High-frequency electron dynamics in thin film superconductors and applications to fast, sensitive THz detectors.

摘要

This thesis is an experimental study of the dynamics and noise processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers which will serve as ultra-low noise detectors in THz heterodyne receivers. The conversion efficiency and output noise of devices of varying lengths were measured with rf frequencies between 8 and 40 GHz. The devices studied consist of 100 A thin film Nb bridges connected to thick (1000 A), high conductivity normal metal (Au) leads. The lengths of the devices studied range from 0.08 {dollar}mu{dollar}m to 3 {dollar}mu{dollar}m. For devices longer than the electron-phonon interaction length {dollar}Lsb{lcub}e-ph{rcub}equivsqrt{lcub}Dtausb{lcub}e-ph{rcub}{rcub}{dollar}, with D the diffusion constant and {dollar}tausbsp{lcub}e-ph{rcub}{lcub}-1{rcub}{dollar} the electron-phonon interaction rate, the hot-electrons are cooled dominantly by the electron-phonon interaction, which in Nb is too slow for practical applications. If the device length is less than {dollar}pi Lsb{lcub}e-ph{rcub} ({lcub}approx{rcub}1 mu{dollar}m at 4.2 K), then out-diffusion of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency (IF) bandwidth is found to vary as {dollar}Lsp{lcub}-2{rcub}{dollar}, with L the bridge length, as expected for diffusion cooling. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-{dollar}Tsb{lcub}c{rcub}{dollar} superconducting bolometric mixer. The component of the output noise not due to Johnson noise decreases with frequency in the same manner as the conversion efficiency, consistent with a model based on thermal fluctuations. The noise bandwidth is 1.4-9.4 times larger than the gain bandwidth, and the mixer noise is low, ranging from 120-530 K (double side-band).; The crossover from phonon dominated to diffusion dominated behavior is also demonstrated using noise thermometry measurements in the normal state. Scalar measurements of the device differential impedance in the intermediate state agree with a theoretical model which takes into account the thermal and electrical dynamics.