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SIS mixers

We use Niobium-Aluminumoxide-Niobium (Nb-AlOx-Nb) Superconductor Insulator Superconductor (SIS) tunneljunctions as heterodyne mixers. SIS junctions have a highly non-linear IV-characteristic and are capable of quantum limited mixer performance. The performance of SIS mixers is well understood and described in various review papers. The frequency range where SIS mixers can be applied is limited by the superconducting material of the electrodes.The critical temperature (Tc) of the electrodes (9.2K for Niobium) is related to the gap frequency (fgap) of the material ( fgap=3.5Tc.kB/h, BCS theory), which is around 700 GHz for Niobium. Above the fgap the photon energy is larger than the "binding" energy of the Cooper pairs and the electrodes loose their superconducting properties under radiation. Because an SIS junction contains its non linearity up to the sum of the gap frequecies of both electrodes, the Nb-AlOx-Nb junction can theoretically be used to approximately 1400 GHz (in practice somewhat lower)

280-380 GHz mixer device DC IV-curve with two SIS junctions in parallel. The IV curve without coupled 0.35 THz radiation is shown in black and and the curve with 0.35 THz radiation is in red, clearly showing the photon assisted tunneling steps. Cooper pair tunneling is suppressed by a carefully adjusted magnetic field in the plane of the junction.

Photograph of a part of the device. The dark brown layer is the first Nb layer, the layer with the lightest color is 300 nm SiO2 on top of the first Nb layer.The more gold coloured layer is the second Nb layer. The 2 black dots in the vertical bar are the SIS junctions, 47µm apart. The quarterwave line connecting to the antenna (left) is 80µm long. To the right is the first section of the RF blocking filter.

SIS junctions as THz mixers

For ground based observatories (e.g. NANTEN2, APEX) below 1 THz more observing efficiency can be achieved by dedicated application of sideband separating techniques, by wider instantaneous bandwidths, and, most imporantly, by developing large focal plane arrays with many pixels .At these frequencies SIS mixers are the most sensitive mixers available and therefore a large part of our development is towards state of the art mixers (including e.g. side band separation) of a structural form that is suitable for focal plane arrays, and that can be fabricated in sufficient quantities.

SIS junctions for mixers have an area of the order of 1µm2  and their specific capacitance is approximately 85fF/µm2. Our group develops predominantly waveguide mixers, where the free-space impedance of 377 Ohm is converted to a value around 40 Ohm.( link to HF page). Due to the large capacitance of the SIS junction which is in parallel to the tunnel barrier, the impedance of the SIS junction at THz frequencies is below 1 Ohm, hence the necessity of a matching (tuning) circuit.