Highly sensitive heterodyne receivers for submillimeter-wave astronomy

This paper reports the current status in our development of sensitive heterodyne detectors for the efficient down-conversion of submillimeter-wave signals to D-band. These detectors use the strong non-linearities of Nb/Al/AlOx/Nb tunnel junctions and employ waveguide or quasi-optical techniques to couple local-oscillator and radio-frequency signals to the junctions via a corrugated feed-horn or a log-periodic antenna and quartz hyper-hemispherical lens combination. The fabrication of Nb/Al/AlOx/Nb tunnel junctions is described and measured DC and RF characteristics are presented. Current densities of approximately 3000 A/m(2) and capacitances of 40fF/mu m(2) have been recorded - making such junctions applicable for down-conversion from submillimeter-wave frequencies with the aid of suitable distributed tuning structures. Proof of principle heterodyne mixing with series junctions integrated with planar antennae and planar intermediate frequency circuits has been demonstrated and preliminary results are described herein. Furthermore, the design of a broad-band open structure mixer for use at submillimeter-wave frequencies employing Nb/Al/AlOx/Nb junctions is also described. In particular, the design of optimised junction embedding circuits and planar log-periodic antennae is detailed. The latter has resulted in a planar antenna design which gives broad-band performance in terms of impedance and beam-pattern over the 90-400 GHz band. Junction characteristics are used to calculate the values of the junction embedding impedances needed to realise specific intrinsic cut-off frequencies and an optimised junction/antenna match