Superconducting tunnel junction lasers: photon emission at twice-the-gap energy

Contemporary laser physics spans from black holes to nano-needles. Solid-state lasers cover wide range of photonic spectrum. However, superconducting materials are not yet quite serving this task. Despite remarkable recent progress in the field of coherent photon emission from Josephson junction-based lasers, these devices are not emitting in terahertz range. That is not a positive factor for applications, because existing terahertz lasers are bulky and cannot be used in cryogenic environment, especially when the heat-load on the cold platform is an issue. Thus, having compact and high efficiency lasers can make difference for some demanding applications. Achieving superconducting lasers requires population inversion, or the so called “negative temperature” state. Injection of electron excitations in tunnel junctions can be a solution. However, a great accuracy in applying correct values of pair-breaking voltages in these junctions, as well as choosing right parameters, both material and geometric, are required. We discuss these topics, as well as address competing physical processes at the “negative temperature” state in superconductors.