Critical Temperature Dependence of High Frequency Electron Dynamics in Superconducting Hot-Electron Bolometer Mixers

Critical Temperature Dependence of High Frequency Electron Dynamics in Superconducting Hot-Electron Bolometer Mixers Irfan Siddiqi May, 2002 Superconducting diffusion-cooled hot-electron bolometer (HEB) mixers are ideal candidates for use in terahertz heterodyne spectroscopy (THz). This thesis investigates mixer performance as a function of the superconducting transition temperature (Tc). In the present work, three different materials systems have been used to obtain devices with Tc between 1 and 5.5 K – Al, Nb in a magnetic field, and Nb-Au. The mixing measurements were conducted in a 30 GHz microwave setup with the mixer block at T = 0.22K. Microwave measurements are used to understand the underlying physics of the HEB mixer, and are a guide for the future development of THz receivers. The mixer noise temperature is found to be equal to approximately 25 times Tc, and the dominant noise is assumed to be thermal fluctuations. The LO power is found to equal (0.7 nW/K) · Tc when the HEB is cooled to well below Tc. As for saturation effects, input saturation is expected when the incident noise power is greater than (0.14 nW/K) · Tc, and output saturation should result for incident power greater than (27 pW/K) · Tc. Additional performance limitations are observed in Al devices which have a long coherence length. Superconductivity is suppressed in the ends of the microbridge due to the proximity effect. This results in a minimum bridge length of ~ 6-7 ξ(0) for superconductivity to be observed, therefore imposing an upper limit for the IF bandwidth of approximately (8 GHz/K) · Tc for N-S-N HEB structures. Output saturation is also more likely in Al devices with normal ends. Nb-Au mixers perform the best and are very convenient with their tunable Tc. A mixer Tc of 2 K is predicted to be optimum for many applications. Such devices should have lower noise than Nb and should not exhibit saturation effects. Critical Temperature Dependence of High Frequency Electron Dynamics in Superconducting Hot-Electron Bolometer Mixers A Dissertation Presented to the Faculty of the Graduate School of Yale University in Candidacy for the Degree of Doctor of Philosophy By Irfan Siddiqi Dissertation Director: Professor Daniel E. Prober May, 2002