Studies of Noise in Josephson-Effect Mixers and Their Potential for Submillimeter Heterodyne Detection.

This thesis describes theoretical and experimental investigations into the dynamics and noise processes of Josephson junctions, with the intent of evaluating their potential as mixers in heterodyne instruments for submillimeter -wave detection. Recent progress in the fabrication of high-T_{c} superconductors has led to Josephson-effect devices with I_ {C}R_{N} products of up to ten millivolts, which might be suitable for mixing at frequencies of many terahertz, but an important question is that of the sensitivity which can be attained. Previous experimental work on Josephson mixing, as well as modeling of mixer performance based on the resistively-shunted junction (RSJ) model, suggests the existence of an "excess" noise, which degrades the sensitivity. The origin of this noise was not clearly understood, however, nor was its exact magnitude or expected scaling with frequency or junction characteristics known. In the first part of this thesis research, extensive numerical simulations were performed with the RSJ model, including calculations of mixer noise and conversion efficiency. These calculations have revealed that the source of excess noise is the AC Josephson oscillations of the device, which can be completely incoherent, with a linewidth comparable to their frequency. While this noise is intrinsic and unavoidable, an optimized mixer is still shown to be capable of interesting sensitivity levels, and the excess noise is expected to become relatively less important as the operating frequency is increased. Secondly, a process has been developed for the fabrication of reproducible Josephson devices based on Nb and NbN tunnel junctions, shunted with a AuGe resistor. These devices have non-hysteretic I-V curves, normal-state resistances of about 40 Omega, and I _{C}R_{N} products of about half a millivolt, and should be nearly optimal for mixing at 100 GHz. Heterodyne receiver measurements using these junctions have obtained noise temperatures as low as 190 K (DSB), with -6 dB conversion efficiency at 100 GHz. These results are still a factor of about four higher than predicted by the RSJ simulations. Accurate measurements of the available noise power of the junctions at 1.5 GHz were made, and confirmed that the receivers were limited by elevated junction output noise. The deviations from theoretical predictions are shown to be caused by the nonlinear interaction of the junction with the embedding circuit. While this work points out some of the complexity introduced by the strong nonlinearity, it is still expected that Josephson -effect mixers may be useful for heterodyne detection at very high frequencies.