Engineering physics of superconducting hot-electron bolometer mixers

摘要

Superconducting hot-electron bolometers are presently the best performingmixing devices for the frequency range beyond 1.2 THz, where good qualitysuperconductor-insulator-superconductor (SIS) devices do not exist. Theirphysical appearance is very simple: an antenna consisting of a normal metal,sometimes a normal metal-superconductor bilayer, connected to a thin film of anarrow, short superconductor with a high resistivity in the normal state. Thedevice is brought into an optimal operating regime by applying a dc current anda certain amount of local- oscillator power. Despite this technologicalsimplicity its operation has been found to be controlled by many differentaspects of superconductivity, all occurring simultaneously. A core ingredientis the understanding that there are two sources of resistance in asuperconductor: a charge conversion resistance occurring at annormal-metal-superconductor interface and a resistance due to time- dependentchanges of the superconducting phase. The latter is responsible for the actualmixing process in a non-uniform superconducting environment set up by thebias-conditions and the geometry. The present understanding indicates thatfurther improvement needs to be found in the use of other materials with afaster energy-relaxation rate. Meanwhile several empirical parameters havebecome physically meaningful indicators of the devices, which will facilitatethe technological developments.