Fixed Tuned Broadband Matching Structure for Submillimeter SIS Receivers

We have designed, fabricated, and tested a quasi-optical submillimeter wave receiver with an Nb / AIO(x) / Nb tunnel junction. This design incorporates a hybrid antenna fed by a planar logarithmic spiral structure in order to couple to the radiation field from the telescope. An important aspect is a novel matching circuit which requires several layers of photolithographic processing on top of the actual tunneling device, only possible with a refractory superconductor like Nb. Computer modeling of the device correctly predicted the measured bandwidth and the characteristic frequencies to within 8% as measured by SIS direct detection using a Fourier Transform Spectrometer. We have obtained a good match from 200 to 475 GHz between the antenna and a relatively large area ( 1 mu m(2)) tunnel junction with omega RnC approximate to 2-4. This compares to a matching circuit consisting of a simple open-ended inductive stub that attains only a few percent of total bandwidth in the submillimeter band or inductively tuned SIS arrays where the upper limit of operation is currently below the submillimeter band. Noise measurements were made at 318 GHz, 395 GHz, 426 GHz, and 492 GHz, yielding uncorrected double sideband receiver noise temperatures of 200 K, 230 K, 220 K, and 500 K, respectively. Using the same optics, coupling efficiencies between the receiver and the Caltech Submillimeter Telescope were found to have values approaching those achieved by the best waveguide horn based receiver systems.