Performance of a 700 GHz unilateral finline sis mixer

We present a novel design and the measured performance for a superconductor-insulator-superconductor (SIS) mixer, operating near the superconducting gap of niobium (Nb), in the frequency range of 600-700 GHz. A key feature of the mixer design is the employment of a unilateral finline taper to provide smooth transition from the high-impedance waveguide mode to the low-impedance of planar circuits suitable for the operation of SIS tunnel junction. This geometry of the transition can be electromagnetically modelled and optimised to give a short mixer chip with wide RF bandwidth. The finline taper and all the superconducting transmission lines are integrated on-chip and deposited on a 60 μm thick quartz substrate. This results in an extremely simple mixer block design, comprising a feed horn and a straight waveguide section; no backshort or any mechanical tuning structure is needed. In this paper, we describe the design of the mixer chip, including full electromagnetic simulations of the passive circuits, combined with the heterodyne mixer gain and noise temperature predictions.We have tested the mixer performance from 595 GHz to 702 GHz and measured best receiver noise temperature of 145 K at 600 GHz, corrected for a beam splitter of 75 μm thickness. The need for a thick beam splitter was caused by the lack of sufficient power from the local oscillator (LO) which was optimised between 630-720 GHz and because our mixer was tuned low as a result of larger than designed tunnel junction. Nevertheless, our investigation has demonstrated that superconducting finline mixers work well at high frequencies and have several advantages over other designs, in particular simple mixer block and large substrate for integration of planar circuits.