Submillimeter-wave mixing and noise in HTS Josephson junctions

A Josephson quasioptical detector comprising YBaCuO junction on bicrystal M gO substrate, integrated log-periodic Au antenna, MgO extended hyperhemisph ere lens attached to the substrate and 1.4 GHz cold amplifier with cold cir culator has been designed, fabricated and experimentally studied. We fabric ated several integrated Josephson junctions 1-2 mu m wide which at 4.2 K de monstrate RSJ IV curves with critical currents 100-300 mu A and normal resi stance of 10-20 Ohm. The noise temperature (T-N) at 4.2 K and 430 GHz measu red by a hot/cold load method in fundamental mixing mode with external BWO local oscillator brings T-N=1100 K for the junction shunted by low-inductan ce resistive shunt and T-N=1400 K without shunting. At bath temperatures of 20-40 K the noise temperature was in the range 1500-3000 K, at 50 K the no ise temperature was below 2000 K, and at 60-70 K increased over 3000 K in t he frequency range 410-586 GHz. The noise temperatures measured in the self -pumping mode vary between 580-2300 K with dc bias in the range 0.1-0.5 mV. The linewidth of Josephson oscillations has been extracted from a selectiv e detector response and IF noise voltage dependence. The measured excess no ise can be attributed to shot noise and multiple Andreev reflection in pinh oles with Ss'S structure. We observed small subharmonic steps in IV curve a nd noise dependencies, which are typical for SNS junctions.