctions are integrated with a NbTiN-SiO2-Al microstrip circuit to tune out the junction’s geometrical capacitance and to match the antenna impedance to the junction over a wide frequency range. The direct response, of the mixer measured by means of a Michelson Fourier transform spectrometer (FTS), is presented showing the mixer’s ~30% instantaneous bandwidth. This bandwidth appears to be limited by the material properties of the NbTiN ground plane. Noise temperatures of 245 K @ 850 GHz, 310 K @ 980 GHz and 400 K @ 1020 GHz are presented. This is a two-fold improvement over best previously reported results for frequencies around 1 THz. A far-field antenna beam pattern of the antenna-lens combination is measured at 900 GHz. The 1-st order side lobe level is –(16-18) dB and total power in the cross-polar beam is 0.3 % of the power in the co-polar beam.