Brighter Attosecond X-Ray Pulses

With an eye toward enabling higher-resolution time-resolved photoemission spectroscopy and nanostructure imaging, a research team has achieved a long-held goal of creating much brighter trains of attosecond X-ray pulses (Science 2015, DOI: 10.1126/science.aac9755). The technique involves high-harmonic generation, in which a laser pulse drives an electron out of an atom then pushes the electron back. If the electron recombines with the atom, it releases its kinetic energy as a burst of attosecond-duration X-rays. The X-ray emission per atom is highest when the driver is an ultraviolet laser. But researchers also need to get many atoms to emit X-rays so that their phases match, something that until now has been possible only when the driver is a mid-infrared laser. A team led by Tenio Popmintchev, Henry C. Kapteyn, and Margaret M. Murnane of the physical sciences research institute JILA has now been able to get phase matching by focusing a UV laser on argon gas to create a plasma mixture of Ar and Ar ions. IR pulses can’t penetrate the plasma, but UV light can. The refractive properties of the ions help phase-match the emitted X-rays. The resulting X-ray pulses are not only brighter than those achieved from mid-IR, but they also have a comblike spectrum with narrow line widths that could improve imaging resolution compared with the continuum generated from mid-IR.